#14 Read About Trees on Building Sites
Edited by Gordon Mann and Len Phillips, updated in January 2023
Sections You may go directly to the section by clicking on titles listed here.
Note: Click on green text in each section for more information and photos.
The current development climate in most cities almost always requires trees to be included with new projects and retrofits. In order for a project to be approved, it pretty much has to include preserving or installing trees. While this initially sounds good to tree advocates, the reality is that if existing trees don’t receive the protection they need, and space isn’t created for new trees, the site ends up with infrastructure conflicts, dead trees and most likely trees being removed before they provide the benefits they were intended to provide by their inclusion in the project design. While contractors, design engineers, architects, and landscape architects tend to be cautious due to liability issues, they also tend to repeat designs they think work. These design professionals are willing to look at new concepts from an arborist or a landscape architect on the site, especially if the alternative reduces project costs or improves the aesthetics at a reasonable cost. These professionals also have problems with changes to the design when it is too late in the design stages. Input from the arborist and landscape architect on the site must be made as early into the design process as possible. Whenever the comments can be provided before 60% or 75% design, the better chance they have to be included in the final design. Our experience has shown that the more we can be helpful and not critical, the better chances we have of being included earlier in the process.
This Topic #14 is about tree protection and including trees in projects that every person designing with or working around trees must consider when construction work is being proposed on a property. The approach is that trees are infrastructure and have value like other improvements on the property. As infrastructure, the needs of the trees for long-term growth have to be considered for trees to successfully grow on a site. The focus of tree protection is really on soil protection and what has to be done to protect the soil and the tree roots. The final section of this Topic contains a sample specification that should be adjusted to meet the requirements of a proposed construction plan.
While it is known that construction around trees can lead to decline and death, it is difficult to know the extent and types of injury once the project is completed. Noticeable decline may take several years after the person responsible for the damage has left the site (and probably collected their money). Insufficient rooting space, severed roots, altered drainage, and compaction, can contribute to a weakened root system and result in the decline of the tree. These stressed trees are more susceptible to infestations by diseases and insects.
There is not one correct model for the structure of every tree. If there was, we wouldn’t need arborists during construction around trees. The design of any tree’s root system, and what can be expected underground, cannot be assumed without a professional looking at the tree. At some point, each tree involved with a modification of its growing environment or space will have to react to the changes placed upon it. The arborist or landscape architect has to examine the details of a tree and site, and then make the best decisions for the client, the tree, and the site, to best meet the purpose of the assignment. The arborist or landscape architect should be diligent in the inspection and true to the facts. The thoroughness and clarity in developing that information is the arborists’ responsibility which will help others make an informed decision. The decision about the fate of the tree is usually up to the tree owner or the jurisdiction’s rules.
Protecting Trees during Construction
Shade trees have an extensive root system that is required to maintain the health of the tree. Healthy trees provide benefits and services and add value to the property. Most people think about protecting what they can see – the trunk and branches. They don’t consider protecting what they cannot see – soil and roots. In order to protect and preserve established, healthy shade trees and their root systems we have to protect the soil space the trees are growing in. Any construction work in the area near trees should be done with extreme care. Depending on the area of the country, the location of roots and details of a site, a necessary amount of permeable soil has to be protected. While many roots may extend beyond the dripline, the critical soil and root area needs to be established for the subject tree to protect it from soil compaction and disruption.
The roots usually extend beyond the outer reach of the tree foliage in all directions. Leaning trees have a very distorted dripline. For example, a tree’s branches may extend thirty feet (10 m) from the base of the tree up and down the slope, while the lateral branches may extend only 15 feet (5 m) wide. If we simply protect every tree to the dripline, we just need a tape measure. Creating a reasonable soil space to protect the tree’s roots is much more than a circle on the plan; it must be determined by an arborist who has inspected and determined the tree’s critical root space. We can encroach upon portions of the dripline. If we don’t, either the project is halted or the tree is removed. Finding the happy medium for tree space and site design, with proper tree protection leads to successful projects.
Saving a Tree
Trees are the most difficult site resource to replace in kind. Using a circle on the plans and not providing any follow up preservation efforts does not save a tree. Damaging a tree during construction only to have it die later changes the property and the cost to remove a tree which is passed on to the new owner after construction is completed is usually substantially increased.
Shade trees have an extensive root system that is required to maintain the health of the tree. Healthy trees provide benefits and services and add value to the property. Most people think about protecting what they can see – the trunk and branches. They don’t consider protecting what they cannot see – soil and roots. In order to protect and preserve established, healthy shade trees and their root systems we have to protect the soil space the trees are growing in. Any construction work in the area near trees should be done with extreme care. Depending on the area of the country, the location of roots and details of a site, a necessary amount of permeable soil has to be protected. While many roots may extend beyond the dripline, the critical soil and root area needs to be established for the subject tree to protect it from soil compaction and disruption.
The roots usually extend beyond the outer reach of the tree foliage in all directions. Leaning trees have a very distorted dripline. For example, a tree’s branches may extend thirty feet (10 m) from the base of the tree up and down the slope, while the lateral branches may extend only 15 feet (5 m) wide. If we simply protect every tree to the dripline, we just need a tape measure. Creating a reasonable soil space to protect the tree’s roots is much more than a circle on the plan; it must be determined by an arborist who has inspected and determined the tree’s critical root space. We can encroach upon portions of the dripline. If we don’t, either the project is halted or the tree is removed. Finding the happy medium for tree space and site design, with proper tree protection leads to successful projects.
Saving a Tree
Trees are the most difficult site resource to replace in kind. Using a circle on the plans and not providing any follow up preservation efforts does not save a tree. Damaging a tree during construction only to have it die later changes the property and the cost to remove a tree which is passed on to the new owner after construction is completed is usually substantially increased.
The following items must be considered to decide whether it is better to save the tree and work around it, or remove the tree and replace it after the construction is complete. There is no boiler plate list for each site. The variables of the site will help frame the decision making process.
1. Age: Younger trees will survive soil and grade changes better than older, more mature trees.
2. Disturbance Sensitivity: Trees vary in sensitivity when the soil and their roots are cut or disturbed.
3. Amount of Space Needed: Trees generally become stressed if there is limited space and the soil around
their roots is compacted.
4. Severity of Change: The amount of grade change or root loss is a major factor to consider.
5. Tree Health: If trees are already stressed the likelihood of surviving a major change in root loss or grade change is lessened.
6. Tree Structure: If the tree has poor structure and will be at a high risk once the site is developed, pruning or removal may be necessary.
7. Tree Population Size: The number of trees, the creation of the “edge effect”, and the crown response to
either tight growing space or open growing space. Is it one of many or the only tree left?
8. Ordinances and Development Regulations: There may be administrative limits on how many trees may be removed or have to be retained on a site.
1. Age: Younger trees will survive soil and grade changes better than older, more mature trees.
2. Disturbance Sensitivity: Trees vary in sensitivity when the soil and their roots are cut or disturbed.
3. Amount of Space Needed: Trees generally become stressed if there is limited space and the soil around
their roots is compacted.
4. Severity of Change: The amount of grade change or root loss is a major factor to consider.
5. Tree Health: If trees are already stressed the likelihood of surviving a major change in root loss or grade change is lessened.
6. Tree Structure: If the tree has poor structure and will be at a high risk once the site is developed, pruning or removal may be necessary.
7. Tree Population Size: The number of trees, the creation of the “edge effect”, and the crown response to
either tight growing space or open growing space. Is it one of many or the only tree left?
8. Ordinances and Development Regulations: There may be administrative limits on how many trees may be removed or have to be retained on a site.
Preventing Injury to Trees
Preventing damage is less costly and more sustainable than correcting it. Few developers and builders are aware of the way tree roots grow and what is needed to protect them. They seem to think of the tree as a telephone pole, not a wineglass. Before the design process begins, the arborist, landscape architect and planner must meet to decide which trees can be saved and what design modifications must be taken to help preserve the trees that are to remain.
Before any construction activity begins, the plans have to show the tree protection that will be in place during the project. The protection may change during different phases, such as demolition or grading, construction, and landscape installation (yes, tree protection has to be in place even during landscape installation where unfortunately most of the root injury can occur by trenching for irrigation). The protection specifications may vary depending on the phase and activities during a project. The protection must be in place prior to all construction activities commencing.
Tips for Tree Care Before and After Construction
The best time to prepare a tree for construction activities, within the root zone, is a year before the work begins. Trees store water and nutrients for times of need. These times can be the spring growth surge, during droughts, after a hailstorm, etc. So extra stores of water and nutrients will create a healthier, stronger tree and will increase survivability following the stress of potentially damaging work. Trees should be watered a few days before work is to begin and again as soon as possible after the work is done. Watering should be done thoroughly and deeply, allowing the soil to partially dry before re-watering. Care also is needed because moist soil compacts more than dry soil.
Always remember to dial 8-1-1 to contact the local utility companies to find out where it is safe to dig before starting construction and soil work around the trees to be preserved. Not only for safety, it's the law.
Protecting Trees Associated with Construction
When construction is occurring within 10 feet (3 m) of a tree’s dripline, the following guidelines should apply:
Install highly-visible barricades and signs around the trees and root zone areas to be protected. The optimal size of barricaded areas varies according to tree species, size, and construction project. For example:
Types of tree injury caused by mechanized equipment
Root Protection
Roots are protected by guarding the soil from compaction. To protect the roots:
Tree Removal Requirements
It may be difficult to estimate how many roots are to be removed. The closer the roots to be severed are to the trunk, the greater the amount of the root system that will be lost. If more than 50% of the tree's roots have to be removed, or if more than 30% have to be removed from one side of the tree, consideration should be given to relocating or removing and replacing the entire tree or tunneling directly under the tree to minimize the loss of roots.
Roots under Sidewalks
The same considerations should apply when pruning roots during curb or sidewalk repairs. Pruning too many roots can impact the tree’s stability. If removal of the tree is required under life-threatening or related emergency situations, remove it carefully and immediately.
Locating Roots
Most roots are not visible from the surface. Learning where roots are growing requires some excavation in a manner that doesn’t destroy the roots. Soil is best moved using air excavation tools. The least damaging approaches should be specified and followed to learn where the roots are growing and how work can proceed without root damage.
Once roots are exposed, they need to be kept moist either by backfilling with soil or covered with moist fabric. The roots need to be kept moist until the backfill is watered in when the work is completed.
Locating utilities
During the design phase, utilities should be located far enough from trees that the utility can be repaired without requiring the removal of any trees. This should apply to the existing and as well as new trees being planned.
Prior to excavating in any area, always contact the local utility locating system (dial 811 in most parts of the US) to find out where it is safe to dig. Most utility operators perform the utility locating service at no cost to users and in most states – contacting them is the law.
Preventing damage is less costly and more sustainable than correcting it. Few developers and builders are aware of the way tree roots grow and what is needed to protect them. They seem to think of the tree as a telephone pole, not a wineglass. Before the design process begins, the arborist, landscape architect and planner must meet to decide which trees can be saved and what design modifications must be taken to help preserve the trees that are to remain.
Before any construction activity begins, the plans have to show the tree protection that will be in place during the project. The protection may change during different phases, such as demolition or grading, construction, and landscape installation (yes, tree protection has to be in place even during landscape installation where unfortunately most of the root injury can occur by trenching for irrigation). The protection specifications may vary depending on the phase and activities during a project. The protection must be in place prior to all construction activities commencing.
Tips for Tree Care Before and After Construction
The best time to prepare a tree for construction activities, within the root zone, is a year before the work begins. Trees store water and nutrients for times of need. These times can be the spring growth surge, during droughts, after a hailstorm, etc. So extra stores of water and nutrients will create a healthier, stronger tree and will increase survivability following the stress of potentially damaging work. Trees should be watered a few days before work is to begin and again as soon as possible after the work is done. Watering should be done thoroughly and deeply, allowing the soil to partially dry before re-watering. Care also is needed because moist soil compacts more than dry soil.
Always remember to dial 8-1-1 to contact the local utility companies to find out where it is safe to dig before starting construction and soil work around the trees to be preserved. Not only for safety, it's the law.
Protecting Trees Associated with Construction
When construction is occurring within 10 feet (3 m) of a tree’s dripline, the following guidelines should apply:
- Soil excavation work or root cutting should not occur closer than a certain prescribed distance from the tree, which may be defined by using the dripline or by using a multiple of the trunk diameter at breast height (dbh).
- Soil excavation work may be done closer than the distance parameters established above provided all excavation of soil is accomplished by hand, and no roots greater than 2 inches (5 cm) in diameter are severed without further evaluation by a qualified arborist. If it is not feasible to perform the work using these guidelines, options for changing the construction design or relocating the tree may need to be considered.
- Root cutting at the edge of the excavation closest to the tree needs to be done before excavating to avoid tearing roots towards the tree during the excavation. This creates a known limit to the root damage.
Install highly-visible barricades and signs around the trees and root zone areas to be protected. The optimal size of barricaded areas varies according to tree species, size, and construction project. For example:
- For recently installed trees (one to four years), the area to be protected should be within the dripline.
- For minimal protection of trees older than four years, barricades should extend beyond the dripline.
- For additional protection, a temporary layer of wood chips, 4 to 6 inches (10 – 15 cm) deep, should be placed over the soil around each tree (and kept clear of the trunk) prior to placement of the barricades.
- Examine trees and barricades at least once a week during the entire construction time frame; more often during the active construction period.
Types of tree injury caused by mechanized equipment
- Bark removal or "skinning" of the trunk can be caused by any type of equipment. This can easily kill the tree if the damage is extensive around the trunk circumference and if the loose bark is not cut back to connected tissue very soon after the injury. An arborist can replace the bark over the injury using grafting techniques, provided the bark and the trunk are kept moist from the time of the injury until the arborist is on the site.
- Breakage of branches may make the tree unsightly and removing too many leaves will cause stress and future decline of the tree.
- Surface grading that removes surface vegetation and the topsoil that contains a tree's fine absorbing roots and can cause compaction to the soil.
- Trenching that requires cutting through roots for utility or irrigation installations.
Root Protection
Roots are protected by guarding the soil from compaction. To protect the roots:
- fence off the ground underneath the tree’s crown before any construction activity begins including demolition, landscaping, and grading.
- if foot traffic must work in the construction area, protect the soil with a layer of 4 to 6 inch deep mulch. If vehicle equipment must work in the construction area, protect the soil with either a 12" deep gravel or coarse mulch or a 6 inch layer of mulch and steel plates on top of the mulch. A geotextile may be used over the soil before placing the cover material. Temporary construction routes should be located in the approximate location of a permanent road or driveway layout.
- place temporary or permanent sidewalks around or up and over the roots of older trees.
- consider alternative modular or re-usable permeable materials instead of poured concrete or mortared bricks for sidewalks over all tree roots.
- write the specifications to use an air tool, or a pneumatic soil-excavating tool, or an air spade to dig under the tree roots. Digging by hand around the roots is an alternative, but will result in many damaged roots.
- retain the original soil grade over the roots at a maximum of not more than four inches (10 cm) of cut and fill without special precautions to mitigate the oxygen requirements for the roots in the original soil.
- cover bare soil with 4 inches (10 cm) of mulch, especially around younger trees trying to establish their roots.
- whenever possible, all underground utilities should be located within the limits of the final paved surface area such as the road, driveway, or walkway to minimize unnecessary excavations around tree roots that are to be preserved during and after the construction.
Tree Removal Requirements
It may be difficult to estimate how many roots are to be removed. The closer the roots to be severed are to the trunk, the greater the amount of the root system that will be lost. If more than 50% of the tree's roots have to be removed, or if more than 30% have to be removed from one side of the tree, consideration should be given to relocating or removing and replacing the entire tree or tunneling directly under the tree to minimize the loss of roots.
Roots under Sidewalks
The same considerations should apply when pruning roots during curb or sidewalk repairs. Pruning too many roots can impact the tree’s stability. If removal of the tree is required under life-threatening or related emergency situations, remove it carefully and immediately.
Locating Roots
Most roots are not visible from the surface. Learning where roots are growing requires some excavation in a manner that doesn’t destroy the roots. Soil is best moved using air excavation tools. The least damaging approaches should be specified and followed to learn where the roots are growing and how work can proceed without root damage.
Once roots are exposed, they need to be kept moist either by backfilling with soil or covered with moist fabric. The roots need to be kept moist until the backfill is watered in when the work is completed.
Locating utilities
During the design phase, utilities should be located far enough from trees that the utility can be repaired without requiring the removal of any trees. This should apply to the existing and as well as new trees being planned.
Prior to excavating in any area, always contact the local utility locating system (dial 811 in most parts of the US) to find out where it is safe to dig. Most utility operators perform the utility locating service at no cost to users and in most states – contacting them is the law.
Starting Construction
Trenching
Gravity flow such as sewers and storm water pipe need to be located far away from existing trees. Trenching for utilities with a backhoe can cause substantial root damage and should be done as far away as possible from existing trees. Roots should be severed on the trench side of the tree before root removal. In new developments, this can be easily done.
Where the trench must pass under or near an existing tree and there are no alternative layouts, substantial injury can be avoided by using trenchless technology to bore a tunnel under the roots. If backhoe trenching is unavoidable, place the trench as far from the trunk as possible (such as a minimum 8 feet) (2 m), or 3, 4, 5, or 6 times DBH while cutting as few roots as possible. Cleanly cut the roots closest to the trunk side of the trench, with a pruning saw or loppers before pulling them out of the ground. Cover all roots to be saved with wet burlap to prevent excessive moisture loss from the time they are exposed until the time they are recovered with soil. Refill the trench as soon as possible with moist soil.
Root wounds leave the tree highly susceptible to root pathogens and decay fungi. Decline and death can result if more than 40% of the roots are damaged or killed. Stressed trees are also more susceptible to insect attacks such as from bark beetles and borers.
Digging Trenches
Trenches that require root removal should only be dug on one side of the tree within the protected soil zone. To find the minimum distance from the tree trunk that the trench can be dug, measure the diameter, breast height (dbh) and multiply the dbh by three to six (depending on tree age, size, and species) to get minimum distance from tree trunk for the location of the trench. Keep in mind this is the minimum distance and it is always best for the health of the tree to maximize this distance to beyond the dripline. If the necessary location is closer to the tree than the desired minimum distance, a shallow trench shall be hand dug to learn what roots if any are in the location where the trench is to be placed. If there are no large roots greater than 2” (5 cm) in diameter, the trench can be dug with the roots being cleanly cut prior to pulling them out of the ground.
Some recent research has indicated that if the trench is going to remove more than one quarter of the tree's roots, the best place to locate a trench is directly under the tree trunk. This will minimize root loss provided the excavating equipment starts far enough on either side of the trunk (i.e. 3 times DBH) and between major buttressing roots and the tunnel is bored at the bottom of the trench, under the tree trunk and root crown to allow the pipe to run directly under the tree. The utility installation should use materials that will avoid a future pipe failure under the tree so as to avoid having to repair the utility under the trunk.
Augering
The installation of piers in the ground instead of a slab over the ground will reduce soil compaction and damage to roots. When augering for piers, fence posts, signs, etc., the same distance calculation should be used as required for digging trenches. This should be a minimum distance. If possible, extend this distance outward a few feet or better still, outside the dripline. If a pier or post has to be installed closer to the tree, the area proposed to put the hole should be hand dug the first 12 inches (30 cm) deep to learn the presence or lack of roots in this specific area. If during the excavation or drilling process, a substantial root is encountered, the hole should be relocated next to the root. Sometimes pruning is necessary to create space for the augering equipment.
Trenching
Gravity flow such as sewers and storm water pipe need to be located far away from existing trees. Trenching for utilities with a backhoe can cause substantial root damage and should be done as far away as possible from existing trees. Roots should be severed on the trench side of the tree before root removal. In new developments, this can be easily done.
Where the trench must pass under or near an existing tree and there are no alternative layouts, substantial injury can be avoided by using trenchless technology to bore a tunnel under the roots. If backhoe trenching is unavoidable, place the trench as far from the trunk as possible (such as a minimum 8 feet) (2 m), or 3, 4, 5, or 6 times DBH while cutting as few roots as possible. Cleanly cut the roots closest to the trunk side of the trench, with a pruning saw or loppers before pulling them out of the ground. Cover all roots to be saved with wet burlap to prevent excessive moisture loss from the time they are exposed until the time they are recovered with soil. Refill the trench as soon as possible with moist soil.
Root wounds leave the tree highly susceptible to root pathogens and decay fungi. Decline and death can result if more than 40% of the roots are damaged or killed. Stressed trees are also more susceptible to insect attacks such as from bark beetles and borers.
Digging Trenches
Trenches that require root removal should only be dug on one side of the tree within the protected soil zone. To find the minimum distance from the tree trunk that the trench can be dug, measure the diameter, breast height (dbh) and multiply the dbh by three to six (depending on tree age, size, and species) to get minimum distance from tree trunk for the location of the trench. Keep in mind this is the minimum distance and it is always best for the health of the tree to maximize this distance to beyond the dripline. If the necessary location is closer to the tree than the desired minimum distance, a shallow trench shall be hand dug to learn what roots if any are in the location where the trench is to be placed. If there are no large roots greater than 2” (5 cm) in diameter, the trench can be dug with the roots being cleanly cut prior to pulling them out of the ground.
Some recent research has indicated that if the trench is going to remove more than one quarter of the tree's roots, the best place to locate a trench is directly under the tree trunk. This will minimize root loss provided the excavating equipment starts far enough on either side of the trunk (i.e. 3 times DBH) and between major buttressing roots and the tunnel is bored at the bottom of the trench, under the tree trunk and root crown to allow the pipe to run directly under the tree. The utility installation should use materials that will avoid a future pipe failure under the tree so as to avoid having to repair the utility under the trunk.
Augering
The installation of piers in the ground instead of a slab over the ground will reduce soil compaction and damage to roots. When augering for piers, fence posts, signs, etc., the same distance calculation should be used as required for digging trenches. This should be a minimum distance. If possible, extend this distance outward a few feet or better still, outside the dripline. If a pier or post has to be installed closer to the tree, the area proposed to put the hole should be hand dug the first 12 inches (30 cm) deep to learn the presence or lack of roots in this specific area. If during the excavation or drilling process, a substantial root is encountered, the hole should be relocated next to the root. Sometimes pruning is necessary to create space for the augering equipment.
General Rules for Pruning Roots
The following rules will provide guidance to ensure successful root pruning:
The following rules will provide guidance to ensure successful root pruning:
- Provide deep watering around the tree a few days to a week prior to construction activities to soak the soil and hydrate the tree roots, but not within two days before construction begins. The more clay content in the soil, the more days the watering should occur prior to the construction.
- If soil is too wet, any weight on the soil surface will compact the soil pores much worse than dry soil. This will destroy the soil structure and damage the tree's roots.
- Make sure cuts to the roots are done with sharp appropriately sized saws or pruning tools that will make clean cuts. Clean cuts are an absolute necessity for quicker compartmentalization of decay, and increased sprouting of new roots.
- Do not use large equipment like backhoes and bulldozers to tear the roots off the tree. Roots that have been ripped and torn with backhoes typically leave a large amount of root surface exposed to pathogens and unnecessary drying. Also the extent of damage will not be known, since the damage goes beyond the trench wall closer to the tree and it might take years for the damage to become visible in the tree or result in premature death.
- Do not disturb the roots any more than necessary. Twenty percent of the tree's roots can be cut before any signs of stress can appear. Keep in mind that tree roots do extend outside of the protected root zone and beyond the dripline. Compacting soil does disturb roots, even though the roots beneath the compaction may not have been touched.
- During excavation, make sure roots are cut smooth and clean and then covered with soil and wood chips or moist material such as burlap as soon as possible after exposure. Be sure to keep the cover material moist until the soil can be replaced. Depending on site use, mulch can be used for a permanent cover. Small feeder roots can die in less than 10 – 15 minutes with larger roots dying in less than an hour. Hot, dry, and windy conditions require prompt covering of roots to avoid drying.
- Water the tree thoroughly when job is done. This means wetting the top 12 inches (30 cm) of soil within the protected area, and out to the entire dripline if possible, using a slow or low volume water delivery application to seep into the soil.
- Put a permanent layer of landscape mulch, a minimum 3 foot (1m) radius from the trunk around base of the tree. The mulch should be 3-5 inches (8-12 cm) deep but keep the mulch 6 inches (15cm) away from trunk. Mulch near construction areas will need to be 4-6 inches (10-15 cm) deep. Wood chips, bark mulch, ground wood, and ground corn cobs are examples of good products that can be used for mulch. Good mulch products break down over time and add organic matter to the soil, so it is necessary to replenish the mulch from year to year.
Soil Compaction after Planting After a tree is established, any activity that changes the soil pore space or structure can be extremely detrimental to the tree’s health. Construction traffic will compact the soil most severely near the surface where the majority of tree roots may be growing, and where air and water enter the soil profile. Soil compaction decreases soil permeability, reduces the amount of pore space, and interferes with essential gas exchange as well as surface and subsurface drainage. When root growth is restricted by compacted soils, fewer nutrients, air, and water are available for plant growth and soil microorganisms which results in reduced tree vigor and possibly death. Decline and dieback may gradually appear over a period of years because of soil compaction. In construction business transactions, the future owner can unknowingly be the recipient of a declining tree.
It is much easier to avoid soil compaction than to correct it. Keep construction traffic and material storage away from tree root areas and out of the barricaded areas. A 4 to 6 inch (10 – 15 cm) layer of wood chips around all protected trees will help reduce compaction from vehicles that inadvertently cross the barricades. If the soil is wet, compaction is more severe and more permanent, and must be avoided.
When soil or a base material is needed to support a surface treatment such as a walkway, driveway, or patio going over existing tree roots, instead of compacting the soil, a root bridge, a geotextile fabric or other bridging material can be placed over the exiting soil surface to protect the soil containing roots from compaction. The desired improvement can be built with structural integrity without having to implement soil compaction. The geotextile fabric can bridge the loose soil beneath allowing the above materials to stay firmly in place. The use of pervious materials will allow precipitation to flow into the un-compacted soil underneath and recharge the groundwater supply.
It is much easier to avoid soil compaction than to correct it. Keep construction traffic and material storage away from tree root areas and out of the barricaded areas. A 4 to 6 inch (10 – 15 cm) layer of wood chips around all protected trees will help reduce compaction from vehicles that inadvertently cross the barricades. If the soil is wet, compaction is more severe and more permanent, and must be avoided.
When soil or a base material is needed to support a surface treatment such as a walkway, driveway, or patio going over existing tree roots, instead of compacting the soil, a root bridge, a geotextile fabric or other bridging material can be placed over the exiting soil surface to protect the soil containing roots from compaction. The desired improvement can be built with structural integrity without having to implement soil compaction. The geotextile fabric can bridge the loose soil beneath allowing the above materials to stay firmly in place. The use of pervious materials will allow precipitation to flow into the un-compacted soil underneath and recharge the groundwater supply.
Cuts and Fills
Grade Cuts
If more than 12 inches (30 cm) of soil is to be removed from an area containing tree roots, use ground radar to determine the location of all large roots. Then install a retaining wall to reduce the amount of grade change within the protected root zone. If possible create steps of grade changes.
After building the retaining wall, the grade on the tree side should not be changed. After removing the designed amount of soil on the outside of the retaining wall, depending on the remaining soil profile, and if necessary, add topsoil, and place mulch over the soil surface. Soil should not be removed within a specified distance (i.e.3 feet (1 m), or 3, 4, 5, times the DBH) of the trunk. If these distances and tolerances will not work, consider the removal or relocation of the tree and install a replacement tree once the grade change has been completed. If relocation is to be temporary, consider uncovering all the moistened roots with the use of a pneumatic soil-excavating tool, or an air spade then lift the tree out of the way and replant it as soon as possible.
Filling Grade
Before raising the grade within the protected root zone, avoid soil compaction of the existing soil under the fill. No more than 24 inches (65 cm) of soil should be added within the protected root zone. When adding soil to increase the grade within the protected root zone, keep the fill soil a minimum specified distance (i.e. 3 feet (1m) or 3, 4, 5, times the DBH) from the trunk or base of the tree. Soil fill around the trunk will cause root death and wood decay, which can kill the tree no matter how healthy it is.
To keep soil away from the base of the tree, build a retaining wall a minimum specified distance (i.e. 3 feet (1 m), or 3, 4, 5, times the DBH) away from the trunk. The original soil grade between the tree and retaining wall should not be changed. When adding soil, it is advisable to lay perforated drainage pipes in a radial pattern to provide air flow from openings at the wall and the new fill soil should be a mix of permeable fill soil material with high organic matter content. While there is limited research showing the benefits of an aeration system over the soil, there is no other simple cost effective way known at this time to try and provide air and irrigation to the original soil grade once fill has occurred.
If the fill is going to be deeper than suggested above, build the retaining wall as far from the tree trunk as possible, keeping the soil grade around the tree at the original grade, preferably to or outside the dripline. Back-filling should be done as mentioned above, with the lower half the depth being a mix of coarse sand and compost. Aeration placed over the original soil grade is advised when adding large amounts of fill.
Adding or Removing Soil on One Side of the Tree
If it is necessary to cut or fill on only one side of the tree, use the same methods mentioned above. Still do not work closer than 3 feet (1 m) from the trunk of the tree.
Aeration
Aerate within the protected root zone before and after grade changes. Starting a specified distance (i.e. 3 feet (1 m) or 3, 4, 5, times the DBH) from the base of the tree, drill 2-inch (5 cm) diameter holes into the soil about 1 to 1-1/2 feet (30 – 50 cm) deep. Drill the holes in a circle 3 feet (1 m) away from the trunk, spacing the holes two feet (60 cm) apart, and then go out 5 feet (1.5 m) and drill again in a circle. Continue every 2 feet (60 cm) within the protected root zone. Use sand, gravel, or compost to fill the holes.
Another aeration process is radial trenching where it is believed roots are growing. A wheel-spoke pattern from twelve to eighteen inches (30 – 50 cm) from the trunk to the dripline or beyond is created to a depth of 15 inches to 2 feet (30 - 50 cm) without tearing roots. Loosen the soil and fill the trench with a soil and compost mix.
Grade Cuts
If more than 12 inches (30 cm) of soil is to be removed from an area containing tree roots, use ground radar to determine the location of all large roots. Then install a retaining wall to reduce the amount of grade change within the protected root zone. If possible create steps of grade changes.
After building the retaining wall, the grade on the tree side should not be changed. After removing the designed amount of soil on the outside of the retaining wall, depending on the remaining soil profile, and if necessary, add topsoil, and place mulch over the soil surface. Soil should not be removed within a specified distance (i.e.3 feet (1 m), or 3, 4, 5, times the DBH) of the trunk. If these distances and tolerances will not work, consider the removal or relocation of the tree and install a replacement tree once the grade change has been completed. If relocation is to be temporary, consider uncovering all the moistened roots with the use of a pneumatic soil-excavating tool, or an air spade then lift the tree out of the way and replant it as soon as possible.
Filling Grade
Before raising the grade within the protected root zone, avoid soil compaction of the existing soil under the fill. No more than 24 inches (65 cm) of soil should be added within the protected root zone. When adding soil to increase the grade within the protected root zone, keep the fill soil a minimum specified distance (i.e. 3 feet (1m) or 3, 4, 5, times the DBH) from the trunk or base of the tree. Soil fill around the trunk will cause root death and wood decay, which can kill the tree no matter how healthy it is.
To keep soil away from the base of the tree, build a retaining wall a minimum specified distance (i.e. 3 feet (1 m), or 3, 4, 5, times the DBH) away from the trunk. The original soil grade between the tree and retaining wall should not be changed. When adding soil, it is advisable to lay perforated drainage pipes in a radial pattern to provide air flow from openings at the wall and the new fill soil should be a mix of permeable fill soil material with high organic matter content. While there is limited research showing the benefits of an aeration system over the soil, there is no other simple cost effective way known at this time to try and provide air and irrigation to the original soil grade once fill has occurred.
If the fill is going to be deeper than suggested above, build the retaining wall as far from the tree trunk as possible, keeping the soil grade around the tree at the original grade, preferably to or outside the dripline. Back-filling should be done as mentioned above, with the lower half the depth being a mix of coarse sand and compost. Aeration placed over the original soil grade is advised when adding large amounts of fill.
Adding or Removing Soil on One Side of the Tree
If it is necessary to cut or fill on only one side of the tree, use the same methods mentioned above. Still do not work closer than 3 feet (1 m) from the trunk of the tree.
Aeration
Aerate within the protected root zone before and after grade changes. Starting a specified distance (i.e. 3 feet (1 m) or 3, 4, 5, times the DBH) from the base of the tree, drill 2-inch (5 cm) diameter holes into the soil about 1 to 1-1/2 feet (30 – 50 cm) deep. Drill the holes in a circle 3 feet (1 m) away from the trunk, spacing the holes two feet (60 cm) apart, and then go out 5 feet (1.5 m) and drill again in a circle. Continue every 2 feet (60 cm) within the protected root zone. Use sand, gravel, or compost to fill the holes.
Another aeration process is radial trenching where it is believed roots are growing. A wheel-spoke pattern from twelve to eighteen inches (30 – 50 cm) from the trunk to the dripline or beyond is created to a depth of 15 inches to 2 feet (30 - 50 cm) without tearing roots. Loosen the soil and fill the trench with a soil and compost mix.
Care Under the Leaf Foliar Canopy - Summary List
- Do not add more than 6" (15 cm) total of soil and/or mulch.
- Keep mulch 6” (15 cm) away from trunk.
- Avoid tilling soil under canopy.
- Do not overwork or compact the soil under the tree.
- Work soil with hand tools only, no large equipment.
- Preserve the ground cover and/or mulch around the tree to protect its roots and the soil from compaction and maintain soil moisture.
- Do not plant anything within 3 feet (1 m) of the tree trunk that will compete with the tree for moisture and nutrients, or cover and shade the flare, such as another tree, groundcover, or dense shrubs. Ground covers, perennials, and small shrubs can be beneficial to the tree, just not placed too close to the trunk. They act as a barrier to soil compaction and improving moisture content by shielding the soil from direct sun exposure. However they will also compete with the tree for nutrients and moisture, so be sure the tree is healthy enough to compete. Grass or turf is NOT a suitable groundcover. Grass will compete with the tree for nutrients and water. Also control irrigation drainage flow so water does not flow or sit at the base of the tree. The other issue with competing plants around the base of trees in arid climates is they can keep moisture around the flare which should remain dry in dry months.
- Avoid installing any plants under the crown that will grow to more than one-third the mature height of the tree. In Firewise Communities, there should be a distance of 3 times the height of plants under a tree canopy.
- Avoid cutting roots from one inch (2 cm) diameter and larger. If roots are encountered, move the planting hole over until you can dig without encountering large roots. If the root system is so prolific that there are no spaces where roots aren’t present, judicious root pruning can be performed to install the desired plant.
Tips for High Quality Soils
Soil is a combination of particles consisting of any mixture of sand, silt, and clay, plus other materials such as rock, assorted debris, wood, roots and pore space. While we can’t do much to initially change soil particles without replacing them, the most critical element is pore space. Pore space is where the available air and water flow and move in the soil. Like a dry sponge, water may initially run off the soil surface when the soil is very dry or baked by the sun. Compacted soil surfaces also have limited pore space to allow water and air entry. Water should be delivered to soil in low volumes at a low rate over the necessary time so it seeps into the soil to the preferred depth, and does not run off. Placing a slow delivery hose such as soaker, drip or low flow emitters under mulch (when present) helps the water go directly into the soil. The use of soil coverings such as wood chip mulch protects the soil from the sun, breaks down over time to add organic matter which improves the soil quality, and helps reduce evaporation and drying of the soil. Managing watering practices under mulch does require removing the mulch and checking the soil for moisture. Watering frequency should be monitored during the different seasons.
Soil is a combination of particles consisting of any mixture of sand, silt, and clay, plus other materials such as rock, assorted debris, wood, roots and pore space. While we can’t do much to initially change soil particles without replacing them, the most critical element is pore space. Pore space is where the available air and water flow and move in the soil. Like a dry sponge, water may initially run off the soil surface when the soil is very dry or baked by the sun. Compacted soil surfaces also have limited pore space to allow water and air entry. Water should be delivered to soil in low volumes at a low rate over the necessary time so it seeps into the soil to the preferred depth, and does not run off. Placing a slow delivery hose such as soaker, drip or low flow emitters under mulch (when present) helps the water go directly into the soil. The use of soil coverings such as wood chip mulch protects the soil from the sun, breaks down over time to add organic matter which improves the soil quality, and helps reduce evaporation and drying of the soil. Managing watering practices under mulch does require removing the mulch and checking the soil for moisture. Watering frequency should be monitored during the different seasons.
Growing Trees on Public Property
If the project has a requirement to install trees on public property or near other project infrastructure, it is usually to meet a requirement for a number of trees or an amount of canopy cover. The space required for these trees to grow and thrive has to be designed for the mature size of the tree, not the tree size at the time of installation.
Installing New Trees
Prior to installing trees in a limited soil space situation, there are a few design enhancements that can be used to reduce or delay the eventual root damage. Whenever possible select a tree with the mature size that is suitable for the space and soil available. If the width of the space between the curb and sidewalk is only three feet (1m) wide, then a small tree should be installed. If the space is ten feet (3 m) wide or more, then a large shade tree can be considered. However, the available soil space for roots and the quality of the soil are also very important considerations. Additionally, the size and shape of the foliar crown has to be considered adjacent to sidewalks, driveways and roads, as well as adjacent to buildings, street signs and lights.
Just because a person can plant a tree anywhere does not mean the tree is restricted to only growing in the limited space provided. Growing in permitted space is not something we can train trees to do like a Bonsai planting. We have to create the space for the trees to grow to their mature size.
Alternative Planting Spaces
When no trees will properly fit in the tree lawn, verge, or planting strip, a different design and site use should be considered. Moving the sidewalk next to the curb, sometimes referred to as a monolithic sidewalk, can create more tree growing space behind the sidewalk. Other options include narrowing the sidewalk to widen the planting strip.
The use of root barriers along the outer edge of the planting space can delay infrastructure damage. Do not use circular barriers around the root ball. They usually result in planting failures as the roots never leave the barrier and the trees fail, or the roots break the barrier and the infrastructure damage occurs. Smaller planting areas with root barriers while experiencing reduced adjacent damage still reduce the longevity of the tree in a compromised growing space.
It is a challenge to list trees and minimum space in a matrix because the quality and depth of the soil varies from site to site along with compaction and construction practices that alter the soil. These factors make it harder for trees to grow in so many different locations. Also, when a matrix is provided, human nature is to try and stick a desired tree into the next smaller size planting space to see if it will work. Practice and observation have shown that the minimum planter strip dimension for a large-scale tree that can grow to 24 inch (60 cm) dbh or greater is 6 feet (1.8 m) wide. Even at this width, there is still a 40% to 50% chance that when the tree reaches maturity, there will be sidewalk or curb damage.
Alternatives to Concrete
One of the space considerations is the depth of a covering material when creating a driveway, walkway, or patio area. Not only will the soil be covered by the hard surface, many concrete installations require a base or sub-base compaction and no roots can be present in the sub-base. The typical depths for different materials and structures to be clear of roots include:
Surface Feature Base and Sub-base
Concrete curb and gutter 6” to 10” deep
Concrete driveway 4” to 10” deep
Concrete Sidewalk and patios 4” to 8” deep
Tree grate frames 6” to 10” deep
Interlocking pavers 3” to 5” deep
Plastic or rubber preformed pavers 2” deep
Stepping stones 2” to 3” deep
Asphalt 3” to 12” deep
Pervious rubber or wood products 1” to 2” deep
The more soil space available for roots to grow, the less amount of infrastructure disturbance will be caused by roots. Additionally, if the pavements are modular or removable and re-usable, the materials can be lifted up while root management is performed, and the same materials set back in place or re-used for another location. As another benefit, when the product is pervious, the modular materials provide storm water infiltration and water for the tree.
Installing Shade Trees on Private Property
At times there will be limited space for trees in the tree lawn strip, or the trees would have to be installed too close to the sidewalk or curb. In order to avoid future conflicts and damage, a consideration is to install the trees behind the sidewalk. Often there is a larger continuous soil area with less conflicting infrastructure and the trees will have more space to grow. If the trees are large scale, they will still provide the canopy over the street and sidewalk at maturity. In many cases, the trees set back behind the sidewalk will provide a canopy over the street in less time than the tree that is struggling in compromised soil next to the curb.
If the trees are installed on private property, the private property owner is going to be expected to maintain the trees. The investment of public funds for private property trees is still an investment in the community canopy infrastructure. The trees still provide the benefits to the community and the air filtering and stormwater interception does not stop at the property lines. If the choice is between no large trees because public space is not available and low cost subsidizing large trees on private property, it should be an easy decision to make (with permission of state and local laws).
The Trees Usually Win
When a developer or designer proposes installing large trees in limited spaces, three things are very likely to occur:
If the project has a requirement to install trees on public property or near other project infrastructure, it is usually to meet a requirement for a number of trees or an amount of canopy cover. The space required for these trees to grow and thrive has to be designed for the mature size of the tree, not the tree size at the time of installation.
Installing New Trees
Prior to installing trees in a limited soil space situation, there are a few design enhancements that can be used to reduce or delay the eventual root damage. Whenever possible select a tree with the mature size that is suitable for the space and soil available. If the width of the space between the curb and sidewalk is only three feet (1m) wide, then a small tree should be installed. If the space is ten feet (3 m) wide or more, then a large shade tree can be considered. However, the available soil space for roots and the quality of the soil are also very important considerations. Additionally, the size and shape of the foliar crown has to be considered adjacent to sidewalks, driveways and roads, as well as adjacent to buildings, street signs and lights.
Just because a person can plant a tree anywhere does not mean the tree is restricted to only growing in the limited space provided. Growing in permitted space is not something we can train trees to do like a Bonsai planting. We have to create the space for the trees to grow to their mature size.
Alternative Planting Spaces
When no trees will properly fit in the tree lawn, verge, or planting strip, a different design and site use should be considered. Moving the sidewalk next to the curb, sometimes referred to as a monolithic sidewalk, can create more tree growing space behind the sidewalk. Other options include narrowing the sidewalk to widen the planting strip.
The use of root barriers along the outer edge of the planting space can delay infrastructure damage. Do not use circular barriers around the root ball. They usually result in planting failures as the roots never leave the barrier and the trees fail, or the roots break the barrier and the infrastructure damage occurs. Smaller planting areas with root barriers while experiencing reduced adjacent damage still reduce the longevity of the tree in a compromised growing space.
It is a challenge to list trees and minimum space in a matrix because the quality and depth of the soil varies from site to site along with compaction and construction practices that alter the soil. These factors make it harder for trees to grow in so many different locations. Also, when a matrix is provided, human nature is to try and stick a desired tree into the next smaller size planting space to see if it will work. Practice and observation have shown that the minimum planter strip dimension for a large-scale tree that can grow to 24 inch (60 cm) dbh or greater is 6 feet (1.8 m) wide. Even at this width, there is still a 40% to 50% chance that when the tree reaches maturity, there will be sidewalk or curb damage.
Alternatives to Concrete
One of the space considerations is the depth of a covering material when creating a driveway, walkway, or patio area. Not only will the soil be covered by the hard surface, many concrete installations require a base or sub-base compaction and no roots can be present in the sub-base. The typical depths for different materials and structures to be clear of roots include:
Surface Feature Base and Sub-base
Concrete curb and gutter 6” to 10” deep
Concrete driveway 4” to 10” deep
Concrete Sidewalk and patios 4” to 8” deep
Tree grate frames 6” to 10” deep
Interlocking pavers 3” to 5” deep
Plastic or rubber preformed pavers 2” deep
Stepping stones 2” to 3” deep
Asphalt 3” to 12” deep
Pervious rubber or wood products 1” to 2” deep
The more soil space available for roots to grow, the less amount of infrastructure disturbance will be caused by roots. Additionally, if the pavements are modular or removable and re-usable, the materials can be lifted up while root management is performed, and the same materials set back in place or re-used for another location. As another benefit, when the product is pervious, the modular materials provide storm water infiltration and water for the tree.
Installing Shade Trees on Private Property
At times there will be limited space for trees in the tree lawn strip, or the trees would have to be installed too close to the sidewalk or curb. In order to avoid future conflicts and damage, a consideration is to install the trees behind the sidewalk. Often there is a larger continuous soil area with less conflicting infrastructure and the trees will have more space to grow. If the trees are large scale, they will still provide the canopy over the street and sidewalk at maturity. In many cases, the trees set back behind the sidewalk will provide a canopy over the street in less time than the tree that is struggling in compromised soil next to the curb.
If the trees are installed on private property, the private property owner is going to be expected to maintain the trees. The investment of public funds for private property trees is still an investment in the community canopy infrastructure. The trees still provide the benefits to the community and the air filtering and stormwater interception does not stop at the property lines. If the choice is between no large trees because public space is not available and low cost subsidizing large trees on private property, it should be an easy decision to make (with permission of state and local laws).
The Trees Usually Win
When a developer or designer proposes installing large trees in limited spaces, three things are very likely to occur:
- There will be competing infrastructure damage, with the tree causing damage at a high cost to the agency or future property owner.
- The trees will usually be removed before they achieve the size to provide the valuable benefits and services such as carbon sequestration and shade lined streets.
- The intended design will not be achieved. Whether the trees are replaced with smaller trees or continually removed on a pre-mature tree cycle, large sustainable tree canopy will not be achieved or experienced.
Trees and Competing Infrastructure
Trees are infrastructure Tree roots can cause severe damage to sewer and septic lines, storm water drains, water supply lines, building foundations, sidewalks, streets, parking lots, curbs, walls, and swimming pools, and each year the repair of this damage is a major cost to cities and property owners. Species and soils vary, so some arboricultural experience is necessary to manage these different situations. Researchers found some of the most aggressive roots are from trees such as: Italian stone pine (Pinus pinea), sycamore maple (Acer pseudoplatanus), Siberian elm (Ulmus pumila), Dawn redwood (Metasequoia glyptostroboides) and some species of willow (Salix spp.) and poplar (Populus spp.). Researchers also found that some species have roots growing underneath the sidewalk may behave differently than expected. For example, Gleditsia spp. develops a very high number of small diameter roots growing beneath the sidewalk, followed by Zelkova spp. and Koelreuteria spp., with Quercus spp. producing the lowest number. These trees do not produce the damage that the more aggressive trees on the list produced.
Tree Roots in Pipes
While tree roots are blamed for cracking concrete and invading sewer lines, it is equally valid to point out that these structures usually fail because they have not been properly engineered or given enough space to function in a landscape that contains growing trees and their anticipated roots. Unfortunately, the main approach in too many cities has been to remove the trees rather than to find ways to redesign the utilities to be compatible with tree roots. This could be considered a poor decision of infrastructure and investment management as the utilities can be replaced in a short period of time, days, while the trees cannot be replaced in kind for years. For more information on roots in pipes, see Topic #2.
Tree Roots in Sewers
Tree roots are opportunistic and will find air and moisture in the soil, wherever it is. Just as tree roots find air and moisture under a sidewalk pavement, eventually causing it to rise up as a root grows in diameter, tree roots will find the air and moisture that is found along a pipe's outer surface. Even if the pipe has no leakage, roots will be there because the pipe temperature is most likely different from the soil temperature and water vapor in the soil will condense along the pipe surface. Plus, sewage and drainage pipes may not have tight seals at the joints and may leak over time. Air and nutrient rich water can seep from the pipes, creating an ideal growing medium for tree roots. The small root tips will also enter through any pipe leakage points; and the developing root mass can fill the pipe; and as the roots enlarge, they may exert enough pressure to spread the crack, increase the leakage, and worsen the problem. The roots in the pipe, whether it is a storm sewer or septic (sanitary) sewer, can survive because the pipes are always full of water. The water flow is cyclical with the use of either sewer or storm systems.
Whenever possible, those responsible for sewer systems can reduce root intrusion by locating pipes beyond the dripline of existing mature trees, reducing the number of pipe joints, and by ensuring that the pipes are properly installed and joints are properly sealed. Arborists and landscape architects can minimize or reduce root intrusion by installing slower growing species with less-aggressive root systems near sewers. Where trees are installed near existing sewer pipes, closed circuit TV cameras should be used to inspect the pipes in order to determine their internal condition. Once tree root intrusion into sewer pipes has been identified, it may be necessary to repair or replace the pipes. Both the sewerage engineer and the arborist or landscape architect should examine the value of the trees in relation to the pipe damage. Replacement of vegetation is often considerably less expensive than renovating or replacing pipes. However, there are several new trenchless technology techniques mentioned below, which are much less expensive than digging up and replacing the pipe.
Trees should be located far enough from sewer lines so the lines can be dug up and repaired without requiring the removal of the trees. This distance is usually an 8 to 10 feet (2.5 – 3.5m) minimum from the trunk and flares, increasing for trees with larger mature sizes or 10 feet (3.5 m) from the new tree.
Catch Basins and Storm Drain Lines
A permeable basin is designed to allow infiltration of water into the soil around the basin. Tree roots aid in keeping the soil permeable, provided they tolerate wet conditions. If the basin floods for long periods, use trees that tolerate flooding such as bald cypress, alder, red maple, sycamore, and other lowland species.
The storm drain lines are similar to sewer lines in regard to the temperature and condensation opportunities for root growth, with the absence of nutrient rich effluent. The temperature and moisture around the lines can be beneficial to root growth. The trenches created for the installation may provide an easier path for roots to follow if the trenches are not well-compacted or filled with high density fill.
Trees should be located far enough from storm drain lines and catch basins so the lines can be dug up and repaired without requiring the removal of the trees and damage to the other underground utilities. This distance is the same as it is for sewer lines.
Water, Communication, and Gas Lines
Tree roots do not normally affect water, communication, or gas supply lines since these are usually sealed and do not leak. Because the lines are pressurized or solid wires, the design of the pipe delivery and number of bends, level flow, and turns in the lines do not affect the quality of the delivery. Therefore, the design of these utility service wires and lines is easier to move and locate around existing and newly installed trees.
The pipe trench itself may provide better growing conditions than the surrounding soil, allowing the roots to proliferate in this area. Also the same temperature and moisture gradients as found in sewer and storm drain lines may occur around these utility service lines. Over time, roots may grow large enough to exert pressure that bends pipes, causes stress cracks, or breaks nearby joints. However, it is usually a circumstance of pipe deterioration that has been the chief cause of pipe failures, compared to the roots causing pipe damage.
Pipe Repair Solutions
Aggressive roots can penetrate pipe systems when the pipes are in poor condition. Root pruning alone will not keep roots from growing in a cracked pipe. The solution is to replace the pipe and seal the joints. The use of plastic pipes, longer pipe sections with fewer joints, and better joint sealing will help reduce this conflict.
To install or replace pipes, several methods are used to restore the flow or service delivery and preserve tree roots:
Trenchless technology or directional boring is a method of installing underground pipes and conduits along a prescribed path from the surface, with minimal impact to the surrounding area. Directional boring causes less disturbance to the environment, fewer site use inconveniences, and is suitable in a variety of soil conditions. The installation of gas, electric, water, and telecommunication lines is accomplished with directional boring machines. The optimum placement is a distance from the tree so the repair can be made without removing the tree.
Directional boring is used for the following reasons:
It is important to design tree pits sufficiently large in area and depth to allow for healthy root growth and the physical stability of the tree. The surface area of the rooting zone for an individual tree should be approximately equal to the projected foliar crown spread. Smaller pits can be tolerated if the trees can penetrate into the soil underneath adjacent paved footpaths and streets. However, this has become increasingly difficult due to the required compaction under walkways and around underground utilities. CU-Structural Soil and soil cells which can provide more pore space and still achieve construction compaction needs might be an option to consider in these situations.
Efforts should also be made so the tree pit can be covered by a modular and permeable surface that allows infiltration of rainwater and air into the soil containing roots. Where the risk of soil compaction is high, impervious pavements or areas of coarse gravel might be good solutions. The current practice in most urban sidewalks rarely meets this consideration.
The size of the planting pits in Europe varies between 2.6 cubic yards (2 cubic meters) and 13 cy (10 cm) per tree. In the US, tree pits as small as 0.65 c.y. (0.5 c.m.) have been reported as a standard size in too many places. In most urban soils, if the tree pit size can be 15 feet (5 m) wide, there is a 95% chance the adjacent improvements will not be damaged by the tree roots, and if it is damaged, the distance is far enough away the repairs can be completed without having to remove the trees. If the tree pit size is 4 feet wide there is a 95% chance the adjacent infrastructure will be damaged by the tree roots or trunk flare. This substandard distance usually means the tree will have to be removed during a subsequent repair. All tree pit sizes starting from the 4 feet (1.3 m) width up to 15 feet (5 m) width will have varying probabilities of adjacent improvement damage, depending on soils, tree size, and tree age. The farther from the tree the adjacent improvements are placed, targeting a minimum of 4 feet (1.3 M) distance from the tree trunk at the time of maturity, the adjacent improvement damage will be lessened. For example a tree achieving 24” dbh will likely have a 36” diameter base, so the minimum space requirement is 8’ + 3’ or 11’. The soil in the tree pit should not be compacted, and this area can be used for other purposes than open soil with reasonable permeable covering. When covering the soil around a tree, it should be a material that the opening can be easily enlarged as the tree grows.
Urban Soil Quality
Another thing to consider is that urban soils often have low organic matter content, low and unbalanced nutrient content, and/or low nutrient availability with a high soil pH. Impermeable soil layers can also lead to waterlogging. Urban soils should have good water-holding capacity but drain freely and be well-aerated. Trees need to be supplied with a balanced mix of nutrients and microorganisms. Tree pits may be backfilled with a specially designed soil mix, although this practice is not usually recommended unless the entire future root zone can be amended.
Soil Remediation
Dr. Susan Day has introduced a method of soil improvement that digs up and loosens the soil in an urban tree pit and adds amendments and reduces compaction prior to planting a tree in a once compacted soil. This can be performed with a backhoe lifting the soil and dropping it back into the hole, or an air or high pressure water excavation tool on existing trees, but is best used for new tree sites.
An important consideration is the damage that roots can cause to sidewalks and competing infrastructure. Research suggests that the condition of a sidewalk may determine the likelihood of finding roots under the walk within 20 years after planting. This study shows that cracked sidewalks are more likely to favor oxygen dissemination into the soil and that results in increased root growth. This is plausible given that water and air directly beneath sidewalks possess better growing conditions than surrounding soils. The conflicts are so pervasive that in the US, millions of dollars from city budgets are spent annually on sidewalk repairs. This is not a sustainable practice, and it siphons funds that could be used for tree maintenance.
Trees are infrastructure Tree roots can cause severe damage to sewer and septic lines, storm water drains, water supply lines, building foundations, sidewalks, streets, parking lots, curbs, walls, and swimming pools, and each year the repair of this damage is a major cost to cities and property owners. Species and soils vary, so some arboricultural experience is necessary to manage these different situations. Researchers found some of the most aggressive roots are from trees such as: Italian stone pine (Pinus pinea), sycamore maple (Acer pseudoplatanus), Siberian elm (Ulmus pumila), Dawn redwood (Metasequoia glyptostroboides) and some species of willow (Salix spp.) and poplar (Populus spp.). Researchers also found that some species have roots growing underneath the sidewalk may behave differently than expected. For example, Gleditsia spp. develops a very high number of small diameter roots growing beneath the sidewalk, followed by Zelkova spp. and Koelreuteria spp., with Quercus spp. producing the lowest number. These trees do not produce the damage that the more aggressive trees on the list produced.
Tree Roots in Pipes
While tree roots are blamed for cracking concrete and invading sewer lines, it is equally valid to point out that these structures usually fail because they have not been properly engineered or given enough space to function in a landscape that contains growing trees and their anticipated roots. Unfortunately, the main approach in too many cities has been to remove the trees rather than to find ways to redesign the utilities to be compatible with tree roots. This could be considered a poor decision of infrastructure and investment management as the utilities can be replaced in a short period of time, days, while the trees cannot be replaced in kind for years. For more information on roots in pipes, see Topic #2.
Tree Roots in Sewers
Tree roots are opportunistic and will find air and moisture in the soil, wherever it is. Just as tree roots find air and moisture under a sidewalk pavement, eventually causing it to rise up as a root grows in diameter, tree roots will find the air and moisture that is found along a pipe's outer surface. Even if the pipe has no leakage, roots will be there because the pipe temperature is most likely different from the soil temperature and water vapor in the soil will condense along the pipe surface. Plus, sewage and drainage pipes may not have tight seals at the joints and may leak over time. Air and nutrient rich water can seep from the pipes, creating an ideal growing medium for tree roots. The small root tips will also enter through any pipe leakage points; and the developing root mass can fill the pipe; and as the roots enlarge, they may exert enough pressure to spread the crack, increase the leakage, and worsen the problem. The roots in the pipe, whether it is a storm sewer or septic (sanitary) sewer, can survive because the pipes are always full of water. The water flow is cyclical with the use of either sewer or storm systems.
Whenever possible, those responsible for sewer systems can reduce root intrusion by locating pipes beyond the dripline of existing mature trees, reducing the number of pipe joints, and by ensuring that the pipes are properly installed and joints are properly sealed. Arborists and landscape architects can minimize or reduce root intrusion by installing slower growing species with less-aggressive root systems near sewers. Where trees are installed near existing sewer pipes, closed circuit TV cameras should be used to inspect the pipes in order to determine their internal condition. Once tree root intrusion into sewer pipes has been identified, it may be necessary to repair or replace the pipes. Both the sewerage engineer and the arborist or landscape architect should examine the value of the trees in relation to the pipe damage. Replacement of vegetation is often considerably less expensive than renovating or replacing pipes. However, there are several new trenchless technology techniques mentioned below, which are much less expensive than digging up and replacing the pipe.
Trees should be located far enough from sewer lines so the lines can be dug up and repaired without requiring the removal of the trees. This distance is usually an 8 to 10 feet (2.5 – 3.5m) minimum from the trunk and flares, increasing for trees with larger mature sizes or 10 feet (3.5 m) from the new tree.
Catch Basins and Storm Drain Lines
A permeable basin is designed to allow infiltration of water into the soil around the basin. Tree roots aid in keeping the soil permeable, provided they tolerate wet conditions. If the basin floods for long periods, use trees that tolerate flooding such as bald cypress, alder, red maple, sycamore, and other lowland species.
The storm drain lines are similar to sewer lines in regard to the temperature and condensation opportunities for root growth, with the absence of nutrient rich effluent. The temperature and moisture around the lines can be beneficial to root growth. The trenches created for the installation may provide an easier path for roots to follow if the trenches are not well-compacted or filled with high density fill.
Trees should be located far enough from storm drain lines and catch basins so the lines can be dug up and repaired without requiring the removal of the trees and damage to the other underground utilities. This distance is the same as it is for sewer lines.
Water, Communication, and Gas Lines
Tree roots do not normally affect water, communication, or gas supply lines since these are usually sealed and do not leak. Because the lines are pressurized or solid wires, the design of the pipe delivery and number of bends, level flow, and turns in the lines do not affect the quality of the delivery. Therefore, the design of these utility service wires and lines is easier to move and locate around existing and newly installed trees.
The pipe trench itself may provide better growing conditions than the surrounding soil, allowing the roots to proliferate in this area. Also the same temperature and moisture gradients as found in sewer and storm drain lines may occur around these utility service lines. Over time, roots may grow large enough to exert pressure that bends pipes, causes stress cracks, or breaks nearby joints. However, it is usually a circumstance of pipe deterioration that has been the chief cause of pipe failures, compared to the roots causing pipe damage.
Pipe Repair Solutions
Aggressive roots can penetrate pipe systems when the pipes are in poor condition. Root pruning alone will not keep roots from growing in a cracked pipe. The solution is to replace the pipe and seal the joints. The use of plastic pipes, longer pipe sections with fewer joints, and better joint sealing will help reduce this conflict.
To install or replace pipes, several methods are used to restore the flow or service delivery and preserve tree roots:
- Tunneling under roots - trenchless technology like directional boring, moles, and bore & jack methods can be used for construction of a new line or to replace an existing line.
- Hand digging with small tools - is suitable for pipe replacement in short distances, and working around roots during the repair or installation will leave large roots intact.
- Re-lining old pipelines by PipeburstingTM or other in-line expansion techniques such as pipe-pulling, slip-lining, SwageliningTM, or cured-in-place pipe – can be used to replace old, broken pipes without having to excavate and prune roots. Roots growing in the old pipe will be severed and removed during the re-lining operation.
Trenchless technology or directional boring is a method of installing underground pipes and conduits along a prescribed path from the surface, with minimal impact to the surrounding area. Directional boring causes less disturbance to the environment, fewer site use inconveniences, and is suitable in a variety of soil conditions. The installation of gas, electric, water, and telecommunication lines is accomplished with directional boring machines. The optimum placement is a distance from the tree so the repair can be made without removing the tree.
Directional boring is used for the following reasons:
- less traffic disruption when crossing the streets underground,
- lower long-term cost,
- deeper installation,
- longer installation length,
- no access pit required except at the beginning and end and service junctions,
- shorter completion times,
- directional capabilities,
- safer for the environment,
- no tree root damage when going under or near existing trees,
- no trenching is necessary.
It is important to design tree pits sufficiently large in area and depth to allow for healthy root growth and the physical stability of the tree. The surface area of the rooting zone for an individual tree should be approximately equal to the projected foliar crown spread. Smaller pits can be tolerated if the trees can penetrate into the soil underneath adjacent paved footpaths and streets. However, this has become increasingly difficult due to the required compaction under walkways and around underground utilities. CU-Structural Soil and soil cells which can provide more pore space and still achieve construction compaction needs might be an option to consider in these situations.
Efforts should also be made so the tree pit can be covered by a modular and permeable surface that allows infiltration of rainwater and air into the soil containing roots. Where the risk of soil compaction is high, impervious pavements or areas of coarse gravel might be good solutions. The current practice in most urban sidewalks rarely meets this consideration.
The size of the planting pits in Europe varies between 2.6 cubic yards (2 cubic meters) and 13 cy (10 cm) per tree. In the US, tree pits as small as 0.65 c.y. (0.5 c.m.) have been reported as a standard size in too many places. In most urban soils, if the tree pit size can be 15 feet (5 m) wide, there is a 95% chance the adjacent improvements will not be damaged by the tree roots, and if it is damaged, the distance is far enough away the repairs can be completed without having to remove the trees. If the tree pit size is 4 feet wide there is a 95% chance the adjacent infrastructure will be damaged by the tree roots or trunk flare. This substandard distance usually means the tree will have to be removed during a subsequent repair. All tree pit sizes starting from the 4 feet (1.3 m) width up to 15 feet (5 m) width will have varying probabilities of adjacent improvement damage, depending on soils, tree size, and tree age. The farther from the tree the adjacent improvements are placed, targeting a minimum of 4 feet (1.3 M) distance from the tree trunk at the time of maturity, the adjacent improvement damage will be lessened. For example a tree achieving 24” dbh will likely have a 36” diameter base, so the minimum space requirement is 8’ + 3’ or 11’. The soil in the tree pit should not be compacted, and this area can be used for other purposes than open soil with reasonable permeable covering. When covering the soil around a tree, it should be a material that the opening can be easily enlarged as the tree grows.
Urban Soil Quality
Another thing to consider is that urban soils often have low organic matter content, low and unbalanced nutrient content, and/or low nutrient availability with a high soil pH. Impermeable soil layers can also lead to waterlogging. Urban soils should have good water-holding capacity but drain freely and be well-aerated. Trees need to be supplied with a balanced mix of nutrients and microorganisms. Tree pits may be backfilled with a specially designed soil mix, although this practice is not usually recommended unless the entire future root zone can be amended.
Soil Remediation
Dr. Susan Day has introduced a method of soil improvement that digs up and loosens the soil in an urban tree pit and adds amendments and reduces compaction prior to planting a tree in a once compacted soil. This can be performed with a backhoe lifting the soil and dropping it back into the hole, or an air or high pressure water excavation tool on existing trees, but is best used for new tree sites.
An important consideration is the damage that roots can cause to sidewalks and competing infrastructure. Research suggests that the condition of a sidewalk may determine the likelihood of finding roots under the walk within 20 years after planting. This study shows that cracked sidewalks are more likely to favor oxygen dissemination into the soil and that results in increased root growth. This is plausible given that water and air directly beneath sidewalks possess better growing conditions than surrounding soils. The conflicts are so pervasive that in the US, millions of dollars from city budgets are spent annually on sidewalk repairs. This is not a sustainable practice, and it siphons funds that could be used for tree maintenance.
Root Barrier Use
A possible solution to control root growth is the use of root barriers. Although this is not universally accepted, some researchers have suggested that root barriers direct roots downward and too far under the sidewalks to cause damage. We do know that roots cannot grow through solid barriers. They may grow over, under, or around, and may grow through chemical barriers in certain soil conditions. If the root barriers create a space too constricted for root growth, trees may become unstable due to limited root development and inadequate anchorage. One fact often overlooked, is that trees need a suitable soil in which to grow, with or without the barriers.
Root barriers are a physical or chemical impediment intended to block root growth away from competing infrastructure. There are three main classes of root barriers: 1. traps, 2. inhibitors, and 3. deflectors.
2. Inhibitors are used to control root growth by means of chemical intervention. They are composed of a slowly released herbicide (i.e. trifluralin) on a fabric placed in the soil surrounding the root ball. The new growing root tips are inhibited by the chemical and the roots cannot grow past the chemical barrier. The chemical is claimed to have no detrimental environmental impact beyond the root control area.
3. Deflectors are physical impediments to root growth and are often constructed from plastics.They function by blocking root growth from infrastructure by forcing roots to grow along the barrier or downward. Deeper roots growing under the barrier can return to the surface. These roots will have to travel a longer distance through the soil delaying the displacement of the adjacent infrastructure They can be used adjacent to roads, sidewalks, or buildings.
Root barriers are designed to deflect young root growth. Roots cannot grow through a solid panel. If the barriers are in place before the tree roots grow, the barriers will deflect roots sideways or downward. The earlier in the trees’ life that root barriers and root management is initiated, the more effective the process and the lesser amount of root damage will occur to the tree as a result of future root pruning. The fewer roots pruned, and the lesser disturbance to the soil area where roots are growing, the better off the tree will be.
Root barriers are not a maintenance free tool/root management technique. Trees need to grow roots to be healthy. The barrier must be monitored to remove roots that might be growing over the top. If mulch is placed over soil and the root barrier does not extend to the top of the mulch, roots can grow over the barrier. A typical time frame would be every two years to prune roots growing over the top of the barrier. The barrier design should also reduce roots growing around the barrier.
The use of root barriers after roots have already caused damage will only be effective to control the root re-growth after root pruning. Once a root has grown into an area, a root barrier cannot re-direct those roots. If a root is pruned, it will not automatically re-grow downward. In fact most of the time, without a root barrier, the root will regrow into the same location where the root was removed. With a root barrier, the root growth will be directed along the barrier or downward.
The use of root barriers is not an effective strategy for a design that plants large trees in small spaces and has limited root or soil space. Limiting root growth to a restricted area with minimal soil is like installing the tree in a container. At some point, the foliar crown will enlarge to a point where the limited root system cannot support the tree and the tree will fail at the roots. The better solution is to grow smaller sized trees in the limited soil area.
Root barriers are often installed in narrow trenches along the edge of a desired protection zone such as a sidewalk, driveway, or curb. Other barriers have been installed beside the rootball of a newly installed tree, with restricted or limited spaces. The root barrier has to be placed as far from the tree as possible. The barrier should outline the perimeter of the growing space and follow the shape to allow the tree to grow as natural a root system as possible before hitting the barrier. Trees have been observed to grow over circular barriers placed directly outside of the rootball, and break the barrier with the flare expansion. The roots continue to cause damage to the infrastructure the barriers were intended to prevent. Circular installations around the root ball are to be avoided.
A possible solution to control root growth is the use of root barriers. Although this is not universally accepted, some researchers have suggested that root barriers direct roots downward and too far under the sidewalks to cause damage. We do know that roots cannot grow through solid barriers. They may grow over, under, or around, and may grow through chemical barriers in certain soil conditions. If the root barriers create a space too constricted for root growth, trees may become unstable due to limited root development and inadequate anchorage. One fact often overlooked, is that trees need a suitable soil in which to grow, with or without the barriers.
Root barriers are a physical or chemical impediment intended to block root growth away from competing infrastructure. There are three main classes of root barriers: 1. traps, 2. inhibitors, and 3. deflectors.
- Traps do not entirely inhibit root growth; they allow root tips to penetrate small holes, but subsequently preclude radial growth increase because the root becomes girdled as it continues to grow in diameter and is constricted by the size of the hole opening. They are made of woven nylon or copper screen, which are permeable and allow for water movement and gas exchange, but at the same time they severely restrict large root development in one or more directions and may predispose trees to instability until the roots grow deeper into the soil or under the barrier.
2. Inhibitors are used to control root growth by means of chemical intervention. They are composed of a slowly released herbicide (i.e. trifluralin) on a fabric placed in the soil surrounding the root ball. The new growing root tips are inhibited by the chemical and the roots cannot grow past the chemical barrier. The chemical is claimed to have no detrimental environmental impact beyond the root control area.
3. Deflectors are physical impediments to root growth and are often constructed from plastics.They function by blocking root growth from infrastructure by forcing roots to grow along the barrier or downward. Deeper roots growing under the barrier can return to the surface. These roots will have to travel a longer distance through the soil delaying the displacement of the adjacent infrastructure They can be used adjacent to roads, sidewalks, or buildings.
Root barriers are designed to deflect young root growth. Roots cannot grow through a solid panel. If the barriers are in place before the tree roots grow, the barriers will deflect roots sideways or downward. The earlier in the trees’ life that root barriers and root management is initiated, the more effective the process and the lesser amount of root damage will occur to the tree as a result of future root pruning. The fewer roots pruned, and the lesser disturbance to the soil area where roots are growing, the better off the tree will be.
Root barriers are not a maintenance free tool/root management technique. Trees need to grow roots to be healthy. The barrier must be monitored to remove roots that might be growing over the top. If mulch is placed over soil and the root barrier does not extend to the top of the mulch, roots can grow over the barrier. A typical time frame would be every two years to prune roots growing over the top of the barrier. The barrier design should also reduce roots growing around the barrier.
The use of root barriers after roots have already caused damage will only be effective to control the root re-growth after root pruning. Once a root has grown into an area, a root barrier cannot re-direct those roots. If a root is pruned, it will not automatically re-grow downward. In fact most of the time, without a root barrier, the root will regrow into the same location where the root was removed. With a root barrier, the root growth will be directed along the barrier or downward.
The use of root barriers is not an effective strategy for a design that plants large trees in small spaces and has limited root or soil space. Limiting root growth to a restricted area with minimal soil is like installing the tree in a container. At some point, the foliar crown will enlarge to a point where the limited root system cannot support the tree and the tree will fail at the roots. The better solution is to grow smaller sized trees in the limited soil area.
Root barriers are often installed in narrow trenches along the edge of a desired protection zone such as a sidewalk, driveway, or curb. Other barriers have been installed beside the rootball of a newly installed tree, with restricted or limited spaces. The root barrier has to be placed as far from the tree as possible. The barrier should outline the perimeter of the growing space and follow the shape to allow the tree to grow as natural a root system as possible before hitting the barrier. Trees have been observed to grow over circular barriers placed directly outside of the rootball, and break the barrier with the flare expansion. The roots continue to cause damage to the infrastructure the barriers were intended to prevent. Circular installations around the root ball are to be avoided.
Infrastructure Damage
Trees can produce direct or indirect damage to competing infrastructure. Damage is considered direct if it is related to the forces exerted by expanding roots or the flare and typically affects only light infrastructure such as roads, curbs, and sidewalks. Indirect damage is associated with soil movement resulting from changes in moisture content precipitated by roots sometimes referred to as subsistence. The biggest factor in sidewalk related competing infrastructure damage is space. If space is limited, the damage will occur faster.
A recent study compared different methods to reduce sidewalk damage from tree roots. It reported that minimal sidewalk lifting was associated with the Deep Root™ barrier, gravel and foam treatments and that these treatments did not hinder tree growth. Similar results were obtained in a recent study by Terrecon, the manufacturer of Terrewalks™. The root barriers diverted roots from contacting the sidewalks, which is the same benefit as increasing the space between the tree and sidewalk. Terrewalks™ panels are thinner than concrete and provide more space for root growth before contact with the sidewalks would occur. They are also modular and can be lifted up and reused.
Root barriers are more effective as a preventative treatment rather than a therapeutic treatment after root pruning. As a preventative, with adequate space, they limit the opportunities for roots to grow outside of the intended area and protect the adjacent infrastructure from root interference. As a therapeutic treatment, the trees first have to receive root pruning to remove conflicting roots, and the root barrier deflects the root re-growth from the cut ends away from the area they were just removed from. The act of root pruning can cause damage to tree health and stability. If the tree was installed with preventatives to avoid root pruning, the trees will be healthier, and the damage will be reduced.
Research shows that barriers are effective in well-drained, uncompacted soils, which are virtually nonexistent in roadside urban areas. The researchers determined however, that they are usually being used to inhibit root growth or direct roots into deeper, poorer soils typical in urban areas. If linear planting areas have perimeter root barrier confinement, and there is adequate root space, successful tree growth will be achieved. Challenges come into play where there is insufficient root space and efforts are applied to limit root growth in this limited space. The soil volume constriction is the main issue. If the barriers were not in place, competing infrastructure damage would occur, and root pruning or tree removal have been part of the solution to these conflicts.
Trees can produce direct or indirect damage to competing infrastructure. Damage is considered direct if it is related to the forces exerted by expanding roots or the flare and typically affects only light infrastructure such as roads, curbs, and sidewalks. Indirect damage is associated with soil movement resulting from changes in moisture content precipitated by roots sometimes referred to as subsistence. The biggest factor in sidewalk related competing infrastructure damage is space. If space is limited, the damage will occur faster.
A recent study compared different methods to reduce sidewalk damage from tree roots. It reported that minimal sidewalk lifting was associated with the Deep Root™ barrier, gravel and foam treatments and that these treatments did not hinder tree growth. Similar results were obtained in a recent study by Terrecon, the manufacturer of Terrewalks™. The root barriers diverted roots from contacting the sidewalks, which is the same benefit as increasing the space between the tree and sidewalk. Terrewalks™ panels are thinner than concrete and provide more space for root growth before contact with the sidewalks would occur. They are also modular and can be lifted up and reused.
Root barriers are more effective as a preventative treatment rather than a therapeutic treatment after root pruning. As a preventative, with adequate space, they limit the opportunities for roots to grow outside of the intended area and protect the adjacent infrastructure from root interference. As a therapeutic treatment, the trees first have to receive root pruning to remove conflicting roots, and the root barrier deflects the root re-growth from the cut ends away from the area they were just removed from. The act of root pruning can cause damage to tree health and stability. If the tree was installed with preventatives to avoid root pruning, the trees will be healthier, and the damage will be reduced.
Research shows that barriers are effective in well-drained, uncompacted soils, which are virtually nonexistent in roadside urban areas. The researchers determined however, that they are usually being used to inhibit root growth or direct roots into deeper, poorer soils typical in urban areas. If linear planting areas have perimeter root barrier confinement, and there is adequate root space, successful tree growth will be achieved. Challenges come into play where there is insufficient root space and efforts are applied to limit root growth in this limited space. The soil volume constriction is the main issue. If the barriers were not in place, competing infrastructure damage would occur, and root pruning or tree removal have been part of the solution to these conflicts.
Critical Root Zones for Trees
Field investigation and research in the last two decades have destroyed the myth that the root system extends only to the drip line of a tree's canopy. Another false myth is that all mature trees have a taproot that is the most vulnerable portion of the root system. Another perspective that dry climate trees have similar root systems to trees growing in climates that receive rainfall during the growing season has to be re-considered. The trees growing in areas that do not receive rainfall during the growing season have to have root systems that find water below the dry soil in the top 18 inches to two feet (50 – 60 cm) of soil profile. A tree's root system varies in width, depth, and structural characteristics with the species of tree, as well as with the soils and moisture levels. The only way to know where tree roots are growing is to find them through careful excavation, the use of a pneumatic soil-excavating tool or root radar.
Critical Root Zone for Trees
A Critical Root Zone (CRZ) is the area under the tree crown where the most important roots for tree stability and survival are growing. These are the vital roots that collect nutrients and moisture for the tree; they provide stability; and they must be protected for the tree to survive. It is very important to determine the minimum area around the tree trunk that must be left undisturbed in order to preserve a sufficient root mass to give a tree a reasonable chance of survival.
The Health of the CRZ can be Damaged by:
CRZ Size
Some reports indicate that the CRZ is the same as the tree's drip line. Others indicate that the CRZ will typically be represented by a concentric circle centering on the tree's trunk with a radius equal in feet to one and one-half times the number of inches of the trunk diameter. For example: The CRZ radius of a 20-inch (50 cm) diameter tree is 30 feet (10 m).
The other important factor in conserving trees during construction is the design of the structure. Flat slabs compacting and covering the CRZ will cause more damage to the soil and the tree than bridged construction where a few piers are inserted into the ground and the structure rests on the piers instead of the soil. The more soil that can be left uncovered, permeable, and un-compacted, the more groundwater infiltration, roots, and healthier soils will be retained.
CRZ Formula
In an attempt to calculate the location of a tree’s critical root zone using information such as the tree’s age, the trees condition, and the species tolerance for root disturbance, the following CRZ formula was developed:
(dbh of tree) x (Distance from trunk – condition rating from Table) = radius (in feet) of CRZ
Critical Root Zone Determination for Trees with a Condition Rating of 2 – 5 (5 being best)
_______________________________________________________________________________
Species tolerance Tree Age Distance from Trunk / Condition Rating
4 or 5 2 or 3
Good Young (<20% of life expectancy) 0.5’ 0.62’
Mature (20 – 80% of life expectancy 0.75’ 0.94’
Over mature (>80% of life expectancy) 1.0’ 1.25’
Moderate Young 0.75’ 0.94’
Mature 1.0’ 1.25’
Over mature 1.25’ 1.56’
Poor Young 1.0’ 1.25’
Mature 1.25’ 1.56’
Over mature 1.5’ 1.88’
Note: For trees with a poor condition rating of 1, a determination should be made, with assistance of a qualified Arborist, as to whether or not the tree can be saved or is worth investing resources into the effort.
Examples:
2. A 15-year-old, 10” dbh yellow poplar (Liriodendron tulipifera), with a Condition Rating of 2 (poor tolerance, young age): 10 x 1.25 = 12.5’ radius CRZ
Field investigation and research in the last two decades have destroyed the myth that the root system extends only to the drip line of a tree's canopy. Another false myth is that all mature trees have a taproot that is the most vulnerable portion of the root system. Another perspective that dry climate trees have similar root systems to trees growing in climates that receive rainfall during the growing season has to be re-considered. The trees growing in areas that do not receive rainfall during the growing season have to have root systems that find water below the dry soil in the top 18 inches to two feet (50 – 60 cm) of soil profile. A tree's root system varies in width, depth, and structural characteristics with the species of tree, as well as with the soils and moisture levels. The only way to know where tree roots are growing is to find them through careful excavation, the use of a pneumatic soil-excavating tool or root radar.
Critical Root Zone for Trees
A Critical Root Zone (CRZ) is the area under the tree crown where the most important roots for tree stability and survival are growing. These are the vital roots that collect nutrients and moisture for the tree; they provide stability; and they must be protected for the tree to survive. It is very important to determine the minimum area around the tree trunk that must be left undisturbed in order to preserve a sufficient root mass to give a tree a reasonable chance of survival.
The Health of the CRZ can be Damaged by:
- cutting roots,
- excavating soil,
- applying chemicals,
- compacting the soil,
- applying any fill material or vertical barriers in the soil that impedes the flow of water or air to the roots.
CRZ Size
Some reports indicate that the CRZ is the same as the tree's drip line. Others indicate that the CRZ will typically be represented by a concentric circle centering on the tree's trunk with a radius equal in feet to one and one-half times the number of inches of the trunk diameter. For example: The CRZ radius of a 20-inch (50 cm) diameter tree is 30 feet (10 m).
The other important factor in conserving trees during construction is the design of the structure. Flat slabs compacting and covering the CRZ will cause more damage to the soil and the tree than bridged construction where a few piers are inserted into the ground and the structure rests on the piers instead of the soil. The more soil that can be left uncovered, permeable, and un-compacted, the more groundwater infiltration, roots, and healthier soils will be retained.
CRZ Formula
In an attempt to calculate the location of a tree’s critical root zone using information such as the tree’s age, the trees condition, and the species tolerance for root disturbance, the following CRZ formula was developed:
(dbh of tree) x (Distance from trunk – condition rating from Table) = radius (in feet) of CRZ
Critical Root Zone Determination for Trees with a Condition Rating of 2 – 5 (5 being best)
_______________________________________________________________________________
Species tolerance Tree Age Distance from Trunk / Condition Rating
4 or 5 2 or 3
Good Young (<20% of life expectancy) 0.5’ 0.62’
Mature (20 – 80% of life expectancy 0.75’ 0.94’
Over mature (>80% of life expectancy) 1.0’ 1.25’
Moderate Young 0.75’ 0.94’
Mature 1.0’ 1.25’
Over mature 1.25’ 1.56’
Poor Young 1.0’ 1.25’
Mature 1.25’ 1.56’
Over mature 1.5’ 1.88’
Note: For trees with a poor condition rating of 1, a determination should be made, with assistance of a qualified Arborist, as to whether or not the tree can be saved or is worth investing resources into the effort.
Examples:
- A 60-year-old, 32” dbh live oak (Quercus virginiana), with a Condition Rating of 4 (good tolerance, mature age): 32 x 0.75 = 24’ radius CRZ
2. A 15-year-old, 10” dbh yellow poplar (Liriodendron tulipifera), with a Condition Rating of 2 (poor tolerance, young age): 10 x 1.25 = 12.5’ radius CRZ
Preferred CRZ Determination
Relying on formulas or guesswork alone will not suffice when engineering a parking lot to the nearest inch in elevation or the nearest fraction of a foot horizontally for a sewage line. The latest research indicates that in order to successfully determine the extent of the CRZ and to build in close proximity to significant or specimen trees, it is important to have an accurate depiction of a tree's underground structure. Because nature is variable, textbook answers and ballpark formulas will never be sufficient to make decisions on tree conservation. Site investigation, combined with field experience, the use of a pneumatic soil-excavating tool and/or root radar, is the best way to determine the CRZ location. Investigative digging is a common sense approach employed by experienced urban forestry professionals. Sample trees within representative soils and moisture levels are traced to determine representative root widths, depths and responses to particular site conditions.
It’s important to know all aspects about site conditions, the available space, the construction practices, and how to select a tree species that will adapt to the site environment. These need to be based on comprehensive inventories of the urban tree resources and should assess the aesthetic, social, environmental and economic functions of the urban forest. The intent is to match and grow the right tree in the right place for the right purpose.
Tree Protection Specification
The following specification or guideline is an example that will ensure that trees growing on a site, prior to construction, will have the best opportunity to thrive and continue growing into high value property assets. This tree protection specification is intended to guide a construction project to insure that appropriate practices will be implemented in the field to eliminate consequences of tree damage and death that may result from uninformed or careless acts, and conserve trees to be valuable property assets. Site specific variations may be necessary to conserve trees based on the site use, the space available, species, climate, construction design, and the existing conditions of the trees. This specification is duplicated on this website's Specification page.
1. General Tree Protection Instructions
The objective of these tree protection instructions is to minimize the negative impacts of construction activities on trees near the property under construction. With proper planning, many negative impacts to trees can be avoided. Whenever possible, the placement of underground utilities and service connections to new buildings should be designed so that the proposed service lines are installed as far away from existing and new trees as possible. These instructions list minimum distances that the edge of excavation needs to be in order to ensure that there is minimal impact on all the site’s existing trees, and the soil space for new trees.
The municipality may have conditions, restrictions, and preservation requirements that limit the number of trees that can be removed on public and private property. The first step in design is for the applicant to learn the municipality's rules and work to align the project with these requirements. A Tree Removal Permit shall be issued after the municipality finds that all reasonable efforts have been undertaken in the architectural layout and design of the proposed development to preserve the required existing trees. The Tree Removal Permit may require replacement trees and/or compensation for the lost value of trees on or adjacent to the site under construction.
A) Building placement and driveway, walkways, and parking areas shall be designed in such a way as to avoid unnecessary removal of existing trees found worthy of conservation in health and structure. Preferably all access pavements and utilities shall be located in the same general vicinity to avoid excessive tree removals.
B) The proposed placement of all utility service lines, including irrigation and lighting, shall be shown on the site and landscaping plans. Every effort shall be made to protect the existing trees during the placement of utility service lines including the use of trenchless technology, auguring, and/or jacking as opposed to open trench cutting. The landscape and irrigation plans shall demonstrate that every effort was made to avoid damage to existing trees during installation of irrigation, lighting, and plants.
C) If the agency requirements are for canopy cover, or size and number of trees to be conserved on a site, the plans shall demonstrate how the project meets those requirements.
2. Tree Protection Required
A) An applicant shall protect the public trees as well as the private trees on the site under development.
B) An applicant shall submit a tree protection plan incorporating proposed tree protection measures for any existing public trees and private trees on any site or construction project where improvements are proposed in compliance with this specification.
C) The construction plans shall identify any tree proposed for removal and shall provide explanation for the reason the tree must be removed.
D) The construction plans shall identify the protection measures for existing trees that might be damaged from proposed construction activities for all existing trees located on the property and adjacent public or private properties.
E) The applicant shall notify the Municipality within 24 hours of any suspected damage to trees resulting from construction activities. If damage occurs during construction, the applicant shall have the damaged tree repaired as quickly as possible. If the tree must be removed because it presents a risk or is irreparably damaged, the appropriate mitigation shall be performed, which may be restoring the value of the tree or agency replacement requirements, or through mitigation fees.
3. Protection of Existing Trees on the Site
The Contractor must protect all designated trees on the construction site from damage in accordance with the provisions of the Municipal Code and/or development requirements. The Contractor must restore all damaged public property and parkways to their original condition and repair or remove and replace any trees and shrubs damaged as a result of construction activity on public property as determined by the Municipality, at the Contractor’s expense. If any Municipality-owned trees or shrubs damaged by construction activity must be removed and replaced, and trees or shrubs of comparable size, type, and value are unavailable or the time for planting is unsuitable, the Municipality will charge the Contractor the appraised value determined by a qualified arborist, which amount the Municipality will deduct from amounts due the Contractor, or, if no amounts are due, then Contractor must promptly pay the Municipality the amounts determined. If any private trees damaged by construction activity must be removed and replaced, and trees or shrubs of comparable size, type, and value are unavailable or the time for planting is unsuitable, the property owner shall replace their appraised value as determined by a qualified arborist on the property where the damage occurred prior to project acceptance.
At a minimum, any tree greater than 4" D.B.H. that is permanently damaged due to the construction project and not originally marked for removal shall be replaced with as many new trees, with proper spacing, that have a minimum of 4" (10 cm) caliper, to restore the value of the damaged tree. Any damaged tree smaller than 4" (10 cm) caliper measured 6" (15 cm) above the flare shall be replaced in kind, inch for inch. If the site cannot accommodate the number of trees to be replaced, another nearby planting site or a payment to a tree mitigation fund may be required.
A protection barrier or fence of at least 4 feet (1.2m) in height shall be installed around each tree or group of trees to be protected and preserved. The tree protection shall be installed prior to any construction activities start and maintained for the duration of the project through the landscape phase. Construction materials shall not be stored, nor shall equipment operated and/or temporary storage buildings or work trailers be placed within this protection zone.
The protection barrier must be constructed of orange snow fencing or chain link fencing securely fastened to fence posts spaced appropriately on-center to support the fence. The minimum allowable posts are 6 feet (1.8m) in length with 2 feet (61 cm) set into the ground and 4 feet (1.2m) extending above ground. The fencing shall be securely attached to the post. Temporary chain link fence with on-surface bases may be approved as protection barriers. The fencing shall be maintained throughout the construction process.
4. Public Shade Trees
The protection has to be related to the construction project. If repairing sidewalks adjacent to trees, the protection has to allow the work to proceed in a reasonable manner. If working on adjacent property and the need is to protect the trees and planting pit soil, protective fencing should be in place around the installation pit prior to construction.
A) Trees located in Tree Pits – Where trees are located within tree pits, the fencing should be installed at a
minimum distance of the inside dimension of the tree pit opening with a minimum of one stake at each corner
of the opening, and spaced along sides where needed for stability in larger pits.
B) Trees located in boulevards or medians – Trees located in boulevards or streets shall have tree protection
fencing installed out to the drip-line of the tree where the site allows. These locations shall be noted on the
site plans for the proposed construction.
C) Alternatives to Tree Protection – Fencing may be approved and/or be required should existing conditions
and/or proposed work warrant it (limited access to property etc.). Examples of alternatives are:
1) Trunk protection boards tied around the tree trunk with a constructed bridge over the root system of the
tree. Area of bridge will typically cover the area under the dripline for all trees.
2) Plywood instead of snow fencing may be required on projects with a large number of trees and/or where the municipality determines that additional protection is required.
D) Pre-Construction Tree Pruning
Trees within the construction zone, that are to be preserved, shall have their branches raised to an
appropriate height to perform the work without breaking branches with construction equipment. Typical
ranges are a minimum height of 12 feet (4m) and a maximum of 20 feet (6.1m) if the tree crown is expected
to conflict with construction equipment, or structures. All dead branches greater than one inch in diameter
should be removed to reduce the risk of dead branch failure to construction personnel.
All tree pruning shall be performed prior to the start of construction to prevent damage to trees being
preserved and protected. Tree pruning must be performed so as to maintain natural tree form and retaining
the largest foliar crowns possible. Heading cuts directly above the curb or a limit line is strictly prohibited.
Only qualified tree maintenance companies will receive permit approval to perform pruning work on trees. A
certified arborist or qualified tree worker shall perform all tree pruning work.
E) Root Pruning (prior to excavation)
Where the proposed back of curb, driveway or sidewalk edge is located 5 feet (1.5m) or less from the trunk of
an existing tree, root pruning can be a critical operation. The farther from the tree trunk that the roots are cut,
the lesser percentage of the root system will be removed. If possible, expose the roots prior to cutting. Roots
can be exposed by carefully hand digging, pneumatic or pressurized air excavation, and hydro or pressurized
water excavation. If the tree is a municipality-owned tree, a permit will be required to perform this work.
Whenever roots of trees to remain are exposed during construction, the root ends are to be cut off cleanly and
the roots covered by cloth and kept moist for as long as the root is exposed. When roots are to be removed
during excavation, the roots have to be cut on the tree-side of the trench prior to excavating the roots. This
limits the damage to the roots. If roots are torn out of the trench, the damage can be caused much closer to
the tree, and the extent of damage may not be known.
Sources
* Bradshaw A., B., Hunt, & T. Walmsley, “Trees in the Urban Landscape”, Principles and Practice, Spon, London,
1995.
* Bracewell, Sara K. “Protecting Existing Trees during Construction”, University of Minnesota, Department of
Horticultural Science, 2006.
* City of Chicago “Standard Tree Protection Guidelines”, Bureau of Forestry, 2007.
* Coder, K. D., “Root growth control: Managing perceptions and realities”, The Landscape below Ground II,
International Society of Arboriculture, Champaign, IL. pp. 51–81, 1998.
* Coder, Dr. Kim D., “Tree Conservation during Site Development”, Warnell School of Forestry & Natural
Resources, University of Georgia, 2010.
* Costello, L.R., Jones K.S., “Reducing Infrastructure Damage by Tree Roots: A Compendium of Strategies”,
Western Chapter of the International Society of Arboriculture (WCISA), 2003.
* D’Amato, N.E., T.D. Sydnor, R. Hunt, and B. Bishop, “Root growth beneath sidewalks near trees of four
genera”, Journal of Arboriculture 28:283–290, 2002.
* DeepRoot Tree Root Barriers
* Dennis, C., and W.R. Jacobi, “Protecting Trees During Construction”, Colorado State University Extension,
April 2013.
* Ferrini, Francesco and Alessio Fini, “Trees and Infrastructure”, Online Seminars, Archive #42.
* Kielgren R.K., J.R. Clark, “Growth and water relations of Liquidambar styraciflua L. in an urban park and plaza.
Trees”, Journal of Arboriculture, 7:195-201. 1993.
* Kopinga J. Der Wasserverbrauch von Stadtbäumen. “Stadt Osnabrück and Fachhochschule Osnabrück”,
Osnabrücker Baumpflegetage, Conference Proceedings, Osnabrück. 1989.
* Matheny, N., and J. Clark. "Trees and Development. A Technical Guide to Preservation of Trees during Land
Development", International Society of Arboriculture. 183pp. 1998.
* McPherson, G., and P.J. Peper. “Infrastructure repair costs associated with street trees in 15 cities”,
Proceedings of an International Workshop on Trees and Buildings. International Society of Arboriculture,
Champaign, IL. pp. 49–63.1995.
* Minneapolis Park and Recreation Board, "Pruning Regulations to Protect the Critical Root Zone",
http://www.minneapolisparks.org
* Morgenroth, J. “A review of root barrier research”, Arboriculture & Urban Forestry, 34(2):84-88.2008.
* Pauleit S., “Urban street tree plantings: identifying the key requirements”. Municipal Engineer 156. MEI pp. 43-
50, 2003.
* Randrup T.B., “Soil compaction on construction sites”, The Landscape below Ground II, International Society of
Arboriculture, 146-153, 1998.
* Rathjens, Richard G. James A. Doolittle, Greg Mazur and Alan R. Siewert, "Ground-Penetrating Radar as a
Tool for Diagnosing Shade Tree Soil Problems", TCI Magazine , November 2003.
* Smiley T.E., “Comparison of methods to reduce sidewalk damage from tree roots”, Arboriculture and Urban
Forestry, 34(3):179-183, 2008.
* Stokes, Alexia et al, “Root System Investigation By A Radar Ground-Penetrating System", Journal of
Arboriculture, January 2002. 28(1) Pg 2 - 10
* The Care of Trees, " Construction Tree Preservation", http://www.thecareoftrees.com
* Wagar, J.A., and A.L. Franklin. “Sidewalk effects on soil moisture and temperature”, Journal of Arboriculture
20:237–238, 1994.
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 apply the 4.0 CEUs toward Certified Arborist, Utility Specialist, Municipal Specialist, or BCMA science 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.
The following specification or guideline is an example that will ensure that trees growing on a site, prior to construction, will have the best opportunity to thrive and continue growing into high value property assets. This tree protection specification is intended to guide a construction project to insure that appropriate practices will be implemented in the field to eliminate consequences of tree damage and death that may result from uninformed or careless acts, and conserve trees to be valuable property assets. Site specific variations may be necessary to conserve trees based on the site use, the space available, species, climate, construction design, and the existing conditions of the trees. This specification is duplicated on this website's Specification page.
1. General Tree Protection Instructions
The objective of these tree protection instructions is to minimize the negative impacts of construction activities on trees near the property under construction. With proper planning, many negative impacts to trees can be avoided. Whenever possible, the placement of underground utilities and service connections to new buildings should be designed so that the proposed service lines are installed as far away from existing and new trees as possible. These instructions list minimum distances that the edge of excavation needs to be in order to ensure that there is minimal impact on all the site’s existing trees, and the soil space for new trees.
The municipality may have conditions, restrictions, and preservation requirements that limit the number of trees that can be removed on public and private property. The first step in design is for the applicant to learn the municipality's rules and work to align the project with these requirements. A Tree Removal Permit shall be issued after the municipality finds that all reasonable efforts have been undertaken in the architectural layout and design of the proposed development to preserve the required existing trees. The Tree Removal Permit may require replacement trees and/or compensation for the lost value of trees on or adjacent to the site under construction.
A) Building placement and driveway, walkways, and parking areas shall be designed in such a way as to avoid unnecessary removal of existing trees found worthy of conservation in health and structure. Preferably all access pavements and utilities shall be located in the same general vicinity to avoid excessive tree removals.
B) The proposed placement of all utility service lines, including irrigation and lighting, shall be shown on the site and landscaping plans. Every effort shall be made to protect the existing trees during the placement of utility service lines including the use of trenchless technology, auguring, and/or jacking as opposed to open trench cutting. The landscape and irrigation plans shall demonstrate that every effort was made to avoid damage to existing trees during installation of irrigation, lighting, and plants.
C) If the agency requirements are for canopy cover, or size and number of trees to be conserved on a site, the plans shall demonstrate how the project meets those requirements.
2. Tree Protection Required
A) An applicant shall protect the public trees as well as the private trees on the site under development.
B) An applicant shall submit a tree protection plan incorporating proposed tree protection measures for any existing public trees and private trees on any site or construction project where improvements are proposed in compliance with this specification.
C) The construction plans shall identify any tree proposed for removal and shall provide explanation for the reason the tree must be removed.
D) The construction plans shall identify the protection measures for existing trees that might be damaged from proposed construction activities for all existing trees located on the property and adjacent public or private properties.
E) The applicant shall notify the Municipality within 24 hours of any suspected damage to trees resulting from construction activities. If damage occurs during construction, the applicant shall have the damaged tree repaired as quickly as possible. If the tree must be removed because it presents a risk or is irreparably damaged, the appropriate mitigation shall be performed, which may be restoring the value of the tree or agency replacement requirements, or through mitigation fees.
3. Protection of Existing Trees on the Site
The Contractor must protect all designated trees on the construction site from damage in accordance with the provisions of the Municipal Code and/or development requirements. The Contractor must restore all damaged public property and parkways to their original condition and repair or remove and replace any trees and shrubs damaged as a result of construction activity on public property as determined by the Municipality, at the Contractor’s expense. If any Municipality-owned trees or shrubs damaged by construction activity must be removed and replaced, and trees or shrubs of comparable size, type, and value are unavailable or the time for planting is unsuitable, the Municipality will charge the Contractor the appraised value determined by a qualified arborist, which amount the Municipality will deduct from amounts due the Contractor, or, if no amounts are due, then Contractor must promptly pay the Municipality the amounts determined. If any private trees damaged by construction activity must be removed and replaced, and trees or shrubs of comparable size, type, and value are unavailable or the time for planting is unsuitable, the property owner shall replace their appraised value as determined by a qualified arborist on the property where the damage occurred prior to project acceptance.
At a minimum, any tree greater than 4" D.B.H. that is permanently damaged due to the construction project and not originally marked for removal shall be replaced with as many new trees, with proper spacing, that have a minimum of 4" (10 cm) caliper, to restore the value of the damaged tree. Any damaged tree smaller than 4" (10 cm) caliper measured 6" (15 cm) above the flare shall be replaced in kind, inch for inch. If the site cannot accommodate the number of trees to be replaced, another nearby planting site or a payment to a tree mitigation fund may be required.
A protection barrier or fence of at least 4 feet (1.2m) in height shall be installed around each tree or group of trees to be protected and preserved. The tree protection shall be installed prior to any construction activities start and maintained for the duration of the project through the landscape phase. Construction materials shall not be stored, nor shall equipment operated and/or temporary storage buildings or work trailers be placed within this protection zone.
The protection barrier must be constructed of orange snow fencing or chain link fencing securely fastened to fence posts spaced appropriately on-center to support the fence. The minimum allowable posts are 6 feet (1.8m) in length with 2 feet (61 cm) set into the ground and 4 feet (1.2m) extending above ground. The fencing shall be securely attached to the post. Temporary chain link fence with on-surface bases may be approved as protection barriers. The fencing shall be maintained throughout the construction process.
4. Public Shade Trees
The protection has to be related to the construction project. If repairing sidewalks adjacent to trees, the protection has to allow the work to proceed in a reasonable manner. If working on adjacent property and the need is to protect the trees and planting pit soil, protective fencing should be in place around the installation pit prior to construction.
A) Trees located in Tree Pits – Where trees are located within tree pits, the fencing should be installed at a
minimum distance of the inside dimension of the tree pit opening with a minimum of one stake at each corner
of the opening, and spaced along sides where needed for stability in larger pits.
B) Trees located in boulevards or medians – Trees located in boulevards or streets shall have tree protection
fencing installed out to the drip-line of the tree where the site allows. These locations shall be noted on the
site plans for the proposed construction.
C) Alternatives to Tree Protection – Fencing may be approved and/or be required should existing conditions
and/or proposed work warrant it (limited access to property etc.). Examples of alternatives are:
1) Trunk protection boards tied around the tree trunk with a constructed bridge over the root system of the
tree. Area of bridge will typically cover the area under the dripline for all trees.
2) Plywood instead of snow fencing may be required on projects with a large number of trees and/or where the municipality determines that additional protection is required.
D) Pre-Construction Tree Pruning
Trees within the construction zone, that are to be preserved, shall have their branches raised to an
appropriate height to perform the work without breaking branches with construction equipment. Typical
ranges are a minimum height of 12 feet (4m) and a maximum of 20 feet (6.1m) if the tree crown is expected
to conflict with construction equipment, or structures. All dead branches greater than one inch in diameter
should be removed to reduce the risk of dead branch failure to construction personnel.
All tree pruning shall be performed prior to the start of construction to prevent damage to trees being
preserved and protected. Tree pruning must be performed so as to maintain natural tree form and retaining
the largest foliar crowns possible. Heading cuts directly above the curb or a limit line is strictly prohibited.
Only qualified tree maintenance companies will receive permit approval to perform pruning work on trees. A
certified arborist or qualified tree worker shall perform all tree pruning work.
E) Root Pruning (prior to excavation)
Where the proposed back of curb, driveway or sidewalk edge is located 5 feet (1.5m) or less from the trunk of
an existing tree, root pruning can be a critical operation. The farther from the tree trunk that the roots are cut,
the lesser percentage of the root system will be removed. If possible, expose the roots prior to cutting. Roots
can be exposed by carefully hand digging, pneumatic or pressurized air excavation, and hydro or pressurized
water excavation. If the tree is a municipality-owned tree, a permit will be required to perform this work.
Whenever roots of trees to remain are exposed during construction, the root ends are to be cut off cleanly and
the roots covered by cloth and kept moist for as long as the root is exposed. When roots are to be removed
during excavation, the roots have to be cut on the tree-side of the trench prior to excavating the roots. This
limits the damage to the roots. If roots are torn out of the trench, the damage can be caused much closer to
the tree, and the extent of damage may not be known.
Sources
* Bradshaw A., B., Hunt, & T. Walmsley, “Trees in the Urban Landscape”, Principles and Practice, Spon, London,
1995.
* Bracewell, Sara K. “Protecting Existing Trees during Construction”, University of Minnesota, Department of
Horticultural Science, 2006.
* City of Chicago “Standard Tree Protection Guidelines”, Bureau of Forestry, 2007.
* Coder, K. D., “Root growth control: Managing perceptions and realities”, The Landscape below Ground II,
International Society of Arboriculture, Champaign, IL. pp. 51–81, 1998.
* Coder, Dr. Kim D., “Tree Conservation during Site Development”, Warnell School of Forestry & Natural
Resources, University of Georgia, 2010.
* Costello, L.R., Jones K.S., “Reducing Infrastructure Damage by Tree Roots: A Compendium of Strategies”,
Western Chapter of the International Society of Arboriculture (WCISA), 2003.
* D’Amato, N.E., T.D. Sydnor, R. Hunt, and B. Bishop, “Root growth beneath sidewalks near trees of four
genera”, Journal of Arboriculture 28:283–290, 2002.
* DeepRoot Tree Root Barriers
* Dennis, C., and W.R. Jacobi, “Protecting Trees During Construction”, Colorado State University Extension,
April 2013.
* Ferrini, Francesco and Alessio Fini, “Trees and Infrastructure”, Online Seminars, Archive #42.
* Kielgren R.K., J.R. Clark, “Growth and water relations of Liquidambar styraciflua L. in an urban park and plaza.
Trees”, Journal of Arboriculture, 7:195-201. 1993.
* Kopinga J. Der Wasserverbrauch von Stadtbäumen. “Stadt Osnabrück and Fachhochschule Osnabrück”,
Osnabrücker Baumpflegetage, Conference Proceedings, Osnabrück. 1989.
* Matheny, N., and J. Clark. "Trees and Development. A Technical Guide to Preservation of Trees during Land
Development", International Society of Arboriculture. 183pp. 1998.
* McPherson, G., and P.J. Peper. “Infrastructure repair costs associated with street trees in 15 cities”,
Proceedings of an International Workshop on Trees and Buildings. International Society of Arboriculture,
Champaign, IL. pp. 49–63.1995.
* Minneapolis Park and Recreation Board, "Pruning Regulations to Protect the Critical Root Zone",
http://www.minneapolisparks.org
* Morgenroth, J. “A review of root barrier research”, Arboriculture & Urban Forestry, 34(2):84-88.2008.
* Pauleit S., “Urban street tree plantings: identifying the key requirements”. Municipal Engineer 156. MEI pp. 43-
50, 2003.
* Randrup T.B., “Soil compaction on construction sites”, The Landscape below Ground II, International Society of
Arboriculture, 146-153, 1998.
* Rathjens, Richard G. James A. Doolittle, Greg Mazur and Alan R. Siewert, "Ground-Penetrating Radar as a
Tool for Diagnosing Shade Tree Soil Problems", TCI Magazine , November 2003.
* Smiley T.E., “Comparison of methods to reduce sidewalk damage from tree roots”, Arboriculture and Urban
Forestry, 34(3):179-183, 2008.
* Stokes, Alexia et al, “Root System Investigation By A Radar Ground-Penetrating System", Journal of
Arboriculture, January 2002. 28(1) Pg 2 - 10
* The Care of Trees, " Construction Tree Preservation", http://www.thecareoftrees.com
* Wagar, J.A., and A.L. Franklin. “Sidewalk effects on soil moisture and temperature”, Journal of Arboriculture
20:237–238, 1994.
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 apply the 4.0 CEUs toward Certified Arborist, Utility Specialist, Municipal Specialist, or BCMA science 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.