#17 Read About Proper Pruning of Trees
Edited by Gordon Mann and Len Phillips, updated January 2023
Sections You may go directly to the section by clicking on titles listed below.
Edited by Gordon Mann and Len Phillips, updated January 2023
Sections You may go directly to the section by clicking on titles listed below.
Note: Click on green text in each section for more information and photos.
Introduction
Trees growing in natural settings do not need pruning. Trees only need pruning for the benefit to people to either modify form, reduce risk, or fit into a people-occupied area. Trees around people are too often over-pruned. It is based on a misunderstood sight line for sign clearance, a desired poodle formed crown, or the tree grew too close to a structure. Too many trees have been over-pruned which stresses trees and hinders growth. Poor pruning and not improving structure increase the risk of branch and whole tree failure. The intent of this Topic is to guide practitioners to perform better pruning. The simple solution is to give the tree one or two structural prunings two and three years after the tree was installed. The rest of this topic describes other options.
Trees in the forest grow straight, tall trunks as they compete with neighboring trees for sunlight. Occasionally they grow to the side to find light, then straighten upwards to the sun. If a branch breaks in the forest, it lays on the ground and serves as soil amendments. Trees in the urban landscape have less shade competition, so there can be an abundance of sunlight all around the canopy that encourages trees to develop multiple leaders and many branches. Unfortunately, this type of structure can lead to breakage and a shorter life span. When a branch or whole tree breaks in an area around people, there is risk of injury or property damage. Pruning trees to grow one dominant leader and smaller lateral, well-spaced branches are more likely to have fewer damaged branches after a storm. The dominant leader also makes trees better able to retard the spread of decay within the tree. Even decurrent or hardwood trees should retain their central leader until the permanent branches are in place to spread their crown. Video
Objectives
Every wound to a tree hurts the tree. Every pruning cut is a wound. If the result of our pruning does not provide some gain for the tree, we should not be doing it. Too often, people do things to nature because we can, not because we should. Pruning should be performed with specific objectives in mind, and based on what the tree can tolerate to achieve those objectives while continuing to thrive. Examples of objectives include, but are not limited to:
If you look carefully, all of the above items are actually necessary to meet the needs of humans. Trees have grown on this planet for millions of years without pruning in natural settings. The desire to grow trees where people live requires that trees be managed to reduce maintenance costs as well as the consequences of tree failure injuring a person or damaging property.
Pruning large and mature trees focuses primarily on ensuring human “safety” and passage, minimizing branch failure or total-tree failure near targets such as buildings and cars, and maintaining tree health and vigor. This means reducing the risk of branch failure conditions by reducing foliage mass and weight focusing on the ends of the largest and longest branches, and branches with defects that reduce strength. Canopy raising should be performed where needed for clearance or visibility, along with removing dead branches, while retaining small-diameter interior live branches. Most pruning should be to improve structure, reduce risk, and retain the natural appearance of the tree.
Compartmentalization
Trees are the largest, oldest living organisms on the planet. Their individual ability to survive for hundreds of years is based on an incredible defense mechanism called compartmentalization.
When a tree is injured, it walls-off or compartmentalizes the injured area internally by chemically altering the wood in predictable patterns. Trees do not heal like people do. Instead they set up barriers against spreading decay and damage by limiting the extent of the wound as the tree grows new wood around it or to cover it. Sometimes a tree compartmentalizes well and only a small pocket of decay remains sealed off inside the trunk as the tree grows larger. Sometimes the decay spreads up and down in a long column. The only wood that does not decay is wood that develops after an injury has occurred.
The knowledge of this process means that arborists no longer have to dig out the decayed cavity in a tree and fill it like a dentist fills a cavity in a person's tooth. It also means that there is no need to cut back to sound wood or drill a hole to drain water out of the cavity. Both of these activities make brand new wounds that can allow the decay to enter into previously protected areas of the tree. Plus, the cavity can become a water collection location for small tree climbing animals and birds.
The other aspect of compartmentalization is the chemical presence in the wood cells found in the branch connections that inhibits decay. A tree should be pruned so the wound affects the lateral or subordinate branch that was removed, and decay should not enter into the parent branch or trunk. When the parent stem or trunk is wounded during pruning or from other injury, it opens the door for decay organisms to enter.
Introduction
Trees growing in natural settings do not need pruning. Trees only need pruning for the benefit to people to either modify form, reduce risk, or fit into a people-occupied area. Trees around people are too often over-pruned. It is based on a misunderstood sight line for sign clearance, a desired poodle formed crown, or the tree grew too close to a structure. Too many trees have been over-pruned which stresses trees and hinders growth. Poor pruning and not improving structure increase the risk of branch and whole tree failure. The intent of this Topic is to guide practitioners to perform better pruning. The simple solution is to give the tree one or two structural prunings two and three years after the tree was installed. The rest of this topic describes other options.
Trees in the forest grow straight, tall trunks as they compete with neighboring trees for sunlight. Occasionally they grow to the side to find light, then straighten upwards to the sun. If a branch breaks in the forest, it lays on the ground and serves as soil amendments. Trees in the urban landscape have less shade competition, so there can be an abundance of sunlight all around the canopy that encourages trees to develop multiple leaders and many branches. Unfortunately, this type of structure can lead to breakage and a shorter life span. When a branch or whole tree breaks in an area around people, there is risk of injury or property damage. Pruning trees to grow one dominant leader and smaller lateral, well-spaced branches are more likely to have fewer damaged branches after a storm. The dominant leader also makes trees better able to retard the spread of decay within the tree. Even decurrent or hardwood trees should retain their central leader until the permanent branches are in place to spread their crown. Video
Objectives
Every wound to a tree hurts the tree. Every pruning cut is a wound. If the result of our pruning does not provide some gain for the tree, we should not be doing it. Too often, people do things to nature because we can, not because we should. Pruning should be performed with specific objectives in mind, and based on what the tree can tolerate to achieve those objectives while continuing to thrive. Examples of objectives include, but are not limited to:
- enhance the tree's natural form or aesthetics,
- reduce the size to meet a site need,
- improve clearance of a view,
- reduce diseased or damaged branches,
- reduce the risk of storm damage,
- improve structure,
- repair after damage.
If you look carefully, all of the above items are actually necessary to meet the needs of humans. Trees have grown on this planet for millions of years without pruning in natural settings. The desire to grow trees where people live requires that trees be managed to reduce maintenance costs as well as the consequences of tree failure injuring a person or damaging property.
Pruning large and mature trees focuses primarily on ensuring human “safety” and passage, minimizing branch failure or total-tree failure near targets such as buildings and cars, and maintaining tree health and vigor. This means reducing the risk of branch failure conditions by reducing foliage mass and weight focusing on the ends of the largest and longest branches, and branches with defects that reduce strength. Canopy raising should be performed where needed for clearance or visibility, along with removing dead branches, while retaining small-diameter interior live branches. Most pruning should be to improve structure, reduce risk, and retain the natural appearance of the tree.
Compartmentalization
Trees are the largest, oldest living organisms on the planet. Their individual ability to survive for hundreds of years is based on an incredible defense mechanism called compartmentalization.
When a tree is injured, it walls-off or compartmentalizes the injured area internally by chemically altering the wood in predictable patterns. Trees do not heal like people do. Instead they set up barriers against spreading decay and damage by limiting the extent of the wound as the tree grows new wood around it or to cover it. Sometimes a tree compartmentalizes well and only a small pocket of decay remains sealed off inside the trunk as the tree grows larger. Sometimes the decay spreads up and down in a long column. The only wood that does not decay is wood that develops after an injury has occurred.
The knowledge of this process means that arborists no longer have to dig out the decayed cavity in a tree and fill it like a dentist fills a cavity in a person's tooth. It also means that there is no need to cut back to sound wood or drill a hole to drain water out of the cavity. Both of these activities make brand new wounds that can allow the decay to enter into previously protected areas of the tree. Plus, the cavity can become a water collection location for small tree climbing animals and birds.
The other aspect of compartmentalization is the chemical presence in the wood cells found in the branch connections that inhibits decay. A tree should be pruned so the wound affects the lateral or subordinate branch that was removed, and decay should not enter into the parent branch or trunk. When the parent stem or trunk is wounded during pruning or from other injury, it opens the door for decay organisms to enter.
Pruning Benefits
Trees are usually pruned for the benefits to people. Choosing the correct pruning approach is relatively easy, even for the less experienced arborist or landscape architect because the tree guides the decision-making process. The key indicators of tree growth are:
For example, if a tree has dead branches, those branches could be removed. If a tree has long, heavy branch ends, the ends can be shortened or lightened. If a tree has long branches or trunks with little natural taper, the trunk or branch can be shortened or reduced. If a tree is too low, the lower branches should be reduced so the ends raise up or removed to achieve the desired clearance. When trees are already mature, reducing the length of the large branches reduces the leverage and should reduce the risk of branch failure.
Arborists, landscape architects, and tree owners must realize that more is not always better when it comes to pruning. The leaves feed the tree. If too many leaves are removed, the tree’s response is to produce more leaves in quick shoots from latent buds, and these branches over time may be poorly attached. To reduce the reaction of heavy sprouting, only the necessary live branches should be removed to accomplish the objective.
The benefits of pruning can provide:
Pruning to Maintain Landscape Trees
Before removing any branches, several factors must be considered.
The answers to these questions will guide which branches and to what extent the tree is pruned.
When deciding what branches to remove, one must be specific about which branches will be removed. Previous descriptions have stated, “... remove dead, diseased, crossing branches.” By following these descriptions, two branches rub together, and both can be removed and be in compliance with the specifications. Additionally, anthracnose is a disease. One could remove every anthracnose infected branch and stub the tree and be in compliance with the specifications.
Additionally, we should be explaining where in the tree the pruning cuts will occur, and that description will be based on the objective and the assignment. A pruning to perform clearance over a building, around a streetlight or traffic signal, or for a view may limit the pruning to a certain part of the tree.
Pruning for size reduction or branch shortening may work on those portions of the crown that extend beyond the desired canopy size or shape.
Pruning for removal of dead or diseased branches should set the branch diameter to be removed, and if a certain disease such as fire blight, mistletoe, or verticillium wilt, should direct how far below the infection the branches will be removed to.
Trees are usually pruned for the benefits to people. Choosing the correct pruning approach is relatively easy, even for the less experienced arborist or landscape architect because the tree guides the decision-making process. The key indicators of tree growth are:
- aspect ratio – the comparison between the subordinate branch diameter and the parent branch diameter;
- branch length and taper – how the branch diameter is reduced over the length of the branch;
- dead branches – branches that are no longer providing any food production for the tree;
- broken branches – branches that are weak, may fail, or allow insect and disease entry;
- co-dominant stems – two branches growing next to each other with the same stem diameter originating at a very narrow crotch;
- competing leaders – branches that are the same height or have larger leaf mass as the central leader, and can outgrow it;
- rubbing branches – one branch is causing damage to another branch by rubbing or growing into it;
- branches competing for the same space -- the leaves are growing in the same space on two or more branches either crowding or shading the other branch;
- clearance – the branches obstruct passage, block vision, or grow into structures.
For example, if a tree has dead branches, those branches could be removed. If a tree has long, heavy branch ends, the ends can be shortened or lightened. If a tree has long branches or trunks with little natural taper, the trunk or branch can be shortened or reduced. If a tree is too low, the lower branches should be reduced so the ends raise up or removed to achieve the desired clearance. When trees are already mature, reducing the length of the large branches reduces the leverage and should reduce the risk of branch failure.
Arborists, landscape architects, and tree owners must realize that more is not always better when it comes to pruning. The leaves feed the tree. If too many leaves are removed, the tree’s response is to produce more leaves in quick shoots from latent buds, and these branches over time may be poorly attached. To reduce the reaction of heavy sprouting, only the necessary live branches should be removed to accomplish the objective.
The benefits of pruning can provide:
- reduced risk of branch and whole-tree failure,
- reduction of insect or disease infestation,
- better clearance for vehicles and pedestrians,
- improved appearance,
- healthier and longer living trees.
Pruning to Maintain Landscape Trees
Before removing any branches, several factors must be considered.
- Why am I pruning this tree?
- What is my objective?
- What is the condition of the tree?
- What are the landscape functions provided by the tree?
- Will pruning maintain or enhance those functions?
- Are structural defects or storm damaged branches present that should be reduced or removed?
- Are branches interfering with power lines, houses, and walkways?
- Does the tree need to be reduced in size or shape?
- Will the tree benefit from the removal of live branches compared to the wounds we create?
- What are the customer's or the city's expectations and budget?
- Will only defective branches be removed or is there a benefit to removing live intact branches?
- Should the tree remain the same height and spread or are reductions necessary?
- Are low branches interfering with pedestrian or vehicular traffic?
- What is the smallest diameter cut that can be made to accomplish the objective?
- Is there a final diameter cut size limit on branches to be removed?
The answers to these questions will guide which branches and to what extent the tree is pruned.
When deciding what branches to remove, one must be specific about which branches will be removed. Previous descriptions have stated, “... remove dead, diseased, crossing branches.” By following these descriptions, two branches rub together, and both can be removed and be in compliance with the specifications. Additionally, anthracnose is a disease. One could remove every anthracnose infected branch and stub the tree and be in compliance with the specifications.
Additionally, we should be explaining where in the tree the pruning cuts will occur, and that description will be based on the objective and the assignment. A pruning to perform clearance over a building, around a streetlight or traffic signal, or for a view may limit the pruning to a certain part of the tree.
Pruning for size reduction or branch shortening may work on those portions of the crown that extend beyond the desired canopy size or shape.
Pruning for removal of dead or diseased branches should set the branch diameter to be removed, and if a certain disease such as fire blight, mistletoe, or verticillium wilt, should direct how far below the infection the branches will be removed to.
Proper Cuts
There are three ways to remove branches:
1. Branch removal cut – removes the smaller of two branches at a union or parent stem without cutting into
the branch bark ridge or branch collar, or leaving a stub.
a) prune the branch at the collar when one is visible,
b) if there is no visible collar, begin where the top of the branch makes an abrupt turn toward the trunk
and cut outside an imaginary line drawn perpendicular to the branch,
c) if there is included bark in the union, make the pruning cut at the base of the actual connection
between the branch and trunk.
2. Reduction cut – removes the larger of two or more branches or stems, or one or more co-dominant stems,
to a live lateral branch typically at least 1/3 diameter of the stem or branch being removed.
3. Heading cut – removes a branch or stem between nodes (leaving a stub) to a bud, or to a live branch typically less than 1/3 the diameter of the branch or stem being removed. To remove the end of a branch, there are two types of heading cuts, selective and non-selective. Selective cuts are always recommended.
They require removing branches back to a bud or smaller lateral. Correct pruning cuts should be made
just outside the branch collar to avoid wounding the remaining branch. Do not leave stubs and do not
injure the collar. Video
A non-selective cut is basically cutting off the tip or end of a branch at no place in particular. Wherever this cut is made, hidden dormant buds located directly below the cut are stimulated into growing several new branches right at the end of the cut. This new growth is most often skinny, unsightly, and straight water sprouts. Furthermore, the tree speeds up its growth rate, creating weak branches.
Once the pruning actions have been decided, the size of the smallest branch to remove is the next consideration. Typically, the branch diameters that have been used are 1/2”, 1”, 2” or 4” (1, 2.5, 5, or 10 cm). The size of the smallest branch to be pruned should be adjusted for the tree, by request, or the budget. When pruning a small tree such as a Japanese maple, the smallest branch to remove might be specified at 1/2 inch in diameter. This means that the selected branches greater than 1/2 inch are removed. If a 1/4 inch diameter is chosen instead, the time required to complete the task may be doubled or tripled.
There are three ways to remove branches:
1. Branch removal cut – removes the smaller of two branches at a union or parent stem without cutting into
the branch bark ridge or branch collar, or leaving a stub.
a) prune the branch at the collar when one is visible,
b) if there is no visible collar, begin where the top of the branch makes an abrupt turn toward the trunk
and cut outside an imaginary line drawn perpendicular to the branch,
c) if there is included bark in the union, make the pruning cut at the base of the actual connection
between the branch and trunk.
2. Reduction cut – removes the larger of two or more branches or stems, or one or more co-dominant stems,
to a live lateral branch typically at least 1/3 diameter of the stem or branch being removed.
3. Heading cut – removes a branch or stem between nodes (leaving a stub) to a bud, or to a live branch typically less than 1/3 the diameter of the branch or stem being removed. To remove the end of a branch, there are two types of heading cuts, selective and non-selective. Selective cuts are always recommended.
They require removing branches back to a bud or smaller lateral. Correct pruning cuts should be made
just outside the branch collar to avoid wounding the remaining branch. Do not leave stubs and do not
injure the collar. Video
A non-selective cut is basically cutting off the tip or end of a branch at no place in particular. Wherever this cut is made, hidden dormant buds located directly below the cut are stimulated into growing several new branches right at the end of the cut. This new growth is most often skinny, unsightly, and straight water sprouts. Furthermore, the tree speeds up its growth rate, creating weak branches.
Once the pruning actions have been decided, the size of the smallest branch to remove is the next consideration. Typically, the branch diameters that have been used are 1/2”, 1”, 2” or 4” (1, 2.5, 5, or 10 cm). The size of the smallest branch to be pruned should be adjusted for the tree, by request, or the budget. When pruning a small tree such as a Japanese maple, the smallest branch to remove might be specified at 1/2 inch in diameter. This means that the selected branches greater than 1/2 inch are removed. If a 1/4 inch diameter is chosen instead, the time required to complete the task may be doubled or tripled.
Pruning by Age
The following guidelines have been prepared to indicate when a tree should be routinely pruned as preventative maintenance. This guideline will vary depending upon the budget, species, growth rate, local climate, and site where the tree is growing.
When to Prune
Pruning should begin as soon as a tree is established and has resumed normal growth rates after being installed. This generally occurs two years after installation but may be longer on larger size trees.
Inspect newly installed trees on an annual basis for the first ten years after they become established and prune as needed to provide the desirable structure. On small maturing species, the first ten years after establishment is the critical period for structural pruning but on large maturing species, pruning for structure should occur as needed to repair damage for up to 25 years after installation. Pruning cycles generally can be extended to every 2-4 years during the 11-25 year period following establishment.
Planting Time
New trees should be purchased and pruned in accordance with this linked specification or the latest American Standard for Nursery Stock. This Standard provides buyers and sellers of nursery stock uniform sizes and a common terminology in order to standardize all sales of nursery stock.
Pruning young and medium-sized trees should be done when they have recovered from Installation stress. This can help minimize long-term problems by encouraging trees to develop a strong branch structure. The risk of mature tree failure can be minimized with appropriate pruning of trees that are near potential targets such as buildings, sidewalks, and picnic areas.
The First Pruning
The first pruning at the planting site should remove only dead, diseased, and broken branches back to a bud or branch union. Then remove any girdling, circling, deformed, diseased, or damaged roots. Also at the time of installation, select the central leader and remove or shorten the tops of any competing multiple leaders.
Retain all the scaffolding branches or temporary branches on the lower tree trunk. Temporary branches can be shortened, and retained to shade the trunk. These branches help the tree to establish itself in a new location and they should be removed before the branches become half the diameter of the trunk. Pruning at the time of installation may hinder a tree's ability to make food to recover from the shock of installation, so pruning may be delayed until the tree has recovered from the installation. Also delay removing any hanging or drooping branches for a couple of years (weeping trees should not be installed near a sidewalk). The establishment period is complete when new roots have grown beyond the original root ball.
After Two Years
Two or three years after installation, for fast growing species such as willow and poplar and three to five years for slow growing species such as beech and oak, check the tree to remove dead, specific diseases, broken, weak branch unions, water sprouts, and hanging or drooping branches, followed by reducing or removing co-dominant leaders and branch clusters that may have appeared as a result of transplant shock. When removing any diseased branches, consider cleaning pruning equipment after each tree to prevent spreading the disease to other trees.
On co-dominant branches, reduce or remove the less desirable branch. Be sure the tree is developing a proper structure that is typical of the species. This early pruning should have an emphasis on developing a central leader and spacing permanent branches so they are spaced 6” apart and around the trunk on small maturing trees and 12” apart and around the trunk on large maturing trees. Temporary branches on the trunk should be shortened or reduced. This pruning should also occur after mid-summer to prevent re-sprouts. Root suckers should be removed at ground level. However, do not remove more than 20% of the leaves and branches over the entire crown at any one time. Remove even less if the tree is not growing well.
The last priority is to remove half the scaffolding branches from the ground up to eliminate conflict with pedestrians and traffic.
After Five Years
At five years for fast growing species and six to ten years for slow growing species, remove all remaining scaffolding and lower branches toward a long-term goal of 15 ft. of branch-free space. Also remove branches growing in an undesirable direction or rubbing against one another. If possible, remove branches so they are spaced 12” – 18” apart vertically and around the trunk on large tree species and 6-8 inches apart on small mature trees.
After Seven Years
At seven years for fast growing species and ten to fifteen years for slow growing species check the canopy to reduce wind sail if necessary, but never, ever, top the tree. Maintain a balanced radial distribution of branches on the top to distribute weight and stress around the entire tree.
The following guidelines have been prepared to indicate when a tree should be routinely pruned as preventative maintenance. This guideline will vary depending upon the budget, species, growth rate, local climate, and site where the tree is growing.
When to Prune
Pruning should begin as soon as a tree is established and has resumed normal growth rates after being installed. This generally occurs two years after installation but may be longer on larger size trees.
Inspect newly installed trees on an annual basis for the first ten years after they become established and prune as needed to provide the desirable structure. On small maturing species, the first ten years after establishment is the critical period for structural pruning but on large maturing species, pruning for structure should occur as needed to repair damage for up to 25 years after installation. Pruning cycles generally can be extended to every 2-4 years during the 11-25 year period following establishment.
Planting Time
New trees should be purchased and pruned in accordance with this linked specification or the latest American Standard for Nursery Stock. This Standard provides buyers and sellers of nursery stock uniform sizes and a common terminology in order to standardize all sales of nursery stock.
Pruning young and medium-sized trees should be done when they have recovered from Installation stress. This can help minimize long-term problems by encouraging trees to develop a strong branch structure. The risk of mature tree failure can be minimized with appropriate pruning of trees that are near potential targets such as buildings, sidewalks, and picnic areas.
The First Pruning
The first pruning at the planting site should remove only dead, diseased, and broken branches back to a bud or branch union. Then remove any girdling, circling, deformed, diseased, or damaged roots. Also at the time of installation, select the central leader and remove or shorten the tops of any competing multiple leaders.
Retain all the scaffolding branches or temporary branches on the lower tree trunk. Temporary branches can be shortened, and retained to shade the trunk. These branches help the tree to establish itself in a new location and they should be removed before the branches become half the diameter of the trunk. Pruning at the time of installation may hinder a tree's ability to make food to recover from the shock of installation, so pruning may be delayed until the tree has recovered from the installation. Also delay removing any hanging or drooping branches for a couple of years (weeping trees should not be installed near a sidewalk). The establishment period is complete when new roots have grown beyond the original root ball.
After Two Years
Two or three years after installation, for fast growing species such as willow and poplar and three to five years for slow growing species such as beech and oak, check the tree to remove dead, specific diseases, broken, weak branch unions, water sprouts, and hanging or drooping branches, followed by reducing or removing co-dominant leaders and branch clusters that may have appeared as a result of transplant shock. When removing any diseased branches, consider cleaning pruning equipment after each tree to prevent spreading the disease to other trees.
On co-dominant branches, reduce or remove the less desirable branch. Be sure the tree is developing a proper structure that is typical of the species. This early pruning should have an emphasis on developing a central leader and spacing permanent branches so they are spaced 6” apart and around the trunk on small maturing trees and 12” apart and around the trunk on large maturing trees. Temporary branches on the trunk should be shortened or reduced. This pruning should also occur after mid-summer to prevent re-sprouts. Root suckers should be removed at ground level. However, do not remove more than 20% of the leaves and branches over the entire crown at any one time. Remove even less if the tree is not growing well.
The last priority is to remove half the scaffolding branches from the ground up to eliminate conflict with pedestrians and traffic.
After Five Years
At five years for fast growing species and six to ten years for slow growing species, remove all remaining scaffolding and lower branches toward a long-term goal of 15 ft. of branch-free space. Also remove branches growing in an undesirable direction or rubbing against one another. If possible, remove branches so they are spaced 12” – 18” apart vertically and around the trunk on large tree species and 6-8 inches apart on small mature trees.
After Seven Years
At seven years for fast growing species and ten to fifteen years for slow growing species check the canopy to reduce wind sail if necessary, but never, ever, top the tree. Maintain a balanced radial distribution of branches on the top to distribute weight and stress around the entire tree.
Pruning Techniques
The Branch Collar
Subordinate branch wood is different from parent branch or trunk wood. In the spring, a tree will add a layer of wood to a branch. Then the trunk puts out its own new layer of wood. Where the trunk wood meets the base of the branch, it laminates over the branch creating sort of a bulge called the branch collar. The branch is separate tissue from the trunk and is just held in place, buried inside the trunk by each year's new layer of wood over the previous year's wood. This lamination process is repeated every year adding strength to the union as weight is added to the branch.
When pruning off a branch, be careful to cut off only the branch wood and avoid cutting or wounding the parent branch or trunk wood. This means cutting at the collar but not into it, since that would open up the parent branch or trunk to decay. If the branch is cut off too far out, away from the trunk, the branch will die back to the collar, leaving a stub of dead wood. This will slow the closure over the cut off branch. In general, the right place to cut is almost like following a dotted line where the branch starts to get fatter near the trunk. The angle of the cut varies because the collars vary but mostly, the angle is perpendicular to the branch.
Branch Attachments
Select for retention branches that have a protruding branch bark ridge at the trunk. At the same time, reduce or remove any branches that exceed 50% of the trunk diameter at the point of attachment because they are more prone to included bark failure.
To avoid bark tearing or branch bark ridge or collar damage when branches have narrow unions, it is wise to use a multi-cut system of removal. First remove the weight of the branch by cutting it off farther out from the trunk. Start by making an undercut, then saw down from the top of the branch outward side of the undercut. This prevents the saw from getting stuck and the bark from tearing off and leaving a torn bark wound. Tearing lower bark should break at the undercut. Once the weight is gone, a proper pruning cut at the collar can be made. Practice with different species will teach how high to make the undercut to avoid tearing.
Branch Removal Selection
The diameter of all branches and leaders, especially those developing in the lower portion of the crown, should never be larger than half of the diameter of the trunk at the point of attachment. Branches that exceed this guideline, or are growing at a more rapid rate than the trunk, should have leaves removed to slow their growth rate relative to the growth rate of the trunk. This is referred to as subordination.
The leaves feed the tree, and the more leaves the faster the branches should grow. To subordinate a vigorous branch, it should be reduced in length to achieve a reduction in growth. To subordinate the target branch, the removal of branches should be concentrated in the outer half portion of the branch. Subordination of large or rapidly growing branches should continue as necessary for the life of the tree. Permanent branches that are a smaller diameter than the parent branch or trunk are more strongly attached than equal diameter branches. Subordination also encourages development of branch collars that contain protection zones that discourage development of decay in the trunks if branches are ultimately removed. Subordinating rapidly growing temporary branches and leaders prior to removal will reduce the development of decay in parent branches and the trunk following pruning.
Maintain Foliage Distribution
Foliage should cover at least two-thirds of the trunk and each permanent branch. Maintaining lower branches encourages diameter growth that leads to desirable trunk taper. Lower branches also shade the trunk and reduce the risk of sun-scald. While this may be a challenge maintaining clearance on street trees, the foliage should be distributed as low along the trunk as possible.
Correcting Nursery Grown Pruning
Most of the pruning required on young trees is caused by heading cuts performed in the nursery, and the removal of lower branches along the trunk. Natural grown trees usually start out with foliage along the trunk and a central leader or two striving to grow to survive. In the nursery, our marketing culture in the US, and possibly other places, sells young trees that look like mature trees with a straight clean trunk and a foliar crown on top. This is contrary to how the trees grow and what they need to survive, especially in an open setting. The temporary low lateral branches shade and improve trunk taper. These should be retained and shortened to promote the selected central leader. Any competing leaders and co-dominant attachments should be shortened or removed as appropriate for the tree specimen. Fortunately, severe pruning of young trees has not shown to lead to decay, so headed and cuts into the wood that has not formed heartwood yet do not usually cause decay. Once the bark hardens or thickens, those low temporary branches can be removed, as the tree will have better taper and sun protection. This nursery pruning is covered in the objective of pruning trees to improve structure.
The Branch Collar
Subordinate branch wood is different from parent branch or trunk wood. In the spring, a tree will add a layer of wood to a branch. Then the trunk puts out its own new layer of wood. Where the trunk wood meets the base of the branch, it laminates over the branch creating sort of a bulge called the branch collar. The branch is separate tissue from the trunk and is just held in place, buried inside the trunk by each year's new layer of wood over the previous year's wood. This lamination process is repeated every year adding strength to the union as weight is added to the branch.
When pruning off a branch, be careful to cut off only the branch wood and avoid cutting or wounding the parent branch or trunk wood. This means cutting at the collar but not into it, since that would open up the parent branch or trunk to decay. If the branch is cut off too far out, away from the trunk, the branch will die back to the collar, leaving a stub of dead wood. This will slow the closure over the cut off branch. In general, the right place to cut is almost like following a dotted line where the branch starts to get fatter near the trunk. The angle of the cut varies because the collars vary but mostly, the angle is perpendicular to the branch.
Branch Attachments
Select for retention branches that have a protruding branch bark ridge at the trunk. At the same time, reduce or remove any branches that exceed 50% of the trunk diameter at the point of attachment because they are more prone to included bark failure.
To avoid bark tearing or branch bark ridge or collar damage when branches have narrow unions, it is wise to use a multi-cut system of removal. First remove the weight of the branch by cutting it off farther out from the trunk. Start by making an undercut, then saw down from the top of the branch outward side of the undercut. This prevents the saw from getting stuck and the bark from tearing off and leaving a torn bark wound. Tearing lower bark should break at the undercut. Once the weight is gone, a proper pruning cut at the collar can be made. Practice with different species will teach how high to make the undercut to avoid tearing.
Branch Removal Selection
The diameter of all branches and leaders, especially those developing in the lower portion of the crown, should never be larger than half of the diameter of the trunk at the point of attachment. Branches that exceed this guideline, or are growing at a more rapid rate than the trunk, should have leaves removed to slow their growth rate relative to the growth rate of the trunk. This is referred to as subordination.
The leaves feed the tree, and the more leaves the faster the branches should grow. To subordinate a vigorous branch, it should be reduced in length to achieve a reduction in growth. To subordinate the target branch, the removal of branches should be concentrated in the outer half portion of the branch. Subordination of large or rapidly growing branches should continue as necessary for the life of the tree. Permanent branches that are a smaller diameter than the parent branch or trunk are more strongly attached than equal diameter branches. Subordination also encourages development of branch collars that contain protection zones that discourage development of decay in the trunks if branches are ultimately removed. Subordinating rapidly growing temporary branches and leaders prior to removal will reduce the development of decay in parent branches and the trunk following pruning.
Maintain Foliage Distribution
Foliage should cover at least two-thirds of the trunk and each permanent branch. Maintaining lower branches encourages diameter growth that leads to desirable trunk taper. Lower branches also shade the trunk and reduce the risk of sun-scald. While this may be a challenge maintaining clearance on street trees, the foliage should be distributed as low along the trunk as possible.
Correcting Nursery Grown Pruning
Most of the pruning required on young trees is caused by heading cuts performed in the nursery, and the removal of lower branches along the trunk. Natural grown trees usually start out with foliage along the trunk and a central leader or two striving to grow to survive. In the nursery, our marketing culture in the US, and possibly other places, sells young trees that look like mature trees with a straight clean trunk and a foliar crown on top. This is contrary to how the trees grow and what they need to survive, especially in an open setting. The temporary low lateral branches shade and improve trunk taper. These should be retained and shortened to promote the selected central leader. Any competing leaders and co-dominant attachments should be shortened or removed as appropriate for the tree specimen. Fortunately, severe pruning of young trees has not shown to lead to decay, so headed and cuts into the wood that has not formed heartwood yet do not usually cause decay. Once the bark hardens or thickens, those low temporary branches can be removed, as the tree will have better taper and sun protection. This nursery pruning is covered in the objective of pruning trees to improve structure.
Structural Pruning
The most common pruning objective should be to improve the structure of the tree. Pruning to improve structure removes those branches that impede the central leader, have weak attachments, have excessive loading, or if they have a high likelihood of failure, limit the options to prune after the failure. Pruning considerations include the mature size of the tree, its age, its normal and current architecture, species characteristics, and its location in the landscape. The ideal condition of one dominant trunk with smaller branches distributed horizontally and vertically around the dominant trunk is important for the tree to develop a medium to large size at maturity. These trees must support heavy loads that include the branches and foliage, and deal with extremes of wind, rain, ice, and snow.
Proper structural pruning focuses on creating a tree with a single central leader and a strong framework of well-attached, well-spaced scaffold branches. A structurally trained tree generally suffers less breakage and has a longer useful life in the landscape.
Pruning for structure is different than pruning for crown reduction or thinning which focuses on directing growth and developing a framework for the tree to establish its mature form. Pruning for structure should begin when the tree is installed and requires repeat visits to the tree as the tree grows over a period of time to gradually encourage more growth of the selected leader and increase the spacing between the branches. Pruning for structure should be performed on most tree species that become large at maturity to promote longevity, decrease future maintenance costs, and reduce conditions in the tree that could place people or property at risk.
Pruning for structure can avoid or reduce the need for more expensive tree care practices later in the life of the tree and can extend its lifespan by decreasing the likelihood of branch failures. Pruning young, developing trees to improve structure provides a desirable and stable form at maturity and is one of the best investments that arborists and landscape architects can make in a landscape.
Forest Structure versus Urban Structure
In nature, trees tend to develop a structure in response to competition. They usually grow from seeds in close proximity to other trees, and the shade created by the developing forest canopy suppresses growth of lower lateral branches. At the same time, the dominant trees in the forest tend to maintain a single trunk and tall narrow crown as they grow toward sunlight. This results in a reasonably vertical structure in mature forest trees. Because forest trees have limited targets, there is not a high risk when these vertical trees fail.
Conditions are radically different when trees are growing in the urban landscape. The crown is usually exposed to full sun, which encourages a broader, more complex crown than the competitive stands growing in the forest. The trees are usually nursery grown, and pruned to change the natural form. Competing and co-dominant branches may grow very large. Branches may develop in close proximity to one another and multiple trunks can develop. Pruning to improve structure requires developing and maintaining a single dominant trunk/leader, with smaller branches distributed horizontally and vertically around the trunk. Improving the structure reduces the likelihood of tree failure caused by defective attachments and poor weight distribution that usually appears on unpruned trees.
Maintain a Single Central Trunk/Leader
Pruning for the objective of improving structure, the parts of the crown that contribute to competing leaders are pruned in order to direct future growth to the selected central leader. Pruning to shorten long branches can slow branch growth, and remove density and leverage from clumps of branches high in the tree. Rather than a one-time event, structural pruning should be thought of as a training process that improves tree structure over a period of time. Once the structure is completed, other pruning can be performed if needed to achieve other objectives such as clearance and aesthetics.
Unless the tree is intentionally grown as a multi-stemmed specimen as is often done with certain species such as birch (Betula) and serviceberry (Amelanchier), a single trunk should be maintained until the permanent lateral branches forming the crown are in place, usually when the tree is half the eventual mature height. On large maturing species, such as ash (Fraxinus) or maple (Acer), that can eventually reach 100 feet or more in height, a single trunk should be maintained for at least 35 feet before it is allowed to develop permanent lateral branches of approximately equal size. On smaller maturing trees such as dogwood (Cornus) or crabapple (Malus), a single trunk should be maintained for approximately 10 feet.
Trees with decurrent branching habits, such as elm (Ulmus), honeylocust (Gleditsia), and redbud (Cercis), will need a greater emphasis on structural pruning to develop a single central leader than trees with excurrent habits such as conifers, most oak (Quercus) species, and tulip trees (Liriodendron). Certain species, particularly those with a decurrent growth form and with an opposite branching habit, are particularly prone to developing structural defects or weakly attached branches. Opposite branched species such as maple, ash and dogwood also have a greater tendency to develop co-dominant trunks, and narrow crotches at a young age. In some cases, one of the competing leaders can be removed entirely to maintain one central trunk.
This is most appropriate for very young trees that have a single co-dominant leader. On trees with excurrent branching habits that tend to develop multiple leaders, subordinating the competing leaders by reducing and removing foliage is usually the best option rather than pruning off the entire branch. The earlier in the tree’s life the co-dominant leader is reduced or removed, the less chance of a large failure that removes a significant amount of the grown crown.
Pruning to improve structure is accomplished by managing the leader and size relationships of lateral branch growth. Since the leaves feed the trees, the branches with more leaves will grow faster. The longer branches with terminal buds become the primary growth points. By using this treatment, the branch size remains small and cuts to remove branches also remain small. If too many branches are pruned, the tree may respond by sprouting from latent buds on the trunk, and follow-up pruning will be necessary to manage the growth. More time will need to be spent to manage the re-growth and re-establish the leader and subordinates.
When pruning is needed to correct trunk or branch attachment defects on recently installed trees, the pruning should be done soon after the installation so the tree will develop a leader for future branches to grow from.
However, it is important to avoid applying too much pruning on the branches growing from the leader in the upper crown. Removing too much foliage will slow its growth and potentially cause it to lose dominance. If left unpruned, the branches in the lower crown may grow too fast and take over the central leader. It is important to return to this tree every couple of years to remove co-dominant sprouts and prune to reduce branches in competition with the central leader.
Recurring pruning to improve structure on young trees is the best strategy for training trees to grow with a dominant central leader because only small pruning cuts will be necessary. Small cuts promote a faster wound closure, faster crown development and it helps to minimize decay that might develop on a larger pruning wound. If poor form develops because trees were not pruned during the first few years, or were never pruned for structure, the cuts made during the first pruning to improve structure will be larger. Larger cuts will mean a longer time to cover the wounds and a greater chance for disease and insect invasions.
Strategies
There are six main strategies in executing the objective to prune for improved structure. These include the following:
Prune to maintain a dominant leader by reducing the length of or removing competing leaders. This typically means shortening the longest branches with a reduction cut. Ideally, reduction cuts should be no larger than 2-3 inches (5-7 cm) in diameter on poor compartmentalizers and 3-5 inches (7-12 cm) diameter on good compartmentalizers. Larger cuts can lead to significant decay. Do not allow branches with included bark to grow too large because they could split from the tree. This is accomplished by shortening one of the included branches. Regularly reduce the length of low vigorous branches and branches that will be clearance issues and have to be removed. Typically, this means shorten all branches or prune downward growing branches originating from the lower 20 feet (6 m) of the trunk. Strive to prevent all branches on the tree from growing larger than half the trunk diameter. Older trees can be pruned in a similar manner to create or preserve good structure.
The number and/or size of flowers or fruit can also be influenced by pruning. Fruit size can be increased on certain plants such as peach (Prunus) trees by removing some of the developing fruit or flowers. Flower cluster sizes can be increased on crape myrtle (Lagerstroemia) and some other trees by reducing the number of flower buds. Fruit production can be eliminated by removing all flowers.
It is important to describe what is occurring to the tree, and what the tree will look like after the pruning work. The revised ANSI A300 Part 1 pruning standard uses systems to describe what the tree will look like. This helps better define the work and provide guidance for which parts of the tree are to be removed during the pruning. For example, a tree should not be brushing against a building but it should have as large a natural crown as possible for the benefits the tree will provide.
Species Considerations for Pruning to Improve Structure
Different trees have different natural forms. Trees like Weeping Willow (Salix) or Evergreen Chinese Elm (Ulmus) have a form that grows downward and droops. Some trees have multiple leaders in their decurrent forms. These species usually require more maintenance to develop the long-term structure. As these species grow in open areas, they usually require more frequent inspection and greater pruning attention for clearance and to improve structure.
High Maintenance Species
Acer – Maple: These trees tend to develop co‐dominant trunks. Their opposite branching habit results in closely spaced branches, many with poor branch attachments and dense branch ends. Maples have weak wood and are prone to decay. One tip to consider when pruning maples is to imagine the branch sizes 20 years from now when the branches are all 12 inches (30 cm) in diameter and remove the branches that eventually will be too close together, now.
Betula – Birch: These trees tend to develop multiple leaders from the flare. They can be grown as a multi‐stemmed tree or must receive pruning to develop a single trunk. On river birch, the terminal tends to dieback during droughts resulting in co-dominant leaders.
Cercidphyllum – Katsura: This species tends to develop multiple leaders low on the trunk. It can be grown as a multi-stemmed tree or it must receive early pruning to develop a single trunk.
Cercis – Redbud & Cladrastris – Yellowwood: These trees tend to develop multiple leaders that branch at a low height. The leaders have a narrow angle of attachment.
Cornus – Dogwood: This tree tends to develop multiple leaders that begin branching at a low height. Their opposite branching habit results in closely spaced branches.
Fraxinus – Ash: This tree has a narrow angle of branch attachment that produces an upright growth habit. It tends to develop multiple leaders branching at a low height. The opposite branching habit results in closely spaced branches.
Gleditsia – Honeylocust: This tree may develop multiple leaders but failures seldom occur at maturity.
Lagerstroemia – Crape myrtle: This tree tends to develop multiple leaders from the flare. It can be grown as a multi-stemmed tree or it must receive training to develop a single trunk.
Malus – Crabapple: Generally this tree has closely spaced branches and a dense crown. However, many crabapple cultivars are not susceptible to high maintenance concerns.
Poplar – Poplar: This tree has brittle wood and is prone to decay, requiring frequent removals of small branches. It should be monitored for co-dominant trunks.
Pyrus – Pear: This tree is noted for its narrow angles of branch attachment. Its upright growth habit that tends to result in multiple leaders branching at a low height and a common location on the trunk. Multiple leaders on nursery trees are difficult to correct. Pruning for sound structure must begin as early in the tree's life as possible if a sound tree is to result.
Salix – Willow: This tree has branches that tend to develop low on the trunk and grow rapidly.
Sophora – Pagoda Tree: This tree has branches that tend to develop low on the trunk and grow rapidly. This results in dense branch ends and brittle wood.
Tilia – Linden: This tree is known for its co-dominant leaders and closely spaced branches. It also has weak wood and dense branch ends. Some cultivars are prone to narrow branch angles.
Ulmus – Elm: This tree is noted for its narrow angle of branch attachment that result in multiple leaders low on the trunk and dense branch ends.
Zelkova – Zelkova: This is another tree noted for its narrow angle of branch attachment and upright growth habit. It tends to develop multiple leaders branching at a low height and at a common location on the trunk. Multiple leaders on nursery trees are difficult to correct. Pruning for sound structure must begin in the nursery. Newly developed cultivars will also minimize this concern.
Consideration must be given to selecting newer cultivars in all these species. These new cultivars have been bred to minimize structural problems.
Low Maintenance Species
The following species tend to develop good structure in full sun, on their own, with only a minimum of pruning:
The most common pruning objective should be to improve the structure of the tree. Pruning to improve structure removes those branches that impede the central leader, have weak attachments, have excessive loading, or if they have a high likelihood of failure, limit the options to prune after the failure. Pruning considerations include the mature size of the tree, its age, its normal and current architecture, species characteristics, and its location in the landscape. The ideal condition of one dominant trunk with smaller branches distributed horizontally and vertically around the dominant trunk is important for the tree to develop a medium to large size at maturity. These trees must support heavy loads that include the branches and foliage, and deal with extremes of wind, rain, ice, and snow.
Proper structural pruning focuses on creating a tree with a single central leader and a strong framework of well-attached, well-spaced scaffold branches. A structurally trained tree generally suffers less breakage and has a longer useful life in the landscape.
Pruning for structure is different than pruning for crown reduction or thinning which focuses on directing growth and developing a framework for the tree to establish its mature form. Pruning for structure should begin when the tree is installed and requires repeat visits to the tree as the tree grows over a period of time to gradually encourage more growth of the selected leader and increase the spacing between the branches. Pruning for structure should be performed on most tree species that become large at maturity to promote longevity, decrease future maintenance costs, and reduce conditions in the tree that could place people or property at risk.
Pruning for structure can avoid or reduce the need for more expensive tree care practices later in the life of the tree and can extend its lifespan by decreasing the likelihood of branch failures. Pruning young, developing trees to improve structure provides a desirable and stable form at maturity and is one of the best investments that arborists and landscape architects can make in a landscape.
Forest Structure versus Urban Structure
In nature, trees tend to develop a structure in response to competition. They usually grow from seeds in close proximity to other trees, and the shade created by the developing forest canopy suppresses growth of lower lateral branches. At the same time, the dominant trees in the forest tend to maintain a single trunk and tall narrow crown as they grow toward sunlight. This results in a reasonably vertical structure in mature forest trees. Because forest trees have limited targets, there is not a high risk when these vertical trees fail.
Conditions are radically different when trees are growing in the urban landscape. The crown is usually exposed to full sun, which encourages a broader, more complex crown than the competitive stands growing in the forest. The trees are usually nursery grown, and pruned to change the natural form. Competing and co-dominant branches may grow very large. Branches may develop in close proximity to one another and multiple trunks can develop. Pruning to improve structure requires developing and maintaining a single dominant trunk/leader, with smaller branches distributed horizontally and vertically around the trunk. Improving the structure reduces the likelihood of tree failure caused by defective attachments and poor weight distribution that usually appears on unpruned trees.
Maintain a Single Central Trunk/Leader
Pruning for the objective of improving structure, the parts of the crown that contribute to competing leaders are pruned in order to direct future growth to the selected central leader. Pruning to shorten long branches can slow branch growth, and remove density and leverage from clumps of branches high in the tree. Rather than a one-time event, structural pruning should be thought of as a training process that improves tree structure over a period of time. Once the structure is completed, other pruning can be performed if needed to achieve other objectives such as clearance and aesthetics.
Unless the tree is intentionally grown as a multi-stemmed specimen as is often done with certain species such as birch (Betula) and serviceberry (Amelanchier), a single trunk should be maintained until the permanent lateral branches forming the crown are in place, usually when the tree is half the eventual mature height. On large maturing species, such as ash (Fraxinus) or maple (Acer), that can eventually reach 100 feet or more in height, a single trunk should be maintained for at least 35 feet before it is allowed to develop permanent lateral branches of approximately equal size. On smaller maturing trees such as dogwood (Cornus) or crabapple (Malus), a single trunk should be maintained for approximately 10 feet.
Trees with decurrent branching habits, such as elm (Ulmus), honeylocust (Gleditsia), and redbud (Cercis), will need a greater emphasis on structural pruning to develop a single central leader than trees with excurrent habits such as conifers, most oak (Quercus) species, and tulip trees (Liriodendron). Certain species, particularly those with a decurrent growth form and with an opposite branching habit, are particularly prone to developing structural defects or weakly attached branches. Opposite branched species such as maple, ash and dogwood also have a greater tendency to develop co-dominant trunks, and narrow crotches at a young age. In some cases, one of the competing leaders can be removed entirely to maintain one central trunk.
This is most appropriate for very young trees that have a single co-dominant leader. On trees with excurrent branching habits that tend to develop multiple leaders, subordinating the competing leaders by reducing and removing foliage is usually the best option rather than pruning off the entire branch. The earlier in the tree’s life the co-dominant leader is reduced or removed, the less chance of a large failure that removes a significant amount of the grown crown.
Pruning to improve structure is accomplished by managing the leader and size relationships of lateral branch growth. Since the leaves feed the trees, the branches with more leaves will grow faster. The longer branches with terminal buds become the primary growth points. By using this treatment, the branch size remains small and cuts to remove branches also remain small. If too many branches are pruned, the tree may respond by sprouting from latent buds on the trunk, and follow-up pruning will be necessary to manage the growth. More time will need to be spent to manage the re-growth and re-establish the leader and subordinates.
When pruning is needed to correct trunk or branch attachment defects on recently installed trees, the pruning should be done soon after the installation so the tree will develop a leader for future branches to grow from.
However, it is important to avoid applying too much pruning on the branches growing from the leader in the upper crown. Removing too much foliage will slow its growth and potentially cause it to lose dominance. If left unpruned, the branches in the lower crown may grow too fast and take over the central leader. It is important to return to this tree every couple of years to remove co-dominant sprouts and prune to reduce branches in competition with the central leader.
Recurring pruning to improve structure on young trees is the best strategy for training trees to grow with a dominant central leader because only small pruning cuts will be necessary. Small cuts promote a faster wound closure, faster crown development and it helps to minimize decay that might develop on a larger pruning wound. If poor form develops because trees were not pruned during the first few years, or were never pruned for structure, the cuts made during the first pruning to improve structure will be larger. Larger cuts will mean a longer time to cover the wounds and a greater chance for disease and insect invasions.
Strategies
There are six main strategies in executing the objective to prune for improved structure. These include the following:
- develop or maintain a dominant leader (one main trunk),
- identify the lowest permanent branch in the canopy,
- prevent temporary branches below the permanent canopy from growing upright or too large,
- space main branches along the dominant trunk vertically as well as horizontally,
- keep all lateral branches a smaller diameter than the trunk diameter,
- suppress growth on branches with bark inclusions, or remove the branches completely.
Prune to maintain a dominant leader by reducing the length of or removing competing leaders. This typically means shortening the longest branches with a reduction cut. Ideally, reduction cuts should be no larger than 2-3 inches (5-7 cm) in diameter on poor compartmentalizers and 3-5 inches (7-12 cm) diameter on good compartmentalizers. Larger cuts can lead to significant decay. Do not allow branches with included bark to grow too large because they could split from the tree. This is accomplished by shortening one of the included branches. Regularly reduce the length of low vigorous branches and branches that will be clearance issues and have to be removed. Typically, this means shorten all branches or prune downward growing branches originating from the lower 20 feet (6 m) of the trunk. Strive to prevent all branches on the tree from growing larger than half the trunk diameter. Older trees can be pruned in a similar manner to create or preserve good structure.
The number and/or size of flowers or fruit can also be influenced by pruning. Fruit size can be increased on certain plants such as peach (Prunus) trees by removing some of the developing fruit or flowers. Flower cluster sizes can be increased on crape myrtle (Lagerstroemia) and some other trees by reducing the number of flower buds. Fruit production can be eliminated by removing all flowers.
It is important to describe what is occurring to the tree, and what the tree will look like after the pruning work. The revised ANSI A300 Part 1 pruning standard uses systems to describe what the tree will look like. This helps better define the work and provide guidance for which parts of the tree are to be removed during the pruning. For example, a tree should not be brushing against a building but it should have as large a natural crown as possible for the benefits the tree will provide.
Species Considerations for Pruning to Improve Structure
Different trees have different natural forms. Trees like Weeping Willow (Salix) or Evergreen Chinese Elm (Ulmus) have a form that grows downward and droops. Some trees have multiple leaders in their decurrent forms. These species usually require more maintenance to develop the long-term structure. As these species grow in open areas, they usually require more frequent inspection and greater pruning attention for clearance and to improve structure.
High Maintenance Species
Acer – Maple: These trees tend to develop co‐dominant trunks. Their opposite branching habit results in closely spaced branches, many with poor branch attachments and dense branch ends. Maples have weak wood and are prone to decay. One tip to consider when pruning maples is to imagine the branch sizes 20 years from now when the branches are all 12 inches (30 cm) in diameter and remove the branches that eventually will be too close together, now.
Betula – Birch: These trees tend to develop multiple leaders from the flare. They can be grown as a multi‐stemmed tree or must receive pruning to develop a single trunk. On river birch, the terminal tends to dieback during droughts resulting in co-dominant leaders.
Cercidphyllum – Katsura: This species tends to develop multiple leaders low on the trunk. It can be grown as a multi-stemmed tree or it must receive early pruning to develop a single trunk.
Cercis – Redbud & Cladrastris – Yellowwood: These trees tend to develop multiple leaders that branch at a low height. The leaders have a narrow angle of attachment.
Cornus – Dogwood: This tree tends to develop multiple leaders that begin branching at a low height. Their opposite branching habit results in closely spaced branches.
Fraxinus – Ash: This tree has a narrow angle of branch attachment that produces an upright growth habit. It tends to develop multiple leaders branching at a low height. The opposite branching habit results in closely spaced branches.
Gleditsia – Honeylocust: This tree may develop multiple leaders but failures seldom occur at maturity.
Lagerstroemia – Crape myrtle: This tree tends to develop multiple leaders from the flare. It can be grown as a multi-stemmed tree or it must receive training to develop a single trunk.
Malus – Crabapple: Generally this tree has closely spaced branches and a dense crown. However, many crabapple cultivars are not susceptible to high maintenance concerns.
Poplar – Poplar: This tree has brittle wood and is prone to decay, requiring frequent removals of small branches. It should be monitored for co-dominant trunks.
Pyrus – Pear: This tree is noted for its narrow angles of branch attachment. Its upright growth habit that tends to result in multiple leaders branching at a low height and a common location on the trunk. Multiple leaders on nursery trees are difficult to correct. Pruning for sound structure must begin as early in the tree's life as possible if a sound tree is to result.
Salix – Willow: This tree has branches that tend to develop low on the trunk and grow rapidly.
Sophora – Pagoda Tree: This tree has branches that tend to develop low on the trunk and grow rapidly. This results in dense branch ends and brittle wood.
Tilia – Linden: This tree is known for its co-dominant leaders and closely spaced branches. It also has weak wood and dense branch ends. Some cultivars are prone to narrow branch angles.
Ulmus – Elm: This tree is noted for its narrow angle of branch attachment that result in multiple leaders low on the trunk and dense branch ends.
Zelkova – Zelkova: This is another tree noted for its narrow angle of branch attachment and upright growth habit. It tends to develop multiple leaders branching at a low height and at a common location on the trunk. Multiple leaders on nursery trees are difficult to correct. Pruning for sound structure must begin in the nursery. Newly developed cultivars will also minimize this concern.
Consideration must be given to selecting newer cultivars in all these species. These new cultivars have been bred to minimize structural problems.
Low Maintenance Species
The following species tend to develop good structure in full sun, on their own, with only a minimum of pruning:
- Conifers: These trees only need to be monitored for co-dominant leaders.
- Fagus – Beech: This tree is generally maintenance free except to maintain the size of lower branches and raise the lower branches if the site requires it.
- Quercus – Oak: Certain oaks such as live oak tends to develop multiple trunks at a low height and require more attention to structural pruning. All the other oaks almost always develop a good structure in full sun.
- These trees almost always develop a good structure in full sun:
Ginkgo – Ginkgo
Liquidambar – Sweet-gum
Liriodendron – Tulip tree
Magnolia – Magnolia
Platanus – Sycamore
Pruning Specifications
Pruning specifications are used to guide how the pruning is performed. Pruning specifications are more effective when written. They should clearly explain how the work will be performed, and if well explained, the tree should not be pruned differently than desired and still be in compliance with the specifications.
Specifications should be written in accordance with ANSI A300 Standard, Part 1 Pruning. Clearly identify the tree or trees to be pruned, and where they are located. We want to prune the correct tree(s).
The first step is to have a clear objective(s). As already described, why am I touching this tree?
The second step is to select the pruning system – what will the tree look like when we are finished. Basic systems include Natural, Pollarding, Topiary, Espalier, Pleaching or other.
Detail the work to be accomplished – the type of cuts to be made: branch removal, reduction, or heading. Describe where in the tree the work will be focused. Describe the amount of foliage or branches to be removed. This can be done using a % of foliage on a certain branch or portion of the tree, a % of foliage for the whole tree, a size of branches to be removed, and/or the number of branches to be removed, or some combination of descriptions that clearly explains the work. We have found it to be a challenge (even with a before and after viewing) to confirm what % of foliage was removed, it is more effective to count the number of final cuts made and the size of those cuts. This video is a long example of planning the job before working in the tree.
There are three ways to remove a branch in pruning:
2. Reduction – cut where a longer branch is shortened to a lateral branch that is large enough to assume
the role of the growing branch.
3. Heading cuts – where reduction cuts are made on a branch to smaller lateral branches, they usually
require a return follow up visit to manage the re-growth after the severe cut and larger size of branch
wound.
Crown Reduction
Trees usually continue to grow over time. Sometimes they grow larger than desired for aesthetic or risk management reasons. Previous pruning cut terms are: drop crotching, selective heading, de-horning, or reduction pruning. Pruning has always been used to reduce the size of a tree by decreasing the length of the branches. This reduces the height or spread of the crown and individual branches.
The foliage and branches often grow back quickly, so once a tree is reduced, regular pruning will be required to maintain the tree within desired limits. Big trees installed in small places often require reduction to reduce risk of failure or reduce hardscape damage from roots. Storms often break or overturn the largest trees on a property, so reduction of these can be a good strategy for preserving them.
Reduction is best accomplished by cutting branches back to their point of origin or back to a lateral branch capable of sustaining growth and assuming apical dominance of the branch. Reduction cuts shorten main branches back to lateral branches at least one-third the diameter of the cut branch. Because future growth is forced into the unpruned leader and growth on pruned branches is suppressed, the union becomes stronger. The remaining lateral should be growing in a direction that will maintain a desirable shape.
Consideration must also be given to the ability of the species to sustain this type of pruning. Certain species such as beech, birch, and sugar maple respond poorly to reductions.
Very large cuts on very old trees may succeed in constraining the size, but at an unacceptable cost to the health, safety, and longevity of the tree. The resulting wounds can lead to decay, cracks, and sprout development. Even if the tree recovers by growing back foliage, it will be compromised structurally as a result. The lateral branches that take over as the main branches of the tree will likely be weaker and more prone to breakage even thirty years later. Therefore, it is preferable to perform reduction before the tree has become too large. When grown correctly, selective heading should be limited to a relatively few branches of a mature tree.
Clearance
Large, low branches are fine for trees growing in parks and other open landscapes where passage under the canopy is not needed. Clearance is necessary on established trees when lower branches obstruct vision and interfere with mowing, traffic, people, or utilities. A large wound and possible trunk defects could result when large branches have to be removed.
Crown lifting or raising should be performed on young and medium-aged trees to prevent low branches from growing to a large diameter. Crown lifting shortens low branches regularly to suppress their growth. This forces more growth in the upper branches. This form is most suitable for urban landscapes. The shortened branches are later removed to raise the crown for visual and physical clearance. This sustainable technique damages the tree less than allowing low branches to grow to a large size and then deciding to remove them. Keeping low branches small will also increase the value of the lower trunk should the tree be cut into lumber when it is removed. When raising is performed, branch levels are generally left at a uniform height around the tree to provide symmetry.
Clearance can be accomplished by removing weight from the branch tips and smaller downward growing branches. Removing end weight usually allows the branch to raise up, sometimes achieving clearance without the need to remove the whole branch. When the circumstances for truck routes and road clearance are necessary, pruning off branches before a vehicle strikes and tears the branch off the tree is a more proactive approach to achieving clearance.
Pruning specifications are used to guide how the pruning is performed. Pruning specifications are more effective when written. They should clearly explain how the work will be performed, and if well explained, the tree should not be pruned differently than desired and still be in compliance with the specifications.
Specifications should be written in accordance with ANSI A300 Standard, Part 1 Pruning. Clearly identify the tree or trees to be pruned, and where they are located. We want to prune the correct tree(s).
The first step is to have a clear objective(s). As already described, why am I touching this tree?
The second step is to select the pruning system – what will the tree look like when we are finished. Basic systems include Natural, Pollarding, Topiary, Espalier, Pleaching or other.
Detail the work to be accomplished – the type of cuts to be made: branch removal, reduction, or heading. Describe where in the tree the work will be focused. Describe the amount of foliage or branches to be removed. This can be done using a % of foliage on a certain branch or portion of the tree, a % of foliage for the whole tree, a size of branches to be removed, and/or the number of branches to be removed, or some combination of descriptions that clearly explains the work. We have found it to be a challenge (even with a before and after viewing) to confirm what % of foliage was removed, it is more effective to count the number of final cuts made and the size of those cuts. This video is a long example of planning the job before working in the tree.
There are three ways to remove a branch in pruning:
- Branch removal – cut where a branch can be removed back to its point of origin from the parent branch. The final cut needs to be made at the outer edge of the branch bark ridge or shoulder collar.
2. Reduction – cut where a longer branch is shortened to a lateral branch that is large enough to assume
the role of the growing branch.
3. Heading cuts – where reduction cuts are made on a branch to smaller lateral branches, they usually
require a return follow up visit to manage the re-growth after the severe cut and larger size of branch
wound.
Crown Reduction
Trees usually continue to grow over time. Sometimes they grow larger than desired for aesthetic or risk management reasons. Previous pruning cut terms are: drop crotching, selective heading, de-horning, or reduction pruning. Pruning has always been used to reduce the size of a tree by decreasing the length of the branches. This reduces the height or spread of the crown and individual branches.
The foliage and branches often grow back quickly, so once a tree is reduced, regular pruning will be required to maintain the tree within desired limits. Big trees installed in small places often require reduction to reduce risk of failure or reduce hardscape damage from roots. Storms often break or overturn the largest trees on a property, so reduction of these can be a good strategy for preserving them.
Reduction is best accomplished by cutting branches back to their point of origin or back to a lateral branch capable of sustaining growth and assuming apical dominance of the branch. Reduction cuts shorten main branches back to lateral branches at least one-third the diameter of the cut branch. Because future growth is forced into the unpruned leader and growth on pruned branches is suppressed, the union becomes stronger. The remaining lateral should be growing in a direction that will maintain a desirable shape.
Consideration must also be given to the ability of the species to sustain this type of pruning. Certain species such as beech, birch, and sugar maple respond poorly to reductions.
Very large cuts on very old trees may succeed in constraining the size, but at an unacceptable cost to the health, safety, and longevity of the tree. The resulting wounds can lead to decay, cracks, and sprout development. Even if the tree recovers by growing back foliage, it will be compromised structurally as a result. The lateral branches that take over as the main branches of the tree will likely be weaker and more prone to breakage even thirty years later. Therefore, it is preferable to perform reduction before the tree has become too large. When grown correctly, selective heading should be limited to a relatively few branches of a mature tree.
Clearance
Large, low branches are fine for trees growing in parks and other open landscapes where passage under the canopy is not needed. Clearance is necessary on established trees when lower branches obstruct vision and interfere with mowing, traffic, people, or utilities. A large wound and possible trunk defects could result when large branches have to be removed.
Crown lifting or raising should be performed on young and medium-aged trees to prevent low branches from growing to a large diameter. Crown lifting shortens low branches regularly to suppress their growth. This forces more growth in the upper branches. This form is most suitable for urban landscapes. The shortened branches are later removed to raise the crown for visual and physical clearance. This sustainable technique damages the tree less than allowing low branches to grow to a large size and then deciding to remove them. Keeping low branches small will also increase the value of the lower trunk should the tree be cut into lumber when it is removed. When raising is performed, branch levels are generally left at a uniform height around the tree to provide symmetry.
Clearance can be accomplished by removing weight from the branch tips and smaller downward growing branches. Removing end weight usually allows the branch to raise up, sometimes achieving clearance without the need to remove the whole branch. When the circumstances for truck routes and road clearance are necessary, pruning off branches before a vehicle strikes and tears the branch off the tree is a more proactive approach to achieving clearance.
Pruning Considerations
From Worst to Best When Tree Pruning:
Other pruning activities that require more training than how to make the proper cuts include crown restoration, vista pruning, young tree pruning, espalier, pollarding and palm pruning.
Included Bark
Inclusions in the branch union make a weak union between a small branch and a larger branch or the trunk. This means that pruning may be required to reduce the weight on the end of one branch or remove one of the branches entirely, back to the base of the included bark. The need is to remove some lateral branches or shorten one branch with a reduction cut. Another option is the installation of a cable system as supplemental support, but cabling is a supplement, not a replacement for tree structure.
Dead Branches
Remove detached, dead, and broken branches, especially those more than one inch (2.5 cm) in diameter. Cut outside the swollen expanding collar growing around the base of the dead branch. If two branches touch, remove or shorten one of the pair so they no longer touch. Removing dead branches makes it easier to monitor the rate of further dieback by the tree.
Diseased Branches
Trees are attacked by many diseases that affect foliage, buds, and branches. Not all diseased branches are removed. For example, Anthracnose is a very common fungal disease and affects most of a canopy on many trees such as Ash, Maple, and Sycamore. We would not remove all those branches. However, some diseases that kill branches and are infectious should be removed from the tree, and in certain instances such as Fire Blight or Mistletoe the diseased branch should be removed at least 2 feet (60cm) below the infection point. Verticillium Wilt branches should be removed from the tree. The specific disease and where to remove the branches must be clearly described or Anthracnose attacked trees will be hat-racked (improper cutting branches with no concern for tree survival) and align with a specification to remove diseased branches.
Sprouts
Many branches and trunks sprout new shoots in response to a stress such as root loss, root damage, storm damage, loss of foliage and branches, topping, disease, over-pruning, improper thinning, drought, and a lot of other reasons. These should be allowed to grow for a couple of years until the tree appears to be recovering. Then, remove about one-third of the sprouts, reduce another third of them and leave those that remain well-spaced to become the new branches.
There are situations where removal of water-sprouts is beneficial. For example, removing sprouts on dogwoods in areas where Discula anthracnose is present is recommended to reduce risk of cankers on larger branches.
Suckers
Removing suckers from the base of the tree or from the roots is recommended. Spray and paint treatments containing NAA (synthetic auxin) applied to the pruning cuts can help reduce subsequent suckering on some species. The treatment is not a pruning wound cover, it is to reduce re-sprouting from the cut wound.
Poor Pruning
There are several consequences of not conducting a regular pruning program or conducting a pruning program with untrained tree workers. These include:
Pruning Dose
The pruning dose is the estimated amount of foliage or buds removed during pruning compared with the total amount on the tree prior to pruning. The ANSI A300 Part 1, Pruning Standard states: “Pruning options should remove no more living tissue than what is necessary to achieve specified objectives. Species, size, age, condition, and site shall be considered when specifying the location and amount of live branches to be removed.” In the past an amount of 25% of the foliage was set as the maximum foliage removal. However healthier and younger trees can tolerate greater amounts of live branch removal than do mature or unhealthy trees. On some trees in a compromised health, removing 10% of the live foliage may be too much. Larger doses can be applied to certain branches or sections of the crown to reduce leverage, re-direct growth, special clearance needs, or for other reasons.
Topping
Topping is the removal of live foliage to large diameter branches, and usually results in the removal of greater than 50% of the foliage. In response to the number of leaves removed, trees will develop water sprouts and shoots following the removal of too much foliage. Sprouting is an attempt by the tree using stored energy to replace the photosynthesis generating foliage that was suddenly removed. The trained arborist will have to make several annual visits to the topped tree to provide the correct treatment for the tree's growth response. Sprouting requires energy and depletes the tree’s reserves, so sprouts should be allowed to grow for a few years as they are gradually reduced in number. The sprouts that grow from the cuts are poorly attached for at least a period of several years. The goal in restoring these trees is to prune so that eventually there is only one main leader at the location of each topping cut. Maintain the sprouts as mentioned above.
Lions Tailing
Lions tailing strips all of the interior foliage and retains only the foliage on the branch tips. This impacts the tree in 4 ways: It removes a large amount of photosynthesis producing foliage; it exposes branches to sun burn or sunscald; it reduces lateral branches to shorten long branches; it results in less taper and more end weight leverage increasing the likelihood of branch failure. For all these reasons it is a bad practice.
Tipped Trees
Tipping removes the ends of branches to create a smaller tree. It removes a large amount of terminal buds and can impact tree growth. This is another bad practice.
Both Lions Tailing and Tipping create a challenge for the arborist and take several years to correct. Allow the recently improperly pruned tree to grow for a couple years to regain energy and strength. Then begin the process to remove some of the sprouts and shorten others two or three years after the tree was over-pruned. Finally, proceed with restoration, similar to the procedure for topped trees.
When people prune trees in these abusive manners, excessive live tissue is removed from the tree and the structure is compromised. This creates poor form and numerous wounds, and the tree becomes more prone to stress, failure and premature death.
Pruning Strategies
From Worst to Best When Tree Pruning:
- the worst action is doing no pruning and ignoring targets
- the next worst is topping
- lions tailing
- severe reductions
- not managing the length of branches
- removing one of a co-dominant branch
- shortening outer branches and leave interior branches
- pruning one of a co-dominant branch
Other pruning activities that require more training than how to make the proper cuts include crown restoration, vista pruning, young tree pruning, espalier, pollarding and palm pruning.
Included Bark
Inclusions in the branch union make a weak union between a small branch and a larger branch or the trunk. This means that pruning may be required to reduce the weight on the end of one branch or remove one of the branches entirely, back to the base of the included bark. The need is to remove some lateral branches or shorten one branch with a reduction cut. Another option is the installation of a cable system as supplemental support, but cabling is a supplement, not a replacement for tree structure.
Dead Branches
Remove detached, dead, and broken branches, especially those more than one inch (2.5 cm) in diameter. Cut outside the swollen expanding collar growing around the base of the dead branch. If two branches touch, remove or shorten one of the pair so they no longer touch. Removing dead branches makes it easier to monitor the rate of further dieback by the tree.
Diseased Branches
Trees are attacked by many diseases that affect foliage, buds, and branches. Not all diseased branches are removed. For example, Anthracnose is a very common fungal disease and affects most of a canopy on many trees such as Ash, Maple, and Sycamore. We would not remove all those branches. However, some diseases that kill branches and are infectious should be removed from the tree, and in certain instances such as Fire Blight or Mistletoe the diseased branch should be removed at least 2 feet (60cm) below the infection point. Verticillium Wilt branches should be removed from the tree. The specific disease and where to remove the branches must be clearly described or Anthracnose attacked trees will be hat-racked (improper cutting branches with no concern for tree survival) and align with a specification to remove diseased branches.
Sprouts
Many branches and trunks sprout new shoots in response to a stress such as root loss, root damage, storm damage, loss of foliage and branches, topping, disease, over-pruning, improper thinning, drought, and a lot of other reasons. These should be allowed to grow for a couple of years until the tree appears to be recovering. Then, remove about one-third of the sprouts, reduce another third of them and leave those that remain well-spaced to become the new branches.
There are situations where removal of water-sprouts is beneficial. For example, removing sprouts on dogwoods in areas where Discula anthracnose is present is recommended to reduce risk of cankers on larger branches.
Suckers
Removing suckers from the base of the tree or from the roots is recommended. Spray and paint treatments containing NAA (synthetic auxin) applied to the pruning cuts can help reduce subsequent suckering on some species. The treatment is not a pruning wound cover, it is to reduce re-sprouting from the cut wound.
Poor Pruning
There are several consequences of not conducting a regular pruning program or conducting a pruning program with untrained tree workers. These include:
- increased risk of tree or branch failure,
- development of large, low branches,
- formation of co-dominant stems,
- defects such as included bark and dead branches,
- obstructed views.
Pruning Dose
The pruning dose is the estimated amount of foliage or buds removed during pruning compared with the total amount on the tree prior to pruning. The ANSI A300 Part 1, Pruning Standard states: “Pruning options should remove no more living tissue than what is necessary to achieve specified objectives. Species, size, age, condition, and site shall be considered when specifying the location and amount of live branches to be removed.” In the past an amount of 25% of the foliage was set as the maximum foliage removal. However healthier and younger trees can tolerate greater amounts of live branch removal than do mature or unhealthy trees. On some trees in a compromised health, removing 10% of the live foliage may be too much. Larger doses can be applied to certain branches or sections of the crown to reduce leverage, re-direct growth, special clearance needs, or for other reasons.
Topping
Topping is the removal of live foliage to large diameter branches, and usually results in the removal of greater than 50% of the foliage. In response to the number of leaves removed, trees will develop water sprouts and shoots following the removal of too much foliage. Sprouting is an attempt by the tree using stored energy to replace the photosynthesis generating foliage that was suddenly removed. The trained arborist will have to make several annual visits to the topped tree to provide the correct treatment for the tree's growth response. Sprouting requires energy and depletes the tree’s reserves, so sprouts should be allowed to grow for a few years as they are gradually reduced in number. The sprouts that grow from the cuts are poorly attached for at least a period of several years. The goal in restoring these trees is to prune so that eventually there is only one main leader at the location of each topping cut. Maintain the sprouts as mentioned above.
Lions Tailing
Lions tailing strips all of the interior foliage and retains only the foliage on the branch tips. This impacts the tree in 4 ways: It removes a large amount of photosynthesis producing foliage; it exposes branches to sun burn or sunscald; it reduces lateral branches to shorten long branches; it results in less taper and more end weight leverage increasing the likelihood of branch failure. For all these reasons it is a bad practice.
Tipped Trees
Tipping removes the ends of branches to create a smaller tree. It removes a large amount of terminal buds and can impact tree growth. This is another bad practice.
Both Lions Tailing and Tipping create a challenge for the arborist and take several years to correct. Allow the recently improperly pruned tree to grow for a couple years to regain energy and strength. Then begin the process to remove some of the sprouts and shorten others two or three years after the tree was over-pruned. Finally, proceed with restoration, similar to the procedure for topped trees.
When people prune trees in these abusive manners, excessive live tissue is removed from the tree and the structure is compromised. This creates poor form and numerous wounds, and the tree becomes more prone to stress, failure and premature death.
Pruning Strategies
- Remove live foliage from a mature or over-mature tree only for good reason to accomplish your objectives.
- Refrain from removing any live foliage from a stressed tree because they need as much photosynthetic generating capacity as possible.
- Removing live tissue on a mature tree forces it to react and expend energy unnecessarily.
- To thin an old tree, make cuts on smaller branches only in the canopy edge.
- Removing primary branches such as scaffold branches and secondary branches growing from scaffold branches may leave large pruning wounds and remove too much live tissue.
- Branches that are more than a third the diameter of the trunk, and those that are more than 15 years old may have a poor ability to restrict the spread of decay following these branch removals.
- Consider shortening or thinning the branch by removing smaller branches instead of the entire large branch.
Stages of Tree Growth
Youth – Tree management begins in the early stages when most of the energy produced by a tree is used for growth. There is a rapid increase in size as the crown and leaf area grows from a seedling to a fully mature tree.
Mature – When the optimum crown size is reached the amount of food produced from the leaves remains much the same each year and results in a more or less constant volume of wood being generated throughout the tree. However, as the tree gets larger, this volume is spread thinly, and the successive increments of growth added to the tree is seen as the leveling off in width of the rings in the trunk.
Ancient – The final stage is reached when the rings of wood have a reducing cross-sectional area, and the tree is still increasing in girth very slowly. The crown begins to die back and branches may be lost. Damage and decay also reduce productivity of the tree’s functions. The result is that as the leaf area declines, less new photosynthetic material is produced and the tree is less able to maintain a complete cover of woody material over the trunk. However, despite the age, as long as the tree is adding rings to the trunk, the tree is still growing.
Retrenchment
When ancient trees lose branches and size, it is referred to as retrenchment. This does not mean that the tree is about to die, but it is a condition that can last for many decades or even centuries. Retrenchment is brought on by drought, disease, decay, insect damage, root disturbance, or pollution. The response from the tree results in a new balance between the area of woody material and that of the leaves. A tree in the last phase of its life that has retrenched can be very healthy and vigorous despite extensive decay and dieback. This stage may also be the longest stage in the life of the tree. The ancient stage can be further subdivided into three phases.
Tree species vary in the proportion of time they spend in each of these retrenchment phases. All the phases are a continuous process and of variable length.
Ancient Tree Management
Ancient trees are those that have reached a mature size and have started the final phase of natural processes. This final process includes shedding larger branches, developing cavities, and become shorter. Trees in this process must economize their energy allocation and transport distances. This decline or retrenchment allows the tree to continue living for many more years. As it declines the tree expands its level of biodiversity for the benefit of animals, birds, and insects.
Other Aspects of the Ancient Stage
Ancient trees that are retrenching tend to show a diminished growth rate and a drop in reproductive output. They are also slower to cover over wounds if damaged. They tend to develop other features and characteristics such as cavities in the trunk, dead loose bark, dead wood in the canopy and physical damage. One important point to note is that, as the tree ages it becomes more valuable for a wide range of other organisms and its habitat value increases.
A normal tree reaches the ancient stage when the size of the crown is not large enough to produce enough food to maintain the same cross-sectional area for each annual ring. During the process of retrenchment, the photosynthetic area is reduced, as is the surface area of the woody branches, so that less food is required by the tree. Reducing the size of the crown at intervals delays the ancient stage in a tree’s life when the demand for water and nutrients outstrips its ability to increase the root area to absorb them. A reduced crown also reduces the risk of wind throw, owing to the relatively lower stature because of shorter branch lengths and a smaller wind sail area of leaves.
Wounds to the tree result in the drying out of an area of wood, causing decay. The larger the amount of wounding such as found on an old tree with all its branches removed, will result in more drying out and dieback. This will increase the chance of infection by micro-organisms. The ability of the tree to compartmentalize will be reduced because of its severely reduced photosynthetic area. Very few trees are able to cope with this situation. If some branches are retained on the tree, the amount of exposure, drying out, and infection by micro-organisms is decreased. However, these areas will be restricted to strips of xylem and phloem associated with the remaining bark. For this reason and to keep the sap wood active, it is best to retain good connections of xylem and phloem throughout the tree, thus maintaining channels of living tissue between the roots and shoots. In some old trees that have been cut back heavily on one side, this connection has been broken and the tree has died back completely thus resulting in a lop-sided tree.
The process of decay in wood is a complex subject and the details are only just starting to be understood. There are many different agents involved which make it very difficult to establish the relative importance of each. What is clear however, is that fungi have a fundamental role in the process.
It is thought that the sapwood of a healthy tree has a high enough moisture content that it is unsuitable for the growth of most fungi. However, when the tree is mechanically damaged or is stressed in some way, parts of it may become more suitable for fungal growth. The loss of a branch, for example, allows air in and causes drying of the wood around the wound, and this enables fungal growth. Stress brought on by drought or the severing of roots may cause the tree to stop photosynthesizing from a branch. This branch then dies back and dries out because the flow of sap is no longer as strong as normal. The drier conditions activate some of the latent fungi or fungi entering via the dead or broken wood. Most of the fungi capable of causing extensive decay depend on wounds or dead branches or roots as entry points. Some of these decay species grow only in heartwood, while others are confined to sapwood or are able to colonize in either. A wide range of factors determine whether or not decay becomes extensive enough to weaken the tree significantly. Some pathogenic fungal species are able to cause death or dysfunction to parts of the tree even without stress or major injury. An example might be some honey fungus species or the fungus that causes Dutch elm disease or oak wilt. This relative minority of species are a primary cause of dysfunction in the sapwood or death of the cambium.
As the fruiting bodies of the fungi are the only parts that are usually noticed they are often misinterpreted. A small number of species can cause the death of a tree, but a much larger number produce fruiting bodies only when the tree (or that part with fungal fruiting bodies) has died from other causes. This leads to many misconceptions as to the role of fungi.
Youth – Tree management begins in the early stages when most of the energy produced by a tree is used for growth. There is a rapid increase in size as the crown and leaf area grows from a seedling to a fully mature tree.
Mature – When the optimum crown size is reached the amount of food produced from the leaves remains much the same each year and results in a more or less constant volume of wood being generated throughout the tree. However, as the tree gets larger, this volume is spread thinly, and the successive increments of growth added to the tree is seen as the leveling off in width of the rings in the trunk.
Ancient – The final stage is reached when the rings of wood have a reducing cross-sectional area, and the tree is still increasing in girth very slowly. The crown begins to die back and branches may be lost. Damage and decay also reduce productivity of the tree’s functions. The result is that as the leaf area declines, less new photosynthetic material is produced and the tree is less able to maintain a complete cover of woody material over the trunk. However, despite the age, as long as the tree is adding rings to the trunk, the tree is still growing.
Retrenchment
When ancient trees lose branches and size, it is referred to as retrenchment. This does not mean that the tree is about to die, but it is a condition that can last for many decades or even centuries. Retrenchment is brought on by drought, disease, decay, insect damage, root disturbance, or pollution. The response from the tree results in a new balance between the area of woody material and that of the leaves. A tree in the last phase of its life that has retrenched can be very healthy and vigorous despite extensive decay and dieback. This stage may also be the longest stage in the life of the tree. The ancient stage can be further subdivided into three phases.
- Early ancient – When, over a period of years, the amount of dieback exceeds growth.
- Mid-ancient – When the annual rings cannot form all the way round the trunk because of discontinuities. Once a tree has reached the mid-ancient stage, nothing should be done to encourage the speeding up of the aging process and the aim should be to keep it in this phase for as long as possible.
- Senescent – The terminal decline of the tree, leading to death.
Tree species vary in the proportion of time they spend in each of these retrenchment phases. All the phases are a continuous process and of variable length.
Ancient Tree Management
Ancient trees are those that have reached a mature size and have started the final phase of natural processes. This final process includes shedding larger branches, developing cavities, and become shorter. Trees in this process must economize their energy allocation and transport distances. This decline or retrenchment allows the tree to continue living for many more years. As it declines the tree expands its level of biodiversity for the benefit of animals, birds, and insects.
Other Aspects of the Ancient Stage
Ancient trees that are retrenching tend to show a diminished growth rate and a drop in reproductive output. They are also slower to cover over wounds if damaged. They tend to develop other features and characteristics such as cavities in the trunk, dead loose bark, dead wood in the canopy and physical damage. One important point to note is that, as the tree ages it becomes more valuable for a wide range of other organisms and its habitat value increases.
A normal tree reaches the ancient stage when the size of the crown is not large enough to produce enough food to maintain the same cross-sectional area for each annual ring. During the process of retrenchment, the photosynthetic area is reduced, as is the surface area of the woody branches, so that less food is required by the tree. Reducing the size of the crown at intervals delays the ancient stage in a tree’s life when the demand for water and nutrients outstrips its ability to increase the root area to absorb them. A reduced crown also reduces the risk of wind throw, owing to the relatively lower stature because of shorter branch lengths and a smaller wind sail area of leaves.
Wounds to the tree result in the drying out of an area of wood, causing decay. The larger the amount of wounding such as found on an old tree with all its branches removed, will result in more drying out and dieback. This will increase the chance of infection by micro-organisms. The ability of the tree to compartmentalize will be reduced because of its severely reduced photosynthetic area. Very few trees are able to cope with this situation. If some branches are retained on the tree, the amount of exposure, drying out, and infection by micro-organisms is decreased. However, these areas will be restricted to strips of xylem and phloem associated with the remaining bark. For this reason and to keep the sap wood active, it is best to retain good connections of xylem and phloem throughout the tree, thus maintaining channels of living tissue between the roots and shoots. In some old trees that have been cut back heavily on one side, this connection has been broken and the tree has died back completely thus resulting in a lop-sided tree.
The process of decay in wood is a complex subject and the details are only just starting to be understood. There are many different agents involved which make it very difficult to establish the relative importance of each. What is clear however, is that fungi have a fundamental role in the process.
It is thought that the sapwood of a healthy tree has a high enough moisture content that it is unsuitable for the growth of most fungi. However, when the tree is mechanically damaged or is stressed in some way, parts of it may become more suitable for fungal growth. The loss of a branch, for example, allows air in and causes drying of the wood around the wound, and this enables fungal growth. Stress brought on by drought or the severing of roots may cause the tree to stop photosynthesizing from a branch. This branch then dies back and dries out because the flow of sap is no longer as strong as normal. The drier conditions activate some of the latent fungi or fungi entering via the dead or broken wood. Most of the fungi capable of causing extensive decay depend on wounds or dead branches or roots as entry points. Some of these decay species grow only in heartwood, while others are confined to sapwood or are able to colonize in either. A wide range of factors determine whether or not decay becomes extensive enough to weaken the tree significantly. Some pathogenic fungal species are able to cause death or dysfunction to parts of the tree even without stress or major injury. An example might be some honey fungus species or the fungus that causes Dutch elm disease or oak wilt. This relative minority of species are a primary cause of dysfunction in the sapwood or death of the cambium.
As the fruiting bodies of the fungi are the only parts that are usually noticed they are often misinterpreted. A small number of species can cause the death of a tree, but a much larger number produce fruiting bodies only when the tree (or that part with fungal fruiting bodies) has died from other causes. This leads to many misconceptions as to the role of fungi.
Seasons to Prune
By Steve Gardner
When is the best time to prune a tree? There are lots of factors to consider before pruning. I always answer with a smile, “When I have the money.” Normally, unless the target trees are pines or species with active bark beetle activity, they can be pruned any time of the year. The time of year for pruning affects the growth response. Summer pruning tends toward restricting growth or dwarfing, but is usually better for shaping internally. Summer pruning also helps limit sucker growth. Winter pruning promotes growth at the cut site, so it can be better for leader or branch end pruning. Below here are several very common landscape tree species and a general breakdown of when and how to prune them that best suits their needs and avoids pest problems.
Oaks (Quercus)
Pruning young oaks is important in order to start the tree with proper structure and a well-balanced shape. As they age, pruning involves removing dead and diseased wood rather than maintaining its shape. If the tree is within an oak wilt quarantine area, oak trees have a very specific window of when you should NOT prune them. Oak wilt is caused by a fungus (Ceraticystis fagacearum) that is spread by Nitidulid (sap) beetles. These beetles are most active during the spring months, generally from February through May. By not pruning oak trees during this period, sap from the pruning will not attract the insects and provide an easy access for the fungus to get inside the tree. The best time to prune oaks is November through January when the beetles are least active. If making a pruning cut during sap flow is necessary, cover the wound with a sealant like a warm wax or pitch specially made for sealing tree grafts and wounds. Susceptible trees should not be pruned during active transmission periods.
Maples (Acer)
Maples are quite forgiving and do not necessarily have any special considerations for pruning. The only exceptions are to avoid pruning maples in the early spring or when they are not healthy. Maple trees move large amounts of sap in the spring and if a cut is made during this time, the tree will bleed heavily. Later in the spring as the tree is budding and growing new leaves, they will not put as much energy into healing the wound as they put into growing their leaves, which could result in disease problems. This also applies to when maples drop their leaves in the autumn. Silver maples (Acer saccharinum) sucker often if the tree is over pruned and those suckers are pruned at the wrong time of year.
In addition to the maples, birches (Betula spp.) also drip sap when pruned in the early spring when sap flow is heavy. Although unattractive, sap drainage has little negative effect on tree growth. Some of the sap dripping can be avoided by pruning in summer or at other times of the year.
Other trees that drip sap when pruned in late winter/early spring include:
Avocado (Persea americana)
Flowering dogwood (Cornus)
Hackberry (Celtis)
Honeylocust (Gleditsia)
Magnolia (Magnolia)
Walnut (Juglans)
Willow (Salix)
Willow trees in general are one tree that can be pruned just about any time and anywhere. But like most trees, it’s usually best to prune when the tree is dormant in winter.
Keep in mind when pruning a willow:
Elm (Ulmus)
Elm trees need a lot of special consideration when it comes to pruning them because of Dutch elm disease (DED). The Elm Bark Beetle is attracted to pruning wounds and live tissue on trees should not be pruned when the bark beetles are active. In fact, lots of cities have laws regarding when elms can be pruned to help contain the disease. The exception to timing of pruning is that trees with visible Dutch elm disease symptomatic branches should have the diseased branches removed as soon as a branch shows flagging. Elm trees are pruned very similar to how to other trees are pruned. Remove old, dead wood, crossing and suckering branches, and other unsightly, unbalanced growth. Good pruning at the right time can actually help the elm fight off DED and recover from infections.
Poplar (Populus)
Poplar trees are best pruned when they are entering dormancy in the late summer and autumn. This time is also good because they will not put forth the energy to grow suckers. When pruning Poplars, take great care to approach cuts with an “easy-does-it” attitude. Avoid tearing bark. To prune poplar correctly, cut the weight off of the branch gradually before making the final cut. Regular pruning of these fast-growing trees will extend their life and create a nice mature specimen.
Conifers
Conifers are generally trees that do not require a lot of pruning, and do not recover well after pruning. Most conifers do not sprout new foliage once foliage is removed. The two main reasons to prune a conifer are for health (removing of old, dead, diseased growth) and for size control (which is avoidable by installing a conifer at a place that is the right size for the mature tree).
When and how a conifer is pruned depends on the species.
Fruit Trees
Fruit trees depend on proper pruning to keep them productive, healthy, and balanced. Fruit trees are also pruned to allow easier harvest. Fruit trees can be pruned during the dormant season to enhance structure and distribute fruiting wood, and they are pruned after bloom to thin fruit.
Hardwood Trees and Shrubs Without Showy Flowers
Prune these trees in the dormant season to:
Flowering Trees and Shrubs
Flowering can be prevented or enhanced by pruning at the appropriate time of the year. However, to preserve the current year's flower crop, prune according to the following schedule:
Plants that bloom on last season’s wood, such as Prunus, should be pruned just after bloom in order to preserve the flower display.
Other Trees
The best time to prune live branches may depend on the desired results. Beyond the exceptions noted in the species descriptions above, growth is maximized and defects are easier to see on deciduous trees if live-branch pruning is done just before growth resumes in early spring. Pruning when trees are dormant can reduce the risk of pest problems associated with wounding and allow trees to take advantage of the full growing season to begin closing and compartmentalizing wounds.
Removal of dying, diseased, broken, or dead branches can be accomplished at any time with little negative effect on the tree. Plant growth can be reduced if live-branch pruning takes place during or soon after the initial growth flush. However, since this is when trees have just expended a great deal of stored energy to produce roots, foliage, and early shoot growth, pruning at this time is usually not recommended due to the potential stresses. Stressed trees should not be pruned at this time.
By Steve Gardner
When is the best time to prune a tree? There are lots of factors to consider before pruning. I always answer with a smile, “When I have the money.” Normally, unless the target trees are pines or species with active bark beetle activity, they can be pruned any time of the year. The time of year for pruning affects the growth response. Summer pruning tends toward restricting growth or dwarfing, but is usually better for shaping internally. Summer pruning also helps limit sucker growth. Winter pruning promotes growth at the cut site, so it can be better for leader or branch end pruning. Below here are several very common landscape tree species and a general breakdown of when and how to prune them that best suits their needs and avoids pest problems.
Oaks (Quercus)
Pruning young oaks is important in order to start the tree with proper structure and a well-balanced shape. As they age, pruning involves removing dead and diseased wood rather than maintaining its shape. If the tree is within an oak wilt quarantine area, oak trees have a very specific window of when you should NOT prune them. Oak wilt is caused by a fungus (Ceraticystis fagacearum) that is spread by Nitidulid (sap) beetles. These beetles are most active during the spring months, generally from February through May. By not pruning oak trees during this period, sap from the pruning will not attract the insects and provide an easy access for the fungus to get inside the tree. The best time to prune oaks is November through January when the beetles are least active. If making a pruning cut during sap flow is necessary, cover the wound with a sealant like a warm wax or pitch specially made for sealing tree grafts and wounds. Susceptible trees should not be pruned during active transmission periods.
Maples (Acer)
Maples are quite forgiving and do not necessarily have any special considerations for pruning. The only exceptions are to avoid pruning maples in the early spring or when they are not healthy. Maple trees move large amounts of sap in the spring and if a cut is made during this time, the tree will bleed heavily. Later in the spring as the tree is budding and growing new leaves, they will not put as much energy into healing the wound as they put into growing their leaves, which could result in disease problems. This also applies to when maples drop their leaves in the autumn. Silver maples (Acer saccharinum) sucker often if the tree is over pruned and those suckers are pruned at the wrong time of year.
In addition to the maples, birches (Betula spp.) also drip sap when pruned in the early spring when sap flow is heavy. Although unattractive, sap drainage has little negative effect on tree growth. Some of the sap dripping can be avoided by pruning in summer or at other times of the year.
Other trees that drip sap when pruned in late winter/early spring include:
Avocado (Persea americana)
Flowering dogwood (Cornus)
Hackberry (Celtis)
Honeylocust (Gleditsia)
Magnolia (Magnolia)
Walnut (Juglans)
Willow (Salix)
Willow trees in general are one tree that can be pruned just about any time and anywhere. But like most trees, it’s usually best to prune when the tree is dormant in winter.
Keep in mind when pruning a willow:
- that whatever branch is cut off of a willow, many more will take its place.
- to aim for very basic of results such as balancing the weight of the tree, remove dead branches, and remove suckers.
- be ready to accept whatever shape the tree takes and give the tree lots of room to grow.
Elm (Ulmus)
Elm trees need a lot of special consideration when it comes to pruning them because of Dutch elm disease (DED). The Elm Bark Beetle is attracted to pruning wounds and live tissue on trees should not be pruned when the bark beetles are active. In fact, lots of cities have laws regarding when elms can be pruned to help contain the disease. The exception to timing of pruning is that trees with visible Dutch elm disease symptomatic branches should have the diseased branches removed as soon as a branch shows flagging. Elm trees are pruned very similar to how to other trees are pruned. Remove old, dead wood, crossing and suckering branches, and other unsightly, unbalanced growth. Good pruning at the right time can actually help the elm fight off DED and recover from infections.
Poplar (Populus)
Poplar trees are best pruned when they are entering dormancy in the late summer and autumn. This time is also good because they will not put forth the energy to grow suckers. When pruning Poplars, take great care to approach cuts with an “easy-does-it” attitude. Avoid tearing bark. To prune poplar correctly, cut the weight off of the branch gradually before making the final cut. Regular pruning of these fast-growing trees will extend their life and create a nice mature specimen.
Conifers
Conifers are generally trees that do not require a lot of pruning, and do not recover well after pruning. Most conifers do not sprout new foliage once foliage is removed. The two main reasons to prune a conifer are for health (removing of old, dead, diseased growth) and for size control (which is avoidable by installing a conifer at a place that is the right size for the mature tree).
When and how a conifer is pruned depends on the species.
- Arborvitaes (Thuja), Chamaecyparis, and junipers (Juniperus) should be pruned to keep their size only. Best done in late spring and early summer. The old wood underneath green growth will not grow back.
- Pines (Pinus) should be pruned by cutting off the candles in late spring to encourage bud formation for next year’s new growth.
- Hemlocks (Tsuga), yews (Taxus), firs (Abies), and spruces (Picea) should be pruned from late summer to late winter when the tree is dormant. Prune to control height by cutting the leader about a half inch above a bud on the leader. This will encourage branching while controlling height.
Fruit Trees
Fruit trees depend on proper pruning to keep them productive, healthy, and balanced. Fruit trees are also pruned to allow easier harvest. Fruit trees can be pruned during the dormant season to enhance structure and distribute fruiting wood, and they are pruned after bloom to thin fruit.
Hardwood Trees and Shrubs Without Showy Flowers
Prune these trees in the dormant season to:
- easily visualize the structure of the plant,
- maximize wound closure in the growing season after pruning,
- reduce the chance of transmitting disease,
- discourage excessive sap flow from the pruning wounds.
Flowering Trees and Shrubs
Flowering can be prevented or enhanced by pruning at the appropriate time of the year. However, to preserve the current year's flower crop, prune according to the following schedule:
- To retain the most flowers on landscape trees that bloom on current season’s growth, such as crape-myrtle (Lagerstroemia spp.) or linden (Tilia spp.), these trees are pruned in winter, prior to leaf emergence, or in the summer just after bloom.
- Trees and shrubs that flower in spring such as redbud (Cercis) and dogwood (Cornus), etc. should be pruned immediately after flowering.
- Trees and shrubs that flower in the summer or autumn should be pruned during the dormant season. Flower buds will form on new twigs during the next growing season.
- Dead branches can be removed any time of the year.
- Many flowering trees are susceptible to fire-blight, a bacterial disease that can be spread by pruning. These trees include many varieties of crab-apple (Malus), hawthorn (Crataegus), pear (Pyrus), mountain ash (Sorbus), flowering quince (Cydonia) and Pyracantha. They should be pruned during the dormant season.
Plants that bloom on last season’s wood, such as Prunus, should be pruned just after bloom in order to preserve the flower display.
Other Trees
The best time to prune live branches may depend on the desired results. Beyond the exceptions noted in the species descriptions above, growth is maximized and defects are easier to see on deciduous trees if live-branch pruning is done just before growth resumes in early spring. Pruning when trees are dormant can reduce the risk of pest problems associated with wounding and allow trees to take advantage of the full growing season to begin closing and compartmentalizing wounds.
Removal of dying, diseased, broken, or dead branches can be accomplished at any time with little negative effect on the tree. Plant growth can be reduced if live-branch pruning takes place during or soon after the initial growth flush. However, since this is when trees have just expended a great deal of stored energy to produce roots, foliage, and early shoot growth, pruning at this time is usually not recommended due to the potential stresses. Stressed trees should not be pruned at this time.
Municipal Pruning Options
By Len Phillips
There are many options available for municipalities to consider how their public shade and park trees get pruned. One factor to consider is to determine how often the trees get pruned and what pruning cycle is the best for the city’s trees as well as what the city can afford. For example, ginkgo trees may need pruning every 15 years, while a large pin oak might need pruning every 3 to 4 years. Most municipal arborists and landscape architects will agree that ideally a complete pruning cycle for all city trees should be 8 years. What the actual pruning cycle is usually ranges between 10 and 20+ years. With a 4-year pruning cycle, the trees do not grow all that much so branches at risk are minimal and the pruning cycle is short and storm damage Is minimal. The long-term cycles of more than every 10 years result in crews spending days working on a single street and storm damage is severe. Obviously, the short cycles are preferred but getting there takes a great deal of effort, time, and money, and the assistance of contracted or extra crews may be necessary.
Who Does the Work
The next factor to consider is how the pruning gets done. Are contractors doing all the work, some of the work, or are city workers sharing the load or doing all of the work? This is basically a policy decision each municipality must make. Costs will vary depending on your location, salaries, benefits, and competition for the work. Most of the time the costs are slightly lower by using city workers, plus you have employees who will have to live with their mistakes and they generate pride in their best work. In addition, these workers are available for special needs and emergencies, anywhere and anytime. Contractors are best used for specific activities such as tree installation projects or pruning trees on the less traveled roads, pruning large trees only, or doing line pruning for the local utilities. Some agencies use contractors for block pruning and agency staff for special projects.
Some cities set policies where the city workers prune all public trees up to 10 inches in diameter and a contractor handles all the larger trees. The advantage of this option is that the city can provide the pruning necessary to form the tree with the proper branching structure and straight upright leaders while it is young. These cities will prune newly installed trees early in their life and then prune the trees on a 7 to 10-year cycle. The correct pruning at the earliest stages of tree development will reduce the long-term pruning needs as the tree gets larger.
Public Trees Versus Private Trees
Most communities have from 20% to 50% of the canopy on public property. There is always a comparison of the pruning of public trees versus private trees. Most cities will assume responsibility of trees on the public way and not on private property. The questions come up when private trees produce branches at risk that grow over a property line. However, if it is a risk, the city is responsible to take some action especially when the branch is hanging over the street. Tree care on a private road is almost always the landowner’s responsibility, with the exception of utility line clearance.
A bigger question comes up where the city installs the tree on private property, who maintains the tree then? The city where I worked for most of my career had a policy to install almost all trees on private property so the maintenance of that tree would be the homeowner’s responsibility. This policy was in accordance with state law. My city was also recognized as being a wealthy city where most of the residents could afford to have their “tree guy” provide proper care as the "city installed" tree grew.
My city did have a municipal pruning crew that maintained all trees and branches within the public way and an occasional small tree on private property that had been installed by the city within the past couple of years. The city was able to keep on a 4-year pruning cycle for most of the time I worked there. We also had a separate crew that was assigned, trained, and equipped, to provide line clearance service for our municipal electric light department. This second crew also maintained a 4-year pruning cycle on the overhead wires side of the street.
The major disadvantage of this concept is that the private homeowner may not have the training or funds to properly care for these "city installed" trees that are now on their property. The result is that in 20 to 30 years, when these trees are mature, but not properly pruned, are subject to severe damage in storms. This often results in the complete destruction of the tree, just when it should be getting to the size where it can produce some benefit to combating climate change.
This raises a lot of questions from municipal arborists who feel they should be responsible for all trees on the public way as well as the branches that hang over the street and could become an issue during a storm. One valid argument for this option deals with sustainability. Instead of – say – 5 private contractors coming with equipment and manpower to deal with 5 private landowner neighbors, at 5 different times, the city crew could do all 5 at one time with one visit and use a lot less energy and fewer resources. Furthermore, the municipal arborist would not have to spend the day making inspections and sending out certified letters to those residents where the trees are not maintained and pose a threat to public safety. One city forester told me several years ago that he was spending an average of $25 per street tree to prune it while on their 6-year pruning cycle. No homeowner could even come close to matching that price. He said, “I know it sounds crazy, but perhaps government can actually do some things more efficiently than individuals can!”.
Citizen Pruners
The City of Ithaca, NY pioneered a successful Citizen Pruners program for providing structural pruning on young trees. Trained volunteers work once a week from May to October. Since 1991, Cornell Cooperative Extension and the City of Ithaca provides training programs that cover subjects such as unusual urban trees, evaluating tree structure and needs, and actual hands-on structural pruning of trees and shrubs. The city trains interested citizens on how to properly prune small trees correctly, from the ground, not from ladders or tree climbing. Trees that are appropriately pruned while they are young will require little corrective pruning when they mature, which will save the city money on maintenance. When the training is completed, the citizens are assigned to areas where young trees were recently installed and they do their pruning in teams of two people. Often the citizens are actually able to do as good as or even better than the city staff can do.
This concept has become quite popular and has spread to other cities. California's Redwood City’s nonprofit CityTrees started pruning the trees they installed after the second year of operation. There are two benefits of the volunteer young tree care pruning:
2. The volunteer pruners will use their skills to properly prune the trees on private property reducing the disparity of care on private trees alluded to above.
Final Note
Do not worry about what your city’s pruning cycle might be. Plan and strategize. One city told me that they discovered that their pruning cycle the first time around was about 25 years. The second time around they were able to do it in 13 years. Their goal is to reach 8 years by the fourth time around.
Another city told me that they divided the crews into two groups. The ground crew prunes everything they can from the ground with no climbing. They get around the city once every 5 years. The bucket crew takes 17 to 20 years to complete a cycle. They anticipate in a few years, the bucket crew will greatly improve their cycle time as the young trees that have been properly pruned, begin to reach maturity.
These choices require major decisions by the city staff and its citizens. Just remember, the goal is to grow trees to their optimum size so they will provide maximum benefits to the city and its residents. When the community is involved in the tree care decisions, the opportunities increase for more funding and higher priority in the City’s budget process.
Pruning Near Utility Lines
Electric utility contractors prune trees to ensure safe, reliable service to their customers and to gain access to utility poles and wires. The practice is referred to as line clearance. Homeowners, horticulturists, and other persons without Electrical Hazard Awareness training must leave this pruning to a line clearance tree worker or trainee. The utility company has a contractor or a utility arborist on staff to do this challenging hazardous work. Electric utility companies generally have a goal to remove all branches within 10 feet of the utility conductors, for most residential services. The goal is not to prune the trees, the goal is to clear the wires.
Transmission lines tend to require much more clear space around the wires. Ice, snow, and wind can break branches growing above and among wires and cause the wires to break or blow the circuit breakers. This disrupts electric service to homeowners and businesses. Therefore, trees are pruned to direct their growth away from the wires. Controversy sometimes arises when property owners think that their trees were pruned inappropriately or too severely. Though sometimes mistakes are made, the contractor performing the work is typically following specifications as directed by the utility. Utilities should be following ANSI A300 pruning standards and the Best Management Practices for utility pruning as published by the International Society of Arboriculture (ISA).
Directional pruning (also called lateral pruning) guides the tree to grow in a certain direction by removing live branches from portions of the tree and leaving branches that are growing in the desired direction. Reduction and removal cuts are typically employed in directional pruning. This is especially useful near buildings, roads, walks, streetlights, and overhead utility wires. As with other tree pruning, directional pruning usually provides for only a temporary solution. Ample access to sunlight usually means that branches will return to the portion of the tree that was reduced.
Ending Note
This topic was intended to provide a background about tree pruning for the private tree care company, the landscape architect, and the agency arborists. While our assignments and roles are different, the pruning of trees should not be that different.
If you don’t have the latest copy of the ANSI A300 Part 1 Pruning, revised in 2017, or the ISA Best Management Practices for Pruning, please consider adding those documents to your library and share this information with a colleague.
The trees will be better off for it.
Sources
By Len Phillips
There are many options available for municipalities to consider how their public shade and park trees get pruned. One factor to consider is to determine how often the trees get pruned and what pruning cycle is the best for the city’s trees as well as what the city can afford. For example, ginkgo trees may need pruning every 15 years, while a large pin oak might need pruning every 3 to 4 years. Most municipal arborists and landscape architects will agree that ideally a complete pruning cycle for all city trees should be 8 years. What the actual pruning cycle is usually ranges between 10 and 20+ years. With a 4-year pruning cycle, the trees do not grow all that much so branches at risk are minimal and the pruning cycle is short and storm damage Is minimal. The long-term cycles of more than every 10 years result in crews spending days working on a single street and storm damage is severe. Obviously, the short cycles are preferred but getting there takes a great deal of effort, time, and money, and the assistance of contracted or extra crews may be necessary.
Who Does the Work
The next factor to consider is how the pruning gets done. Are contractors doing all the work, some of the work, or are city workers sharing the load or doing all of the work? This is basically a policy decision each municipality must make. Costs will vary depending on your location, salaries, benefits, and competition for the work. Most of the time the costs are slightly lower by using city workers, plus you have employees who will have to live with their mistakes and they generate pride in their best work. In addition, these workers are available for special needs and emergencies, anywhere and anytime. Contractors are best used for specific activities such as tree installation projects or pruning trees on the less traveled roads, pruning large trees only, or doing line pruning for the local utilities. Some agencies use contractors for block pruning and agency staff for special projects.
Some cities set policies where the city workers prune all public trees up to 10 inches in diameter and a contractor handles all the larger trees. The advantage of this option is that the city can provide the pruning necessary to form the tree with the proper branching structure and straight upright leaders while it is young. These cities will prune newly installed trees early in their life and then prune the trees on a 7 to 10-year cycle. The correct pruning at the earliest stages of tree development will reduce the long-term pruning needs as the tree gets larger.
Public Trees Versus Private Trees
Most communities have from 20% to 50% of the canopy on public property. There is always a comparison of the pruning of public trees versus private trees. Most cities will assume responsibility of trees on the public way and not on private property. The questions come up when private trees produce branches at risk that grow over a property line. However, if it is a risk, the city is responsible to take some action especially when the branch is hanging over the street. Tree care on a private road is almost always the landowner’s responsibility, with the exception of utility line clearance.
A bigger question comes up where the city installs the tree on private property, who maintains the tree then? The city where I worked for most of my career had a policy to install almost all trees on private property so the maintenance of that tree would be the homeowner’s responsibility. This policy was in accordance with state law. My city was also recognized as being a wealthy city where most of the residents could afford to have their “tree guy” provide proper care as the "city installed" tree grew.
My city did have a municipal pruning crew that maintained all trees and branches within the public way and an occasional small tree on private property that had been installed by the city within the past couple of years. The city was able to keep on a 4-year pruning cycle for most of the time I worked there. We also had a separate crew that was assigned, trained, and equipped, to provide line clearance service for our municipal electric light department. This second crew also maintained a 4-year pruning cycle on the overhead wires side of the street.
The major disadvantage of this concept is that the private homeowner may not have the training or funds to properly care for these "city installed" trees that are now on their property. The result is that in 20 to 30 years, when these trees are mature, but not properly pruned, are subject to severe damage in storms. This often results in the complete destruction of the tree, just when it should be getting to the size where it can produce some benefit to combating climate change.
This raises a lot of questions from municipal arborists who feel they should be responsible for all trees on the public way as well as the branches that hang over the street and could become an issue during a storm. One valid argument for this option deals with sustainability. Instead of – say – 5 private contractors coming with equipment and manpower to deal with 5 private landowner neighbors, at 5 different times, the city crew could do all 5 at one time with one visit and use a lot less energy and fewer resources. Furthermore, the municipal arborist would not have to spend the day making inspections and sending out certified letters to those residents where the trees are not maintained and pose a threat to public safety. One city forester told me several years ago that he was spending an average of $25 per street tree to prune it while on their 6-year pruning cycle. No homeowner could even come close to matching that price. He said, “I know it sounds crazy, but perhaps government can actually do some things more efficiently than individuals can!”.
Citizen Pruners
The City of Ithaca, NY pioneered a successful Citizen Pruners program for providing structural pruning on young trees. Trained volunteers work once a week from May to October. Since 1991, Cornell Cooperative Extension and the City of Ithaca provides training programs that cover subjects such as unusual urban trees, evaluating tree structure and needs, and actual hands-on structural pruning of trees and shrubs. The city trains interested citizens on how to properly prune small trees correctly, from the ground, not from ladders or tree climbing. Trees that are appropriately pruned while they are young will require little corrective pruning when they mature, which will save the city money on maintenance. When the training is completed, the citizens are assigned to areas where young trees were recently installed and they do their pruning in teams of two people. Often the citizens are actually able to do as good as or even better than the city staff can do.
This concept has become quite popular and has spread to other cities. California's Redwood City’s nonprofit CityTrees started pruning the trees they installed after the second year of operation. There are two benefits of the volunteer young tree care pruning:
- Most agencies cannot afford to prune the young trees, they focus on the risk management needs of larger trees, so the younger trees never get pruned.
2. The volunteer pruners will use their skills to properly prune the trees on private property reducing the disparity of care on private trees alluded to above.
Final Note
Do not worry about what your city’s pruning cycle might be. Plan and strategize. One city told me that they discovered that their pruning cycle the first time around was about 25 years. The second time around they were able to do it in 13 years. Their goal is to reach 8 years by the fourth time around.
Another city told me that they divided the crews into two groups. The ground crew prunes everything they can from the ground with no climbing. They get around the city once every 5 years. The bucket crew takes 17 to 20 years to complete a cycle. They anticipate in a few years, the bucket crew will greatly improve their cycle time as the young trees that have been properly pruned, begin to reach maturity.
These choices require major decisions by the city staff and its citizens. Just remember, the goal is to grow trees to their optimum size so they will provide maximum benefits to the city and its residents. When the community is involved in the tree care decisions, the opportunities increase for more funding and higher priority in the City’s budget process.
Pruning Near Utility Lines
Electric utility contractors prune trees to ensure safe, reliable service to their customers and to gain access to utility poles and wires. The practice is referred to as line clearance. Homeowners, horticulturists, and other persons without Electrical Hazard Awareness training must leave this pruning to a line clearance tree worker or trainee. The utility company has a contractor or a utility arborist on staff to do this challenging hazardous work. Electric utility companies generally have a goal to remove all branches within 10 feet of the utility conductors, for most residential services. The goal is not to prune the trees, the goal is to clear the wires.
Transmission lines tend to require much more clear space around the wires. Ice, snow, and wind can break branches growing above and among wires and cause the wires to break or blow the circuit breakers. This disrupts electric service to homeowners and businesses. Therefore, trees are pruned to direct their growth away from the wires. Controversy sometimes arises when property owners think that their trees were pruned inappropriately or too severely. Though sometimes mistakes are made, the contractor performing the work is typically following specifications as directed by the utility. Utilities should be following ANSI A300 pruning standards and the Best Management Practices for utility pruning as published by the International Society of Arboriculture (ISA).
Directional pruning (also called lateral pruning) guides the tree to grow in a certain direction by removing live branches from portions of the tree and leaving branches that are growing in the desired direction. Reduction and removal cuts are typically employed in directional pruning. This is especially useful near buildings, roads, walks, streetlights, and overhead utility wires. As with other tree pruning, directional pruning usually provides for only a temporary solution. Ample access to sunlight usually means that branches will return to the portion of the tree that was reduced.
Ending Note
This topic was intended to provide a background about tree pruning for the private tree care company, the landscape architect, and the agency arborists. While our assignments and roles are different, the pruning of trees should not be that different.
If you don’t have the latest copy of the ANSI A300 Part 1 Pruning, revised in 2017, or the ISA Best Management Practices for Pruning, please consider adding those documents to your library and share this information with a colleague.
The trees will be better off for it.
Sources
- "All Things Trees Blog”, American Tree Service Supply, 2013.
- Fraedrich, Bruce R. PhD, “Structural Pruning of Young Trees”, Bartlett Tree Experts Research Library.
- Gilman, Edward F., “Landscape Plants”, IFAS, University of Florida, 2015.
- Gilman, Edward, Brian Kempf, Nelda Maheny, and Jim Clark, “The Structural Approach to Pruning for the Commercial Arborist”, Tree Care Industry, May 2015.
- Government of the United Kingdom, “Veteran Trees: A guide to good management” (IN13), February, 2000.
- Smiley, E. Thomas PhD. and Bruce R. Fraedrich, PhD., Plant Pathologists, “Pruning Standard to Maintain Landscape Trees”, Bartlett Tree Experts Research Library.
- Sutton, Michelle, “Pruning Research Update: Dr. Ed Gilman”, City Trees, Vol. 51 No. 3, May/June 2015.
The test that follows contains 80 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
ISA will award 4.0 CEUs* for a passing grade. SAF members will earn 1.0 Cat. 1-CT credit for a passing grade. The cost for taking this test is $20 per credit. If you purchase an annual subscription for 15 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report all passing test scores to ISA and/or SAF. If you are a member of ISA and SAF we will report your passing test scores to both for no additional cost. Tests with passing scores may be submitted only once to each organization.
*Members of ISA may earn 3.5 CEUs for BCMA practice and 0.5 CEUs for BCMA management credits. The 4 CEU's may also be applied toward Certified Arborist, Municipal Specialist, Tree Worker Specialist, and Aerial Lift Specialist credits.
California UFC members will receive credit for passing the test. Please add your CaUFC number after your ISA and/or SAF certification number.
ASCA and MTOA members may submit your ISA certification record to these organizations and receive credits one for one.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment online securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to Merchant' / gibneyCE.com. That will take you to the test sign in page followed by the test. Members with certifications from both ISA and SAF, please be sure to add both of your certification numbers. These numbers are important for reporting purposes.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see our Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks* after that. ISA maintains a record of CEU credits on their website *SAF requires 5 passing test scores before reporting.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.