LA Course #GCE-6-1402
About Root Damage
Edited by Len Phillips and Richard Gibney, updated in January 2022
Sections Go directly to the section by clicking on the title below
About Root Damage
Edited by Len Phillips and Richard Gibney, updated in January 2022
Sections Go directly to the section by clicking on the title below
Note: Click on green text in each section for more information.
Tree Root Damage
The survival of urban trees depends on the health of their roots. Roots supply water and nutrients to the shoots, and get back sugar and other compounds they need to live. Roots also store food, synthesize hormones, and provide structural support. Research has shown that tree roots can travel a distance away from the trunk that is over 2½ times the height of the tree, and more than 85% of root growth occurs within 18 inches (0.5 m) of the soil surface.
Concerns
Trees often grow roots that may interfere with adjacent infrastructure, presenting a risk when installed too close to sidewalks and curbs. Roots searching for oxygen, water, and nutrients will easily lift sidewalks. For more information see Topic #18. When someone or something cuts the roots of trees to stop them from growing, it causes root injury and threatens the life of the tree. Through a variety of injuries caused by construction, roots can sustain extensive damage:
Symptoms
Root damage symptoms are manifested in the crown by poor growth, thinning, chlorosis of the foliage, defoliation, and crown dieback. Alternatively, crown symptoms do not necessarily mean that the root system has been damaged; it could be that the tree may have a problem with its vascular system. Visible signs of root damage include:
Generally, healthy roots have intact bark and are whitish when the root bark is removed. Rotting roots are usually black or brown and have loose, peeling bark. Trees that have lost more than 50% of their root systems should be considered at risk.
While it is known that construction around trees can lead to decline and death, it is difficult to know the different kinds of injury. The decline may not be noticeable until long after the problem has begun below ground. Insufficient rooting space, severed roots, poor drainage, compaction, and an inability to take up nutrients can contribute to a weakened root system and decline of the tree. As a result, these stressed trees are more susceptible to infestations by diseases and insects.
Tree Establishment
Trees can lose as much as 95% of their root system during transplanting B&B and are subject to water stress. When selecting trees, choose nurseries that provide high quality trees. It is best to find nurseries that will grow their trees up to 2" caliper in containers that encourage fibrous root systems. These efforts encourage the tree to develop a dense root system and this reduces transplanting shock while making the tree roots more adaptable to urban soils.
Root replacement after planting is a gradual process that can take several years. The compacted, poorly drained soils of many urban sites pose special problems. Techniques are being studied to increase root regeneration and reduce transplanting stress. Fortunately roots are good at compartmentalizing damage to prevent soil borne diseases from entering at these wounds.
Root/Crown Imbalances
Diseases of mature trees result from slowly developing imbalances between the crown and roots. As urban trees grow, root development is often stagnated by restricted spaces and poor quality soils. The crown continues to increase disproportionately in size until a problem occurs. Current research is focusing on methods to improve the root environment through mulching, improving soil quantity, quality and elimination of competition. Simultaneously, growth regulators are being studied for their potential in reducing crown growth while enhancing root development.
Root Damage from Construction
During construction, roots can be protected by:
For more on this subject, see LA Courses About Trees On Building Sites and Sidewalk and Tree Roots Conflict
Wind Damage to Roots
When wind loads a tree, the force is transmitted from the leaves and branches through the stem, and to the roots and soil. If the forces are strong enough, the tree can blow over because:
Since the weight of a tree helps keep it in the ground, the frictional force will be decreased when pruning only one side of the canopy. If the tree leans from vertical, greater shear stresses will occur on one side of the root plate. Both of the occurrences can facilitate tree failure.
Soil frequently shows signs of root problems. For example:
Inspections
While tree roots compartmentalize decay very well, decay sometimes extends from the buttress roots into the trunk. Decay can be measured in roots with the same tools used to measure decay in stems. The most practical tools are penetrometers or battery-powered drills. To inspect for decay in the lower trunk and buttress areas, the hole should be drilled in between buttress roots. Since tree roots generally grow with an elliptical shape, the hole should be drilled into the top of the root more than the width of the root. It is important to realize that tree roots generally decay from the root tips back to the buttress roots. Furthermore, roots generally decay from the bottom of the root upwards. If decay is detected in the top half of the root, it is safe to assume that the remainder of the root is compromised below and beyond where the decay was found.
Root Inspections
The proper inspection of trees for risk should take place at regular, scheduled intervals. In most urban, suburban, and park-like settings, tree inspections should be done at a minimum of once a year. The inspector should be trained in the process of inspecting the whole tree, from roots to crown, for risk that could cause a serious problem.
Checklist for inspecting root flares & root zone:
Tree Root Damage
The survival of urban trees depends on the health of their roots. Roots supply water and nutrients to the shoots, and get back sugar and other compounds they need to live. Roots also store food, synthesize hormones, and provide structural support. Research has shown that tree roots can travel a distance away from the trunk that is over 2½ times the height of the tree, and more than 85% of root growth occurs within 18 inches (0.5 m) of the soil surface.
Concerns
Trees often grow roots that may interfere with adjacent infrastructure, presenting a risk when installed too close to sidewalks and curbs. Roots searching for oxygen, water, and nutrients will easily lift sidewalks. For more information see Topic #18. When someone or something cuts the roots of trees to stop them from growing, it causes root injury and threatens the life of the tree. Through a variety of injuries caused by construction, roots can sustain extensive damage:
- when large buttress roots are severed close to the trunk, the tree loses substantial support and can become prone to windfall.
- cutting the roots that supply water makes the tree vulnerable to drought and also to pests that attack water-stressed trees.
- covering root injuries hides the extent of the damage. This creates an especially dangerous situation, since a casual examination can easily overlook root problems.
- roots must grow to take up many nutrients and when the soil is compacted, roots are unable to penetrate it.
- roots must be able to breathe to use the food they get from the leaves.
- tree roots are subject to frequent wounding by freezing and thawing soil particles around the roots.
- wet and dry cycles will also damage fine roots, as will burrowing animals.
- healthy roots and sturdy trees are necessary for the safety of workers in the trees and people on the ground, enjoying the tree's benefits.
Symptoms
Root damage symptoms are manifested in the crown by poor growth, thinning, chlorosis of the foliage, defoliation, and crown dieback. Alternatively, crown symptoms do not necessarily mean that the root system has been damaged; it could be that the tree may have a problem with its vascular system. Visible signs of root damage include:
- bleeding wounds on the trunk,
- loose, peeling bark or sunken areas around the flare,
- girdling roots and adventitious roots growing above the flare,
- cracks extending into the stem from the soil line.
Generally, healthy roots have intact bark and are whitish when the root bark is removed. Rotting roots are usually black or brown and have loose, peeling bark. Trees that have lost more than 50% of their root systems should be considered at risk.
While it is known that construction around trees can lead to decline and death, it is difficult to know the different kinds of injury. The decline may not be noticeable until long after the problem has begun below ground. Insufficient rooting space, severed roots, poor drainage, compaction, and an inability to take up nutrients can contribute to a weakened root system and decline of the tree. As a result, these stressed trees are more susceptible to infestations by diseases and insects.
Tree Establishment
Trees can lose as much as 95% of their root system during transplanting B&B and are subject to water stress. When selecting trees, choose nurseries that provide high quality trees. It is best to find nurseries that will grow their trees up to 2" caliper in containers that encourage fibrous root systems. These efforts encourage the tree to develop a dense root system and this reduces transplanting shock while making the tree roots more adaptable to urban soils.
Root replacement after planting is a gradual process that can take several years. The compacted, poorly drained soils of many urban sites pose special problems. Techniques are being studied to increase root regeneration and reduce transplanting stress. Fortunately roots are good at compartmentalizing damage to prevent soil borne diseases from entering at these wounds.
Root/Crown Imbalances
Diseases of mature trees result from slowly developing imbalances between the crown and roots. As urban trees grow, root development is often stagnated by restricted spaces and poor quality soils. The crown continues to increase disproportionately in size until a problem occurs. Current research is focusing on methods to improve the root environment through mulching, improving soil quantity, quality and elimination of competition. Simultaneously, growth regulators are being studied for their potential in reducing crown growth while enhancing root development.
Root Damage from Construction
During construction, roots can be protected by:
- fencing of the ground underneath the tree's crown before construction begins. If traffic must go through that area, one option to consider might be to first put down a 12" (30 cm) layer of gravel or coarse mulch that vehicles will use.
- working with the utility company (or appropriate department) to tunnel their service lines under tree roots, when appropriate. If excavation is required, it should be right under or beside the trunk to minimize root damage and no major roots should be cut.
- laying sidewalks around (or up and over) the roots of older trees.
- considering the option of using rubber sidewalks or concrete paving blocks set in sand temporarily or permanently.
- not raising the soil grade over roots more than a few inches without special precautions to make sure they can get the oxygen they need.
- mulching wide and deep, especially for younger trees trying to establish their roots.
For more on this subject, see LA Courses About Trees On Building Sites and Sidewalk and Tree Roots Conflict
Wind Damage to Roots
When wind loads a tree, the force is transmitted from the leaves and branches through the stem, and to the roots and soil. If the forces are strong enough, the tree can blow over because:
- the roots themselves fail from a structural defect such as root decay, or the wood is simply not strong enough to endure the internal stresses caused by the wind force.
- the force of the wind overcomes the shear strength of the soil. In this instance, the roots do not
break, but either pull out of the soil or the soil itself breaks apart. Shallow-rooted trees can be particularly troublesome in this type of failure. Sandy or wet soils, that have weaker cohesive forces between soil
particles, facilitate this type of failure. - the roots have been damaged by construction or soil compaction resulting in poor or no root growth.
Since the weight of a tree helps keep it in the ground, the frictional force will be decreased when pruning only one side of the canopy. If the tree leans from vertical, greater shear stresses will occur on one side of the root plate. Both of the occurrences can facilitate tree failure.
Soil frequently shows signs of root problems. For example:
- soil heaving or mounding behind a leaning tree,
- cracks in the soil or broken roots protruding through the soil.
Inspections
While tree roots compartmentalize decay very well, decay sometimes extends from the buttress roots into the trunk. Decay can be measured in roots with the same tools used to measure decay in stems. The most practical tools are penetrometers or battery-powered drills. To inspect for decay in the lower trunk and buttress areas, the hole should be drilled in between buttress roots. Since tree roots generally grow with an elliptical shape, the hole should be drilled into the top of the root more than the width of the root. It is important to realize that tree roots generally decay from the root tips back to the buttress roots. Furthermore, roots generally decay from the bottom of the root upwards. If decay is detected in the top half of the root, it is safe to assume that the remainder of the root is compromised below and beyond where the decay was found.
Root Inspections
The proper inspection of trees for risk should take place at regular, scheduled intervals. In most urban, suburban, and park-like settings, tree inspections should be done at a minimum of once a year. The inspector should be trained in the process of inspecting the whole tree, from roots to crown, for risk that could cause a serious problem.
Checklist for inspecting root flares & root zone:
- Presence/absence of flare
- Dead or loose bark
- Recent landscaping or paving
- Construction damage to trunk
- Soil compaction
- Trenching nearby
- Underground irrigation systems
- Size of crown in proportion to available root space
- Limited root space – curbs, walks, and planters
- Saturated or high-water table soils
- Sandy soils
- Exposed or windy sites
- Leaning tree – especially with soil mounding and cracks in the soil
- Roots causing soil or pavement upheaval
- Root disease and decay
- Fungal fruiting bodies – on trunk, roots, or growing in the soil
- Cracks in the soil near flare
Tree Root Stress
To understand tree stress, it is important to understand the fundamental needs of trees. A tree must have space, heat, light, water, oxygen, carbon dioxide, and nutrients. Most trees require symbiotic fungi and bacteria. All of these constituents must be available in adequate amounts to assure the tree's long-term survival. Either excess or deficiency of any of these components will cause stress or tree death.
Almost all soils provide the elements necessary for the production of carbohydrates. The loss of carbohydrates reduces reserves, causing stress on mature trees. The application of fertilizer on mature trees, to correct stress, should only be done with caution and for a reason.
Insects and diseases are usually the result of damage and stress caused by environmental factors. While selective or exclusive insects feed on specific species and stressed trees, non-selective insects will infest and feed on any healthy tree. Many diseases result from slowly developing imbalances between the tree crown and roots.
As urban trees grow, their root development is often restricted by limited space and poor quality soils. The crown continues to increase disproportionately in size until stress occurs. Current research projects focus on improving the quality of the root environment through mulching, soil replacement, and elimination of competition. Simultaneously, growth regulators and soil amendments are being studied for their potential in reducing crown growth while enhancing root development.
Drought Stress
Trees can lose up to 95% of their roots during balled and burlapped installation and are consequently subject to water stress. Root replacement is a gradual process requiring several years of care. The most common stress for trees is caused by either a lack of, or a surplus of, water in the soil.
The lack of water shuts down the movement of materials in the tree and causes such severe osmotic pressure imbalances that living cell membranes simply collapse inside and the cell dies. If enough cells die, the whole tree will die. Root growth stops in most species when soil moisture is reduced to 12% – 14%. The deposition of a waterproof layer in the walls of cells near the root surface is accelerated in dry soil. As the absorbing surface is diminished, the roots do not regain their capacity for water uptake until new root tips are produced at least one week later. If the soil becomes too dry, some of the smaller roots may die.
Excess water displaces oxygen and results in a build up of methane in the soil. Oxygen starved roots are weakened and are often invaded by organisms that can move into other parts of the tree. Excess water can also cause extreme osmosis, which saturates succulent tissue, weakens membranes, and causes cell death. An increase in moisture content above 40% induces almost no additional root growth.
Oxygen Stress
Oxygen starved roots cannot respire efficiently and energy reserves cannot be utilized, which results in root death. If large roots die, decay fungi invade and structural integrity is then compromised. Weakened by the progressive loss of roots, a tree canopy slowly begins to deteriorate, from the top down. The minimum soil atmosphere for good root growth is 8 – 10% oxygen.
Trees that Tolerate Low Oxygen
alder Alnus
birch Betula
ash Fraxinus (use in non-EAB areas)
tupelo Nyssa
poplar Populus
willow Salix
baldcypress Taxodium
Nutrient Stress
There are 17 essential elements required for healthy trees. The most essential elements, in order of quantity needed are: nitrogen (N), potassium (K), calcium (Ca), phosphorous (P), magnesium (Mg), sulfur (S), iron (Fe) chlorine (Cl), copper (Cu), manganese (Mn), zinc (Zn), molybdenum (Mo), boron (B), sodium (Na), and cobalt (Co). Trees use nutrients to feed themselves by making sugar (carbohydrates) in the leaves that can be used right away, or stored as starch for future needs. Nutrients are released from the soil particles by soil organisms that break down and release the nutrients found in organic matter and by the natural effects of sun, wind, freezing, and rain that make minerals available.
All of these treatments listed below must be on an as needed basis. The landscape architect/arborist can solve nutrient problems by specifying and applying the following treatments:
It is important to keep newly planted trees watered and pruned and to keep weeds away from their bases to avoid excess stress.
Fertilization
Trees in urban and suburban environments are often under high stress conditions. Fertilizer applications may reduce, but cannot eliminate, environmental stresses.
Temperature Stress
Root growth starts just before shoot growth in the spring. Maximal root growth in most tree species occurs in early summer when temperatures reach 65° – 89°F (18° – 32°C) depending on species, with maximum temperatures for active growth reported at 77° – 100°F (25° – 38°C). Roots of most woody species are killed at temperatures higher than 104°F (40°C). This will become a concern with global warming in the future. Fortunately the tree branches will shade the soil, however attention should also be given to grow trees that will tolerate the warming temperatures. Active growth resumes in the late summer or early autumn when soil moisture and temperature become favorable. In temperate climates, root growth slows in the autumn as the soil cools and plants enter dormancy. Minimum temperatures for root growth ranging from 35° – 52°F (2° – 11°C) have been reported. Substantial root growth can continue in areas with mild winter temperatures. Roots can continue to grow in non-frozen soil, but cold soil temperatures will reduce the rate of growth. Root tissues of woody plants can be killed at soil temperatures of 23° to -4°F (-5° to -20°C).
Compaction Stress
The compacted, poorly drained soils at many urban sites pose compaction stress on tree roots. Correcting this compaction stress requires attention to the root-shoot ratio in order to make the soils more suitable for tree growth.
The root-shoot ratio is usually given as the ratio of the weight of the roots to the weight of the top of a plant. For most trees under normal conditions, the root-shoot ratio is 1:5 to 1:6; i.e. the top is 5 to 6 times heavier than the roots. If it were not for the weight of the trunk, however, the top and roots would weigh about the same. Proportionately more roots than top growth is thought to be in the best interests of a tree. Species with high root-to-shoot ratios seem to have a greater ability to penetrate hard soil layers. Roots also tend to follow the path of least resistance.
To penetrate the deeper, denser soils, roots often use old root channels, animal tunnels, pockets of loose soil, cracks, and fissures. To improve dense soils during installation, it might be advisable to dig the pit deeply past the dense layers and at the bottom drill long holes under the sidewalk or beyond the compact soil. Holes can also be drilled using a high pressure nozzle on the end of a hose, letting the water dig the hole. The pit can be refilled with loose soil or replaced with the soil dug from the hole with sand or soil amendments added. After the tree is installed, compaction should be prohibited and only water should be applied to settle the soil.
A cultural practice that brings about a reduction in the root-shoot ratio of a tree is commonly thought to be detrimental for the well being of the tree. That is, proportionately more top than root growth is thought not to be in the best interests of a tree. However, any factor which improves growing conditions, such as favorable weather, loosening the soil, fertilization, irrigation, aeration, or pest control, results in a reduced root-shoot ratio.
Diagnosis of Problems
The first sign of any serious root problem is top decline. Look for signs to determine whether a tree is growing well by observing the amount of stem growth over the past few years. Most trees will show anywhere from 6 – 18 inches (15 – 46 cm) of twig growth in one year according to species and sun orientation on the tree. If the tree shows wide variation of growth in the past three years, it is safe to say that the tree is under stress. Cankers on the stems, stem tip dieback, off-color foliage, early fall color, and early defoliation are also signs that a tree may be stressed by root or soil problems.
Insufficient rooting space, roots severed during transplanting or construction, poor soil drainage, soil compaction, and an inability to take nutrients up from the soil can contribute to a weakened root system and eventual decline of the tree.
One of the more difficult problems to eliminate is root rot. To detect rots, look for mushroom-like fungi growing at the base of the tree. In the case of wood rot fungi, the conks may be found growing out of the trunk or main branches in the canopy. Wet weather often triggers the formation of these fungi. They can easily be confused with fungi growing on dead organic debris near a tree. Trees may survive for many years with wood or root rots. A tree should not be removed simply because it has a conk. If the tree becomes a threat to life or property because of potential to fall or blow over, remove the tree as soon as possible.
By carefully digging in the root zone, it is possible to determine the health of the roots. The best approach is to use a pneumatic soil excavating tool to remove the soil from the surface of the roots in several locations around the tree. This should be done near the drip line at two or three spots. Healthy roots are brown on the outside and white internally or at the very tips of the roots. If the roots have a soft, brown outer layer that easily pulls off the white center of the root, then a root rot may be involved.
There are presently no chemicals that can help a tree in decline. The use of approved cultural practices to improve tree vitality is recommended, including weekly watering of 1 – 2 inches (2 – 4 cm) of water during periods of extended drought, and if a soil test indicates a problem, the tree should be fertilized in late fall or early spring as needed. For very old or large trees, fertilization and watering may have no benefit. Also, pruning out dead branches in the dormant season and keeping foot traffic off of the root system is recommended. These measures may help the tree continue to live for many years.
To understand tree stress, it is important to understand the fundamental needs of trees. A tree must have space, heat, light, water, oxygen, carbon dioxide, and nutrients. Most trees require symbiotic fungi and bacteria. All of these constituents must be available in adequate amounts to assure the tree's long-term survival. Either excess or deficiency of any of these components will cause stress or tree death.
Almost all soils provide the elements necessary for the production of carbohydrates. The loss of carbohydrates reduces reserves, causing stress on mature trees. The application of fertilizer on mature trees, to correct stress, should only be done with caution and for a reason.
Insects and diseases are usually the result of damage and stress caused by environmental factors. While selective or exclusive insects feed on specific species and stressed trees, non-selective insects will infest and feed on any healthy tree. Many diseases result from slowly developing imbalances between the tree crown and roots.
As urban trees grow, their root development is often restricted by limited space and poor quality soils. The crown continues to increase disproportionately in size until stress occurs. Current research projects focus on improving the quality of the root environment through mulching, soil replacement, and elimination of competition. Simultaneously, growth regulators and soil amendments are being studied for their potential in reducing crown growth while enhancing root development.
Drought Stress
Trees can lose up to 95% of their roots during balled and burlapped installation and are consequently subject to water stress. Root replacement is a gradual process requiring several years of care. The most common stress for trees is caused by either a lack of, or a surplus of, water in the soil.
The lack of water shuts down the movement of materials in the tree and causes such severe osmotic pressure imbalances that living cell membranes simply collapse inside and the cell dies. If enough cells die, the whole tree will die. Root growth stops in most species when soil moisture is reduced to 12% – 14%. The deposition of a waterproof layer in the walls of cells near the root surface is accelerated in dry soil. As the absorbing surface is diminished, the roots do not regain their capacity for water uptake until new root tips are produced at least one week later. If the soil becomes too dry, some of the smaller roots may die.
Excess water displaces oxygen and results in a build up of methane in the soil. Oxygen starved roots are weakened and are often invaded by organisms that can move into other parts of the tree. Excess water can also cause extreme osmosis, which saturates succulent tissue, weakens membranes, and causes cell death. An increase in moisture content above 40% induces almost no additional root growth.
Oxygen Stress
Oxygen starved roots cannot respire efficiently and energy reserves cannot be utilized, which results in root death. If large roots die, decay fungi invade and structural integrity is then compromised. Weakened by the progressive loss of roots, a tree canopy slowly begins to deteriorate, from the top down. The minimum soil atmosphere for good root growth is 8 – 10% oxygen.
Trees that Tolerate Low Oxygen
alder Alnus
birch Betula
ash Fraxinus (use in non-EAB areas)
tupelo Nyssa
poplar Populus
willow Salix
baldcypress Taxodium
Nutrient Stress
There are 17 essential elements required for healthy trees. The most essential elements, in order of quantity needed are: nitrogen (N), potassium (K), calcium (Ca), phosphorous (P), magnesium (Mg), sulfur (S), iron (Fe) chlorine (Cl), copper (Cu), manganese (Mn), zinc (Zn), molybdenum (Mo), boron (B), sodium (Na), and cobalt (Co). Trees use nutrients to feed themselves by making sugar (carbohydrates) in the leaves that can be used right away, or stored as starch for future needs. Nutrients are released from the soil particles by soil organisms that break down and release the nutrients found in organic matter and by the natural effects of sun, wind, freezing, and rain that make minerals available.
All of these treatments listed below must be on an as needed basis. The landscape architect/arborist can solve nutrient problems by specifying and applying the following treatments:
- not planting species preferring low pH in soils with a high pH and vice versa,
- improving the drainage,
- modifying the soil pH,
- applying suitable fertilizer,
- applying a chelate as a soil drench if a micro-nutrient is needed,
- applying multiple and repeated injections
It is important to keep newly planted trees watered and pruned and to keep weeds away from their bases to avoid excess stress.
Fertilization
Trees in urban and suburban environments are often under high stress conditions. Fertilizer applications may reduce, but cannot eliminate, environmental stresses.
Temperature Stress
Root growth starts just before shoot growth in the spring. Maximal root growth in most tree species occurs in early summer when temperatures reach 65° – 89°F (18° – 32°C) depending on species, with maximum temperatures for active growth reported at 77° – 100°F (25° – 38°C). Roots of most woody species are killed at temperatures higher than 104°F (40°C). This will become a concern with global warming in the future. Fortunately the tree branches will shade the soil, however attention should also be given to grow trees that will tolerate the warming temperatures. Active growth resumes in the late summer or early autumn when soil moisture and temperature become favorable. In temperate climates, root growth slows in the autumn as the soil cools and plants enter dormancy. Minimum temperatures for root growth ranging from 35° – 52°F (2° – 11°C) have been reported. Substantial root growth can continue in areas with mild winter temperatures. Roots can continue to grow in non-frozen soil, but cold soil temperatures will reduce the rate of growth. Root tissues of woody plants can be killed at soil temperatures of 23° to -4°F (-5° to -20°C).
Compaction Stress
The compacted, poorly drained soils at many urban sites pose compaction stress on tree roots. Correcting this compaction stress requires attention to the root-shoot ratio in order to make the soils more suitable for tree growth.
The root-shoot ratio is usually given as the ratio of the weight of the roots to the weight of the top of a plant. For most trees under normal conditions, the root-shoot ratio is 1:5 to 1:6; i.e. the top is 5 to 6 times heavier than the roots. If it were not for the weight of the trunk, however, the top and roots would weigh about the same. Proportionately more roots than top growth is thought to be in the best interests of a tree. Species with high root-to-shoot ratios seem to have a greater ability to penetrate hard soil layers. Roots also tend to follow the path of least resistance.
To penetrate the deeper, denser soils, roots often use old root channels, animal tunnels, pockets of loose soil, cracks, and fissures. To improve dense soils during installation, it might be advisable to dig the pit deeply past the dense layers and at the bottom drill long holes under the sidewalk or beyond the compact soil. Holes can also be drilled using a high pressure nozzle on the end of a hose, letting the water dig the hole. The pit can be refilled with loose soil or replaced with the soil dug from the hole with sand or soil amendments added. After the tree is installed, compaction should be prohibited and only water should be applied to settle the soil.
A cultural practice that brings about a reduction in the root-shoot ratio of a tree is commonly thought to be detrimental for the well being of the tree. That is, proportionately more top than root growth is thought not to be in the best interests of a tree. However, any factor which improves growing conditions, such as favorable weather, loosening the soil, fertilization, irrigation, aeration, or pest control, results in a reduced root-shoot ratio.
Diagnosis of Problems
The first sign of any serious root problem is top decline. Look for signs to determine whether a tree is growing well by observing the amount of stem growth over the past few years. Most trees will show anywhere from 6 – 18 inches (15 – 46 cm) of twig growth in one year according to species and sun orientation on the tree. If the tree shows wide variation of growth in the past three years, it is safe to say that the tree is under stress. Cankers on the stems, stem tip dieback, off-color foliage, early fall color, and early defoliation are also signs that a tree may be stressed by root or soil problems.
Insufficient rooting space, roots severed during transplanting or construction, poor soil drainage, soil compaction, and an inability to take nutrients up from the soil can contribute to a weakened root system and eventual decline of the tree.
One of the more difficult problems to eliminate is root rot. To detect rots, look for mushroom-like fungi growing at the base of the tree. In the case of wood rot fungi, the conks may be found growing out of the trunk or main branches in the canopy. Wet weather often triggers the formation of these fungi. They can easily be confused with fungi growing on dead organic debris near a tree. Trees may survive for many years with wood or root rots. A tree should not be removed simply because it has a conk. If the tree becomes a threat to life or property because of potential to fall or blow over, remove the tree as soon as possible.
By carefully digging in the root zone, it is possible to determine the health of the roots. The best approach is to use a pneumatic soil excavating tool to remove the soil from the surface of the roots in several locations around the tree. This should be done near the drip line at two or three spots. Healthy roots are brown on the outside and white internally or at the very tips of the roots. If the roots have a soft, brown outer layer that easily pulls off the white center of the root, then a root rot may be involved.
There are presently no chemicals that can help a tree in decline. The use of approved cultural practices to improve tree vitality is recommended, including weekly watering of 1 – 2 inches (2 – 4 cm) of water during periods of extended drought, and if a soil test indicates a problem, the tree should be fertilized in late fall or early spring as needed. For very old or large trees, fertilization and watering may have no benefit. Also, pruning out dead branches in the dormant season and keeping foot traffic off of the root system is recommended. These measures may help the tree continue to live for many years.
Girdling Roots
It is essential that in order to have a well-developed healthy tree, it must have well developed healthy roots. There should be five to seven large roots radiating out from the trunk. If a tree has less than 5 lateral roots or any root defects, the tree can exhibit poor growth, disease and pest susceptibility, and a shortened life span. Trees with girdling roots tend to decline over a 5 – 10 year period. Selecting or purchasing trees with well-distributed root systems that are without kinked or circling roots will avoid future girdling root problems. Problem roots are clearly visible with bare root trees.
Stem girdling roots are those roots that grow either partially or completely against the trunk and compress the stem tissues. Xylem and phloem tissues in the stems become much smaller in diameter at the points of compression, compromising the transport of water, nutrients, and food. Trees become stressed and more vulnerable to secondary problems (drought or insect attacks). Often, the compressed areas of the stems are weak points and far too often are points of failure during windstorms.
Symptoms
The symptoms associated with girdling roots are caused by a weakened root system. The crown of the tree may be thin, with stunted growth. In advanced stages, a girdling root problem will cause the trunk to become flattened along the side where the strangulation is occurring. The problem roots are often found at or within a few inches of the surface. Borers and cankers in the tree are also symptoms of root problems along with a general decline in the tree's vigor. Trees with severe girdling roots may lean or completely break off.
Causes
The development of girdling roots is not well understood, but it is normally thought to be the result of unfavorable conditions that prevent roots from growing out in a normal spreading manner. Girdling roots seldom occur in nature. Girdling roots usually develop at the nursery when a tree has been left growing in a round container for too long. Girdling roots can also be induced through the use of root barriers and containers of soil-less mixes that form an interface discontinuity with the backfill soil after planting. Girdling roots on field grown stock are usually the result of transplanting and the primary roots being cut. In this situation secondary roots begin growing in dominance with the loss of the primary root. Normally these secondary roots would have died, as the primary root grew larger. The secondary roots often grow perpendicular to the primary root and are close to the trunk. Restricted root space, such as tree pits in urban areas, also may result in girdling roots. There is some suggestion also that constant mulching, a desirable practice in many respects, may cause the formation of girdling roots, if the mulch is too close to the trunk.
For trees susceptible to root girdling (see list, below), an inspection should be made several times before the tree is six inches in diameter. Digging the tree up and exposing the roots is the only way to make a positive diagnosis. Using a pneumatic air tool, the soil is carefully removed to a depth of at least 12 inches (30 cm), with care taken to prevent serious mechanical injury to the roots. If girdling roots are found, they must be removed.
If the inspection reveals girdling roots, and a considerable amount of damage, the most prudent move may be to replace the tree. Spending money on a weakened tree that subsequently dies can be an extremely frustrating experience. Because correcting this problem is so labor intensive, the costs and benefits should be weighed carefully prior to making any decisions.
Prevention
Treatment of girdling roots begins with prevention.
Solutions
Root pruning or trenching to correct a problem is costly and often severely affects the tree. Researchers that have attempted to determine how much of a tree's root system may be safely removed, have met with mixed results, probably due to differential sensitivities among tree species. However, without a doubt, major root removal can cause tree decline and often mortality. Removing a girdling root is a wound in its own right. Yet, while the correction of the problem can kill the desirable plant, the likelihood of the tree dying is greater if no action is taken. Research has proven that developing trees with spreading bare roots is the best solution. Before selecting or purchasing a new tree, require proof that it was being grown in containers that air-pruned the roots.
If a large girdling root has grafted to the tree trunk, it is advisable to allow it to remain undisturbed.
The preliminary results of current research are indicating that vertical ribs on root barriers and containers are helping to direct the roots downward instead of circling. Copper based paint was also successfully repelling roots from circling the container or barrier, but this practice is no longer acceptable. Unfortunately, in some cases these containers are encouraging kinked or "J" roots instead. Girdling roots can develop whether the tree is a seedling or a cutting. If a tree being purchased has a girdling root it can be pruned to remove the root completely. However, studies have indicated that the root often grows right back and is even worse than before its removal. This is especially true with Norway maples. Other species of maples that may have had a circling root tendency did not develop the girdling problem with age. Soil amendments have been researched and proven to show no consistent improvement in preventing or correcting the problem. Some researchers have tried mechanical disruption of the root ball by the slashing method described above. This redirects the circling roots and has provided good results.
Researchers tell us that they are close to providing solutions for this problem. However, this may require the purchaser of a tree to pay a little more for girdling root control. However, it will be worth it if the problem is solved and trees will be able to grow better and longer. At the present time, this issue is a major concern and additional research is needed. Seriously weakened or declining trees resulting from girdling roots may need to be removed. Encouraging the development of fibrous roots seems to be the best solution at this time.
Species Susceptible to Girdling Roots Include:
Norway maple Acer platanoides
Red maple Acer rubrum
Sugar maple Acer saccharum
American beech Fagus sp.
Green ash Fraxinus pennsylvanica
Honeylocust Gledistia triacanthos
Pines Pinus sp.
Poplar Poplus sp.
Oak Quercus sp.
Littleleaf linden Tilia cordata
Elm Ulmus sp,
It is essential that in order to have a well-developed healthy tree, it must have well developed healthy roots. There should be five to seven large roots radiating out from the trunk. If a tree has less than 5 lateral roots or any root defects, the tree can exhibit poor growth, disease and pest susceptibility, and a shortened life span. Trees with girdling roots tend to decline over a 5 – 10 year period. Selecting or purchasing trees with well-distributed root systems that are without kinked or circling roots will avoid future girdling root problems. Problem roots are clearly visible with bare root trees.
Stem girdling roots are those roots that grow either partially or completely against the trunk and compress the stem tissues. Xylem and phloem tissues in the stems become much smaller in diameter at the points of compression, compromising the transport of water, nutrients, and food. Trees become stressed and more vulnerable to secondary problems (drought or insect attacks). Often, the compressed areas of the stems are weak points and far too often are points of failure during windstorms.
Symptoms
The symptoms associated with girdling roots are caused by a weakened root system. The crown of the tree may be thin, with stunted growth. In advanced stages, a girdling root problem will cause the trunk to become flattened along the side where the strangulation is occurring. The problem roots are often found at or within a few inches of the surface. Borers and cankers in the tree are also symptoms of root problems along with a general decline in the tree's vigor. Trees with severe girdling roots may lean or completely break off.
Causes
The development of girdling roots is not well understood, but it is normally thought to be the result of unfavorable conditions that prevent roots from growing out in a normal spreading manner. Girdling roots seldom occur in nature. Girdling roots usually develop at the nursery when a tree has been left growing in a round container for too long. Girdling roots can also be induced through the use of root barriers and containers of soil-less mixes that form an interface discontinuity with the backfill soil after planting. Girdling roots on field grown stock are usually the result of transplanting and the primary roots being cut. In this situation secondary roots begin growing in dominance with the loss of the primary root. Normally these secondary roots would have died, as the primary root grew larger. The secondary roots often grow perpendicular to the primary root and are close to the trunk. Restricted root space, such as tree pits in urban areas, also may result in girdling roots. There is some suggestion also that constant mulching, a desirable practice in many respects, may cause the formation of girdling roots, if the mulch is too close to the trunk.
For trees susceptible to root girdling (see list, below), an inspection should be made several times before the tree is six inches in diameter. Digging the tree up and exposing the roots is the only way to make a positive diagnosis. Using a pneumatic air tool, the soil is carefully removed to a depth of at least 12 inches (30 cm), with care taken to prevent serious mechanical injury to the roots. If girdling roots are found, they must be removed.
If the inspection reveals girdling roots, and a considerable amount of damage, the most prudent move may be to replace the tree. Spending money on a weakened tree that subsequently dies can be an extremely frustrating experience. Because correcting this problem is so labor intensive, the costs and benefits should be weighed carefully prior to making any decisions.
Prevention
Treatment of girdling roots begins with prevention.
- When planting, any potentially circling roots should be loosened and straightened.
- The planting hole should be three times the root ball diameter in width, allowing ample room for the root system in all directions.
- The sides of the planting hole should be loosened and roughened, to allow root penetration into surrounding soil.
- The soil type in the root ball should be similar to the soil type at the planting site. If the soils are not similar, the soil from the root ball should be removed and the tree should be installed as bare root.
- Trees should be planted at the proper depth with the flare at or one inch higher than the final grade and mulched lightly.
- Some nurseries recommend root pruning at the time of transplanting as an absolute must for any container grown tree or shrub that has circling roots. Three to five slashes must be made vertically down the root ball and about an inch into the root ball. One or two slashes into the bottom of the root ball must be made at a depth of three to four inches. Some go further by fraying out the pruned roots. If these roots are not pruned at the time of transplanting, inadequate root systems develop. Young trees planted with kinked, entwined, or circling roots continue to grow in the same manner and eventually girdle the tree.
Solutions
Root pruning or trenching to correct a problem is costly and often severely affects the tree. Researchers that have attempted to determine how much of a tree's root system may be safely removed, have met with mixed results, probably due to differential sensitivities among tree species. However, without a doubt, major root removal can cause tree decline and often mortality. Removing a girdling root is a wound in its own right. Yet, while the correction of the problem can kill the desirable plant, the likelihood of the tree dying is greater if no action is taken. Research has proven that developing trees with spreading bare roots is the best solution. Before selecting or purchasing a new tree, require proof that it was being grown in containers that air-pruned the roots.
If a large girdling root has grafted to the tree trunk, it is advisable to allow it to remain undisturbed.
The preliminary results of current research are indicating that vertical ribs on root barriers and containers are helping to direct the roots downward instead of circling. Copper based paint was also successfully repelling roots from circling the container or barrier, but this practice is no longer acceptable. Unfortunately, in some cases these containers are encouraging kinked or "J" roots instead. Girdling roots can develop whether the tree is a seedling or a cutting. If a tree being purchased has a girdling root it can be pruned to remove the root completely. However, studies have indicated that the root often grows right back and is even worse than before its removal. This is especially true with Norway maples. Other species of maples that may have had a circling root tendency did not develop the girdling problem with age. Soil amendments have been researched and proven to show no consistent improvement in preventing or correcting the problem. Some researchers have tried mechanical disruption of the root ball by the slashing method described above. This redirects the circling roots and has provided good results.
Researchers tell us that they are close to providing solutions for this problem. However, this may require the purchaser of a tree to pay a little more for girdling root control. However, it will be worth it if the problem is solved and trees will be able to grow better and longer. At the present time, this issue is a major concern and additional research is needed. Seriously weakened or declining trees resulting from girdling roots may need to be removed. Encouraging the development of fibrous roots seems to be the best solution at this time.
Species Susceptible to Girdling Roots Include:
Norway maple Acer platanoides
Red maple Acer rubrum
Sugar maple Acer saccharum
American beech Fagus sp.
Green ash Fraxinus pennsylvanica
Honeylocust Gledistia triacanthos
Pines Pinus sp.
Poplar Poplus sp.
Oak Quercus sp.
Littleleaf linden Tilia cordata
Elm Ulmus sp,
Tree Roots in Sewers
Tree roots are opportunistic and will seek air and moisture wherever it is. Just as tree roots seek air and moisture under a sidewalk pavement, eventually causing it to rise up as a root grows in diameter, tree roots will seek 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. Drainage pipes usually do not have seals at the joints and will leak over time. Air and nutrient rich water can seep from the pipes, creating an ideal growing medium for roots. The roots will also penetrate any leakage points; fill the pipes with a root mass; and eventually exert enough pressure to spread the crack, increase the leakage, and worsen the problem.
Those responsible for sewer systems can avoid root intrusion by locating pipes away from existing mature trees, by reducing the number of pipe joints, and by ensuring that the pipes are properly installed with sealed joints. Arborists and landscape architects can minimize or prevent root intrusion by planting/specifying only slow growing species with non-aggressive root systems near sewers. Where trees are planted 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 should examine the value of the trees in relation to the damage. Replacement of vegetation is often considerably less expensive than renovating or replacing pipes. However, there are several new techniques that will be studied later in this section, that are much less expensive than digging up and replacing the pipe.
Catch Basins
Catch basins are being redesigned so they will provide stormwater for use by nearby trees. At the catch basin, a perforated pipe is installed at the time of planting the tree, that directs stormwater from the catch basin to the soil around the tree(s). Several feet below this pipe is a second perforated pipe that serves as an under-drain and collects the surplus water and connects to the city storm drainage system. This system not only provides water for the tree, but allows the soil to serve as a water cleaning process that empties clean water into the storm system.
Water Lines
Tree roots do not normally affect water supply lines or gas lines, since these are usually sealed and do not leak. However, the pipe trench itself may provide better growing conditions than the surrounding soil, and roots may proliferate in this area. Over time roots can grow large enough to exert pressure that bends pipes, causes stress cracks, or breaks nearby joints.
Solutions
Aggressive roots can penetrate plumbing when it is in poor condition. Pruning alone will not keep roots from growing in a cracked pipe. The solution is to replace the pipe and tighten the joints. The use of plastic pipes will help alleviate this problem.
With a sewer problem, several methods are used to restore the drain and preserve tree roots:
New Pipes and Conduits
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 minimal disturbance to the environment and is suitable in a variety of soil conditions. The installation of gas, electric, water, and telecommunication lines is accomplished with directional boring machines.
Directional boring is used for the following reasons:
The Jack & Bore method of horizontal earth boring is a process of simultaneously jacking a casing through the earth while removing the spoil inside the encasement by means of a rotating auger. Steerable cutting heads on the end of the boring casing, along with a grade-sensing device, will allow for extreme accuracy. The casing also serves to support the soil around it as the spoil is removed.
Moles and pipe ramming are used for utility installation that eliminates voids created by auger boring under roads. Moles work well in cobble rock and extremely wet conditions. The pipe cracks rocks and displaces or swallows them inside the casing. With pipe ramming, there is no need to have as big a pit as with auger-boring machines.
PipeburstingTM has become an extremely popular, cost-effective, fast, and environmentally superior means of dealing with old pipe. It generally involves reaming out the old pipe with a 'mole' type head. This special bursting head smashes through old service lines while pulling in new replacement lines. A camera follows to identify any low spots that must be corrected. The method can be done without affecting surface structures. The bursting heads are removed at the end of the line, allowing the piercing tool to return to the entrance pit through the new service line. Because PipeburstingTM eliminates an exit pit; excavating costs are cut in half. It is harmless to adjacent utilities because it follows the existing pipe path. The system's capacity can be up-sized with the new service line. Most important, PipeburstingTM saves trees from significant root cutting or removal. Experienced plumbing contractors can use this option for pipes 3 – 4 inch (8 – 10 cm) diameters as well as larger pipelines. It would be totally within reason for a landscape architect/consulting arborist to specify this as a recommendation in a tree preservation report.
A pneumatic soil excavating tool (Air-Tool) is a durable, hand-held tool that digs soil using a "laser-like" jet of air pressurized by a compressor. It is harmless to items like plant roots, buried pipes or cables. The technique has been used by arborists, landscapers, and tree companies for root collar excavation, aeration, radial trenching, vertical mulching, root examinations, soil compaction reduction, plant disease treatment, digging trees for bare root transplanting, and eliminating girdling roots. It is not often used for pipe installation.
Trees That Easily Invade Pipelines:
Populus x canadensis 'Robusta'
P. nigra
P. balsamifera
Salix alba
S. babylonica
S. fragilis
S. elegantisima
Tree roots are opportunistic and will seek air and moisture wherever it is. Just as tree roots seek air and moisture under a sidewalk pavement, eventually causing it to rise up as a root grows in diameter, tree roots will seek 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. Drainage pipes usually do not have seals at the joints and will leak over time. Air and nutrient rich water can seep from the pipes, creating an ideal growing medium for roots. The roots will also penetrate any leakage points; fill the pipes with a root mass; and eventually exert enough pressure to spread the crack, increase the leakage, and worsen the problem.
Those responsible for sewer systems can avoid root intrusion by locating pipes away from existing mature trees, by reducing the number of pipe joints, and by ensuring that the pipes are properly installed with sealed joints. Arborists and landscape architects can minimize or prevent root intrusion by planting/specifying only slow growing species with non-aggressive root systems near sewers. Where trees are planted 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 should examine the value of the trees in relation to the damage. Replacement of vegetation is often considerably less expensive than renovating or replacing pipes. However, there are several new techniques that will be studied later in this section, that are much less expensive than digging up and replacing the pipe.
Catch Basins
Catch basins are being redesigned so they will provide stormwater for use by nearby trees. At the catch basin, a perforated pipe is installed at the time of planting the tree, that directs stormwater from the catch basin to the soil around the tree(s). Several feet below this pipe is a second perforated pipe that serves as an under-drain and collects the surplus water and connects to the city storm drainage system. This system not only provides water for the tree, but allows the soil to serve as a water cleaning process that empties clean water into the storm system.
Water Lines
Tree roots do not normally affect water supply lines or gas lines, since these are usually sealed and do not leak. However, the pipe trench itself may provide better growing conditions than the surrounding soil, and roots may proliferate in this area. Over time roots can grow large enough to exert pressure that bends pipes, causes stress cracks, or breaks nearby joints.
Solutions
Aggressive roots can penetrate plumbing when it is in poor condition. Pruning alone will not keep roots from growing in a cracked pipe. The solution is to replace the pipe and tighten the joints. The use of plastic pipes will help alleviate this problem.
With a sewer problem, several methods are used to restore the drain 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.
- Hand digging with small tools – is suitable for pipe replacement in short distances, and will leave large roots intact.
- Air-trenching tools – can result in roots remaining largely uncompromised. However, these tools are only marginally suitable for tunneling and trenching.
- 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.
New Pipes and Conduits
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 minimal disturbance to the environment and is suitable in a variety of soil conditions. The installation of gas, electric, water, and telecommunication lines is accomplished with directional boring machines.
Directional boring is used for the following reasons:
- less traffic disruption when pipes need to cross streets,
- lower long-term cost,
- deeper installation,
- longer installation length,
- small access pit required,
- shorter completion times,
- directional capabilities,
- safer for the environment.
The Jack & Bore method of horizontal earth boring is a process of simultaneously jacking a casing through the earth while removing the spoil inside the encasement by means of a rotating auger. Steerable cutting heads on the end of the boring casing, along with a grade-sensing device, will allow for extreme accuracy. The casing also serves to support the soil around it as the spoil is removed.
Moles and pipe ramming are used for utility installation that eliminates voids created by auger boring under roads. Moles work well in cobble rock and extremely wet conditions. The pipe cracks rocks and displaces or swallows them inside the casing. With pipe ramming, there is no need to have as big a pit as with auger-boring machines.
PipeburstingTM has become an extremely popular, cost-effective, fast, and environmentally superior means of dealing with old pipe. It generally involves reaming out the old pipe with a 'mole' type head. This special bursting head smashes through old service lines while pulling in new replacement lines. A camera follows to identify any low spots that must be corrected. The method can be done without affecting surface structures. The bursting heads are removed at the end of the line, allowing the piercing tool to return to the entrance pit through the new service line. Because PipeburstingTM eliminates an exit pit; excavating costs are cut in half. It is harmless to adjacent utilities because it follows the existing pipe path. The system's capacity can be up-sized with the new service line. Most important, PipeburstingTM saves trees from significant root cutting or removal. Experienced plumbing contractors can use this option for pipes 3 – 4 inch (8 – 10 cm) diameters as well as larger pipelines. It would be totally within reason for a landscape architect/consulting arborist to specify this as a recommendation in a tree preservation report.
A pneumatic soil excavating tool (Air-Tool) is a durable, hand-held tool that digs soil using a "laser-like" jet of air pressurized by a compressor. It is harmless to items like plant roots, buried pipes or cables. The technique has been used by arborists, landscapers, and tree companies for root collar excavation, aeration, radial trenching, vertical mulching, root examinations, soil compaction reduction, plant disease treatment, digging trees for bare root transplanting, and eliminating girdling roots. It is not often used for pipe installation.
Trees That Easily Invade Pipelines:
Populus x canadensis 'Robusta'
P. nigra
P. balsamifera
Salix alba
S. babylonica
S. fragilis
S. elegantisima
Tree Roots versus Sidewalks
Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in the tree lawn or parkway, or whatever you call that ribbon of grass between the street and the sidewalk. Throughout this article, the term 'tree lawn' will be used. Problems also occur when large trees grow toward the curb or sidewalk and the roots or flare lifts the curb or walk. Photo New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring or removing tree roots. However, sidewalk damage is more often caused by the soil in its expansion and contraction, than by tree roots. For more detailed and additional information see LA Course 'About Sidewalk & Tree Roots Conflict'.
Sidewalk Construction
Sidewalk construction requires compacted soil that will generally deflect tree roots downward. However, in time a small gap develops between the compacted soil and the pavement where groundwater collects from the condensation of soil moisture rising to the surface. Finding water and oxygen in this gap, opportunistic roots will grow and increase in diameter, raising the sidewalk and curb just as roots raise the soil in areas where there is no sidewalk.
When the municipality decides it is time to correct the sidewalk problems, a whole new series of problems with the trees can result. The horizontal roots that are causing the problems are the same roots that make a tree stable against storms and wind. Although these buttress roots supporting the tree cause the worst damage to sidewalks, it is not advisable to prune any of these roots in the interest of saving the walk.
Solutions
The following alternatives are listed from the most to least desirable, in this editor's opinion:
Tree Lawn Width Mature Tree Height
less than 3 feet (0.9 m) No trees
3-5 feet (1 - 1.5 m) 10-30 feet (3 - 9 m) Small trees at maturity are not always bad choices.
5-7 feet (1.5 - 2 m) 30-40 feet (9 - 12 m)
7-9 feet (2 - 3 m) 40-50 feet (12 - 15 m)
9 feet or wider (3 m+) 50 feet or taller (15 m+) Big trees need more space
These recycled pavers have other advantages. - They can be recycled. - They require little maintenance. - There is less trip-risk liability. - They reduce lawsuits caused by pedestrians injuring themselves on concrete. - They are economical and aesthetically pleasing.
The list below indicates some less desirable ways of dealing with sidewalk problems:
*If root pruning is done, the crown of the tree must be reduced to cut back on the wind sail to make the
tree safer if the branches are very thick. In this case, the pruning should be thinning cuts, and not
necessarily on the side of the tree that has had the root severance. Some trees appear to respond with
limb shedding on the same side as the root cutting and some on the opposite side of the root cutting.
Since it may be difficult to predict which branches the tree will shed in response to root cutting, it is
better to avoid pruning until necessary and return later to remove any dieback. Ideally, trees should
have their crown pruned one or two years prior to the root pruning if this pruning is going to become
necessary.
Curb Replacement
Curbs are often replaced during road resurfacing, especially if they are damaged or insufficient in height. The alternatives below will reduce the amount of root damage during the replacement process:
Planting Solutions
The removal and replacement of trees as an alternative to root pruning should be done in accordance with the municipality's street tree policy. Proper tree selection near a sidewalk is critical to long-term success. The amount of space available should determine the size of the tree being planted. Here are general guidelines:
The following lists includes some of the:
More Desirable Trees That Mature at 30 feet: Cultivars of these trees are also acceptable.
Acer campestre
Acer griseum
Acer tartaricum
Amelanchier
Carpinus
Chionanthus
Cornus
Koelreuteria
Maackia
Magnolia
Malus
Ostrya
Oxydendrum
Prunus
Pyrus
Trees and their Cultivars Likely to Have Surface Roots: (Cultivars of these trees are not acceptable.)
Norway Maple Acer platanoides
Red Maple Acer rubrum
Silver Maple Acer saccharinum
Ash Fraxinus spp.
Sweetgum Liquidambar styraciflua
Tuliptree Liriodendron tulipifera
Pin Oak Quercus palustris
Poplar/Cottonwood Populus spp.
Willow Salix spp.
American Elm Ulmus americana
Siberian Elm Ulmus pumila
Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in the tree lawn or parkway, or whatever you call that ribbon of grass between the street and the sidewalk. Throughout this article, the term 'tree lawn' will be used. Problems also occur when large trees grow toward the curb or sidewalk and the roots or flare lifts the curb or walk. Photo New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring or removing tree roots. However, sidewalk damage is more often caused by the soil in its expansion and contraction, than by tree roots. For more detailed and additional information see LA Course 'About Sidewalk & Tree Roots Conflict'.
Sidewalk Construction
Sidewalk construction requires compacted soil that will generally deflect tree roots downward. However, in time a small gap develops between the compacted soil and the pavement where groundwater collects from the condensation of soil moisture rising to the surface. Finding water and oxygen in this gap, opportunistic roots will grow and increase in diameter, raising the sidewalk and curb just as roots raise the soil in areas where there is no sidewalk.
When the municipality decides it is time to correct the sidewalk problems, a whole new series of problems with the trees can result. The horizontal roots that are causing the problems are the same roots that make a tree stable against storms and wind. Although these buttress roots supporting the tree cause the worst damage to sidewalks, it is not advisable to prune any of these roots in the interest of saving the walk.
Solutions
The following alternatives are listed from the most to least desirable, in this editor's opinion:
- Solving the problem of heaving walks is best done when the trees are planted. The community should have solutions like some listed below, built into construction specifications and subdivision regulations.
- There should be at least one foot (30 cm) of soil between the curb and the tree trunk at its maturity. If this space will not be available, create better soil conditions, move the tree to a better site, or select a smaller stature tree with roots that are not as invasive. In other words, plant the right tree in the right place.
- Proper tree selection near a sidewalk is critical to long-term success. The amount of space available should determine the size of the tree being planted. Here are general guidelines:
Tree Lawn Width Mature Tree Height
less than 3 feet (0.9 m) No trees
3-5 feet (1 - 1.5 m) 10-30 feet (3 - 9 m) Small trees at maturity are not always bad choices.
5-7 feet (1.5 - 2 m) 30-40 feet (9 - 12 m)
7-9 feet (2 - 3 m) 40-50 feet (12 - 15 m)
9 feet or wider (3 m+) 50 feet or taller (15 m+) Big trees need more space
- The use of recycled plastics, tire rubber crumb or other modular sidewalk systems can be an appropriate solution. These products use various mixes of rubber tire crumbs, plastic chips or stones mixed with various adhesives to form sidewalk panels. Some of these products are mixed in the field and poured directly on the walk while other products are factory made in segments. The segments are affixed to each other with epoxy or dowels and locked on the sides with anchors. If a segment has to be removed for root pruning or regrading, it can be released and lifted, then anchored back into place. These products do not expand or shrink in weather and are flame resistant. They can be hosed off when they get dirty and are resistant to most compounds except sulfuric acid or long-time exposure to oil based products. These products minimize the effect of aggressive root growth and are durable enough to last for many years longer than concrete. Tests show that after eight years with one product, there is no displacement of the sidewalk by roots. The products porosity allow water and air to pass through the walk. This feature creates an environment where the tree roots develop as fibrous roots instead of large diameter roots that would lift the sidewalk. An advantage of these products is the thin cross section. They are less than two inches thick and can be placed on top of a root if necessary to delay removal or root pruning.
These recycled pavers have other advantages. - They can be recycled. - They require little maintenance. - There is less trip-risk liability. - They reduce lawsuits caused by pedestrians injuring themselves on concrete. - They are economical and aesthetically pleasing.
- Using structural soil under a sidewalk works well. Uniformly sized stone is coated with soil and placed under the sidewalk, curb, and/or street. The large pores between each stone encourage root growth deep under the walk while the rock provides a stable base for the pavement. These structural soils require the removal of all roots in the subgrade area and are only recommended for new installations.
- A favorite of many communities is reversing the sidewalk and tree lawn locations where practical to reduce the need to root prune and provide more space for new trees. With the trees near private property, the roots have plenty of space to develop. The tree can be planted next to the lawn or on private property with permission or an easement. If the tree lawn is less than 3 feet (1 m) wide, use permission or easements to plant trees on private property a minimal distance of 3 feet (1m) beyond the sidewalk.
- New asphalt and concrete walks can be installed with options. They can both be used with new colored and stamped products. They can also use easements to run the sidewalk on private property and increase the tree lawn width. This solution also allows curved sidewalks or curbs to go around trees.
The list below indicates some less desirable ways of dealing with sidewalk problems:
- Using physical root barriers to deflect the roots downward by installing the root barrier at planting time before the roots become a problem.
- Chemically treating the asphalt and concrete walks with herbicide to retard root invasion, provided there is room for roots to grow in a better direction.
- Improving soil conditions to help tree roots grow deeper and further away from the curbs and walks.
- Removing and replacing the tree with deep rooting species such as the oaks, nuts, gums, sassafras, Japanese pagoda, hornbeam and pines.
- Removing and replacing the sidewalks, making them higher than before.
- Using Landscape pavers and bricks to replace the damaged sidewalk. As roots begin to expand,
individual pavers could easily be displaced without much damage to the entire sidewalk.
- Using expansion joints in concrete walks so if a heave occurs, that section is easily replaced after the problem is corrected.
- Drilling a hole in the lower slab and pumping concrete grout to raise the slab level with the adjoining raised slab.
- Using asphalt or concrete ramps to bridge over the problem roots. Such repairs rarely correct the problem, but they do reduce the trip risk to pedestrians.
- Grinding the raised slab to create a ramp is generally limited to slabs with less than 1 inch (2.54 cm) of displacement.
- Installing asphalt pavement over the top of exposed tree roots. Since asphalt is flexible, root expansion would occur only in the area of the root itself.
- The use of poured-in-place rubber as an alternative to rubber pavers. It is installed like concrete and comes in a variety of colors. The disadvantages are that the rubber cannot be reused and that it will not do well with vehicular traffic. Plus it starts to fail after 2 years and then it is not firm or safe. This rubber product is destroyed by tree roots and cannot be easily maintained.
- Root pruning* is the least desirable solution because the roots are cut, removed, and a sidewalk is reinstalled. However, since pruned roots will quickly regrow, the problem will come back unless a barrier is used to deflect roots down and away from the sidewalk. Root pruning can also result in root decay that is difficult to monitor and inspect. It is also a very expensive alternative. For trees 12" in trunk diameter (3 cm) or less, excavation work or root cutting should not occur closer than 3 feet (1 m) from the outer bark of the tree. Trees greater than 12" (3 cm) in trunk diameter should not be cut closer than the distance measured by the circumference of the trunk or a maximum distance of 6 feet (2 m). Soil excavation work is permitted closer if all the excavation of soil is done by hand and no roots greater than 2" (5 cm) in diameter are cut. A trained arborist should do all root pruning. All root-pruned trees should receive annual inspections to check their stability and recovery.
*If root pruning is done, the crown of the tree must be reduced to cut back on the wind sail to make the
tree safer if the branches are very thick. In this case, the pruning should be thinning cuts, and not
necessarily on the side of the tree that has had the root severance. Some trees appear to respond with
limb shedding on the same side as the root cutting and some on the opposite side of the root cutting.
Since it may be difficult to predict which branches the tree will shed in response to root cutting, it is
better to avoid pruning until necessary and return later to remove any dieback. Ideally, trees should
have their crown pruned one or two years prior to the root pruning if this pruning is going to become
necessary.
Curb Replacement
Curbs are often replaced during road resurfacing, especially if they are damaged or insufficient in height. The alternatives below will reduce the amount of root damage during the replacement process:
- The excavation bucket should be only 12 inches (30 cm) wide, for space to set forms or run a curb machine.
- The curb can be removed and replaced at a better height or location.
- The pavement can be ground down with a milling machine so a new layer of pavement can be installed without touching the curb and the curb remains as it was.
Planting Solutions
The removal and replacement of trees as an alternative to root pruning should be done in accordance with the municipality's street tree policy. Proper tree selection near a sidewalk is critical to long-term success. The amount of space available should determine the size of the tree being planted. Here are general guidelines:
The following lists includes some of the:
More Desirable Trees That Mature at 30 feet: Cultivars of these trees are also acceptable.
Acer campestre
Acer griseum
Acer tartaricum
Amelanchier
Carpinus
Chionanthus
Cornus
Koelreuteria
Maackia
Magnolia
Malus
Ostrya
Oxydendrum
Prunus
Pyrus
Trees and their Cultivars Likely to Have Surface Roots: (Cultivars of these trees are not acceptable.)
Norway Maple Acer platanoides
Red Maple Acer rubrum
Silver Maple Acer saccharinum
Ash Fraxinus spp.
Sweetgum Liquidambar styraciflua
Tuliptree Liriodendron tulipifera
Pin Oak Quercus palustris
Poplar/Cottonwood Populus spp.
Willow Salix spp.
American Elm Ulmus americana
Siberian Elm Ulmus pumila
Root Pruning
Prior to root pruning, one must first decide whether or not the tree is worth risking the effects of root cutting. During and after the examination of the tree's health and vigor, structure, shape, and balance, the information should be documented. Some trees will survive root pruning without any additional care while others will die no matter what. The wisdom lies in determining the vigor and stored carbohydrate level of those trees that have the potential for surviving the root pruning but will need follow-up care to do it. Tree inspectors must be knowledgeable in tree risk evaluations and know what to look for, especially when it comes to root examinations.
Reasons to Root Prune
Root pruning will occur in four basic situations:
Whenever a tree will have a substantial portion of its root system removed, a thorough site analysis should be performed. This would include data being taken on windfall potential, direction, and velocity. The distance and location of potential targets should be noted. The site should also be examined for drainage and excessive moisture that can play a significant role in windfall potential and root rots.
A critical concern when dealing with root pruning is whether or not the tree will survive. Arborists and landscape architects must have knowledge of the root spread, growth habit, and root aggressiveness of the species. Root pruning should be done as seldom and as far away from the trunk as possible. Low vigor trees may require substantial increases in the minimum distances required or tree decline and/or liabilities may result. Severe root pruning should only occur on one side of the tree in any one year or tree stability and anchoring will be jeopardized.
Construction Standards
When construction is occurring within 10 feet (3 m) of a tree, the following rules should apply:
If removal of the tree is required under life-threatening or related emergency situations, remove it immediately but carefully. 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, the tree should be removed. The same rules should apply when pruning roots from under a curb or sidewalk. If the tree is in excellent condition, consideration should be given to transplanting the tree.
Root pruning can be accomplished in two methods:
1. Selective root pruning – preferred: excavate the soil away from the roots and carefully select the roots to
cut and prune like the branches of a tree.
2. Non-Selective root pruning – not recommended: no real excavation required; get an effective piece of
equipment and start digging and tearing the roots from the ground.
Cuts
Roots that have been ripped and torn with backhoes typically leave a large amount of root surface exposed to pathogens and unnecessary drying. Clean cuts are an absolute necessity if quick compartmentalization of decay is desired. After pruning, it is essential that cut roots be back-filled as quickly as possible. 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 warrant extreme expediency in backfilling.
Timing
From the standpoint of wound closure and health, the best time to conduct root pruning would be just prior to active root growth which occurs in early spring before bud break and late fall in temperate climates. Root pruning should be avoided during environmentally stressful times such as droughts, floods, active bud break, and shoot growth due to the water and nutrient demands that are placed on the root system during these times.
Porous Species
Due to the differing vascular system of many species, it is important to know the difference between ring-porous and diffuse porous species of trees.
Ring porous trees (such as an elm, ash, oak, chestnut, and black locust), are angiosperms that have large diameter vessels in the first portion of the growth increment and vessels of smaller diameter later in the growth increment. The vessels of a ring porous tree are generally larger and concentrated in the outermost layer of sapwood. These vessels are produced early in the season, laid out in concentric circles, and copiously absorb water in the early growth season after which they close down. Ring porous vascular systems are very efficient, but are much more vulnerable to blockage. For many ring porous trees, severe root pruning on one side of the tree may result 'in a loss of major branches on the same side that the roots were cut rendering the tree aesthetically unsightly. In this situation, if retaining aesthetics are paramount, root pruning should not be done.
Diffuse porous species (such as a birch, maple, cherry, poplar, beech, sycamore, honey locust, tulip tree, pine, spruce, ginkgo, and fir) may also be affected from root pruning but will manifest itself throughout the entire tree. Diffuse porous trees are angiosperms that have vessels of about equal size and diameter arranged at about equal distances from each other throughout the growth increment. Diffuse porous wood has vessels, parenchyma, and fibers of about the same size arranged equally throughout the entire growth increment. These vessels are produced regularly during the growing season. They take up water during the entire growth period.
Follow-up Care
There will need to be a continued commitment for supplemental care and observation after root pruning. Maintaining adequate soil moisture, nutrition, and aeration following root pruning is critical if quick wound closure and root regeneration are desired. The duration of continued maintenance should depend on the tree's root reestablishment rate. Though the roots of many trees can grow up to 15 feet (5 m) per year under ideal conditions, this is rarely the case in most urban soils. Hence, monitoring of vigor should be conducted to determine when supplemental care is no longer needed. Whenever public trees are being severely root pruned, it is wise to provide periodic inspection for usually 1 – 5 years with twig growth and other vigor indicators used to determine adequate reestablishment.
Prior to root pruning, one must first decide whether or not the tree is worth risking the effects of root cutting. During and after the examination of the tree's health and vigor, structure, shape, and balance, the information should be documented. Some trees will survive root pruning without any additional care while others will die no matter what. The wisdom lies in determining the vigor and stored carbohydrate level of those trees that have the potential for surviving the root pruning but will need follow-up care to do it. Tree inspectors must be knowledgeable in tree risk evaluations and know what to look for, especially when it comes to root examinations.
Reasons to Root Prune
Root pruning will occur in four basic situations:
- if a tree is being transplanted,
- if construction is occurring near the tree,
- if the roots are being removed from under a curb or sidewalk,
- to develop a highly fibrous root system for nursery stock and future planting in urban soils.
Whenever a tree will have a substantial portion of its root system removed, a thorough site analysis should be performed. This would include data being taken on windfall potential, direction, and velocity. The distance and location of potential targets should be noted. The site should also be examined for drainage and excessive moisture that can play a significant role in windfall potential and root rots.
A critical concern when dealing with root pruning is whether or not the tree will survive. Arborists and landscape architects must have knowledge of the root spread, growth habit, and root aggressiveness of the species. Root pruning should be done as seldom and as far away from the trunk as possible. Low vigor trees may require substantial increases in the minimum distances required or tree decline and/or liabilities may result. Severe root pruning should only occur on one side of the tree in any one year or tree stability and anchoring will be jeopardized.
Construction Standards
When construction is occurring within 10 feet (3 m) of a tree, the following rules should apply:
- For tree trunks measuring 12 inches (30 cm) in diameter or less, soil excavation work or root cutting should not occur closer than 3 feet (1 m) from the outer bark of the tree.
- For tree trunks measuring greater than 12 inches (30 cm) in diameter, soil excavation work or root cutting should not occur closer than the distance measured by the circumference of the tree trunk or a distance of 6 feet (2 m), whichever is less.
- Soil excavation work may be done closer than the distance parameters established under the above two
categories, provided all excavation of soil is accomplished by hand or with a pneumatic soil-excavating tool and no roots greater than 2 inches (5 cm) in diameter are severed. If it is not feasible to perform any of the requirements established above, stronger consideration should be given to removing the tree.
If removal of the tree is required under life-threatening or related emergency situations, remove it immediately but carefully. 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, the tree should be removed. The same rules should apply when pruning roots from under a curb or sidewalk. If the tree is in excellent condition, consideration should be given to transplanting the tree.
Root pruning can be accomplished in two methods:
1. Selective root pruning – preferred: excavate the soil away from the roots and carefully select the roots to
cut and prune like the branches of a tree.
2. Non-Selective root pruning – not recommended: no real excavation required; get an effective piece of
equipment and start digging and tearing the roots from the ground.
Cuts
Roots that have been ripped and torn with backhoes typically leave a large amount of root surface exposed to pathogens and unnecessary drying. Clean cuts are an absolute necessity if quick compartmentalization of decay is desired. After pruning, it is essential that cut roots be back-filled as quickly as possible. 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 warrant extreme expediency in backfilling.
Timing
From the standpoint of wound closure and health, the best time to conduct root pruning would be just prior to active root growth which occurs in early spring before bud break and late fall in temperate climates. Root pruning should be avoided during environmentally stressful times such as droughts, floods, active bud break, and shoot growth due to the water and nutrient demands that are placed on the root system during these times.
Porous Species
Due to the differing vascular system of many species, it is important to know the difference between ring-porous and diffuse porous species of trees.
Ring porous trees (such as an elm, ash, oak, chestnut, and black locust), are angiosperms that have large diameter vessels in the first portion of the growth increment and vessels of smaller diameter later in the growth increment. The vessels of a ring porous tree are generally larger and concentrated in the outermost layer of sapwood. These vessels are produced early in the season, laid out in concentric circles, and copiously absorb water in the early growth season after which they close down. Ring porous vascular systems are very efficient, but are much more vulnerable to blockage. For many ring porous trees, severe root pruning on one side of the tree may result 'in a loss of major branches on the same side that the roots were cut rendering the tree aesthetically unsightly. In this situation, if retaining aesthetics are paramount, root pruning should not be done.
Diffuse porous species (such as a birch, maple, cherry, poplar, beech, sycamore, honey locust, tulip tree, pine, spruce, ginkgo, and fir) may also be affected from root pruning but will manifest itself throughout the entire tree. Diffuse porous trees are angiosperms that have vessels of about equal size and diameter arranged at about equal distances from each other throughout the growth increment. Diffuse porous wood has vessels, parenchyma, and fibers of about the same size arranged equally throughout the entire growth increment. These vessels are produced regularly during the growing season. They take up water during the entire growth period.
Follow-up Care
There will need to be a continued commitment for supplemental care and observation after root pruning. Maintaining adequate soil moisture, nutrition, and aeration following root pruning is critical if quick wound closure and root regeneration are desired. The duration of continued maintenance should depend on the tree's root reestablishment rate. Though the roots of many trees can grow up to 15 feet (5 m) per year under ideal conditions, this is rarely the case in most urban soils. Hence, monitoring of vigor should be conducted to determine when supplemental care is no longer needed. Whenever public trees are being severely root pruned, it is wise to provide periodic inspection for usually 1 – 5 years with twig growth and other vigor indicators used to determine adequate reestablishment.
Root Barriers
Root barriers are used to deflect tree roots deeper into the soil where they will cause less damage to walks and curbs. In ideal soil conditions, these barriers work very well. In poor soils, the results are not as good. Critics of root barriers insist that tree stability is jeopardized if horizontal roots are not established within 8 feet (2½ m) of the trunk. Furthermore, the buttress roots will be contorted and this can cause a premature collapse of the tree's health and vigor. Proponents of root barriers say that while this is true, it may take 20 – 30 years for the problem to become a concern. Researchers have found that barriers with vertical ribs do in fact direct root growth downward and protect hardscapes but then the roots will grow toward the surface once the barrier is cleared. Root barrier installations are most effective when installed as a preventative measure prior to the need to cut a root that has outgrown its limited soil area. The barriers are quite an improvement over costly repairs because it allows the tree roots to grow naturally, but in a specific direction.
The use of a barrier will be different for each location based on soil type and depth, species, slope, moisture, and other soil environmental factors. Each time a root is cut, a barrier should be placed to deflect the growth of new regenerated roots from the cut end. Otherwise, the roots will grow right back to the areas that they were cut from.
There are some basic types of barriers used to direct roots away from curbs and sidewalks and to encourage the roots to grow down and away from surface hardscape.
Physical Barriers
Years ago, there were many types of barriers in use such as concrete panels, sheets of plastic, tar paper, bottomless containers and chemicals. Then came the development of the 90º, raised rib molded plastic panels. They are generally made of semi-rigid plastic 60 – 80 mm thick. One material is polyethylene, which is considered to have superior durability. Another plastic material is polypropylene, which is harder, and still another is polystyrene, which is an older product that will break down when exposed to the ultraviolet light. All of these plastic barriers are built as panels and are connected together with interlocking couplings, locking strips or are held together with bonding glue. These barriers are placed into planting holes around the tree or in a linear fashion along one side of a planting between 12 – 24 inches (30 – 60 cm) deep. The vertical ribbed panel is the most commonly purchased barrier.
Geo-textile Barriers
Another type of barrier is made of nylon and may require support to stand vertically. The nylon mesh is heat fused and may droop without proper backfilling and support. The geo-textile fabrics may work, but many species' roots commonly engage or escape in the felt fabric rather than being deflected. When inserted vertically in the soil, the material acts as a root barrier and a root conductor. A thick plastic mat covered with filter cloth offers an ideal rooting surface. This product may encourage deeper rooting by creating corridors that increase oxygen availability deeper into the soil.
Chemical Barriers
Flexible fabric barriers impregnated with a chemical are an alternative to rigid barriers. These barriers typically contain herbicide that burn off the root hairs. The herbicide is Trifluralin, also called Treflan, and the fabric is Typar. Trifluralin is released as a vapor around each nodule in time-release form.
The fabric root barrier is a non-woven filter fabric with a coating of herbicide heat-bonded to the fabric. It is non-toxic and doesn’t leach into the soil. Roots are stopped at the barrier, while the fabric allows water and nutrients to pass through. It is very thin and it is glued to the hard surface by a silicon seal. Installation of the product usually requires two people. One will hold the fabric in place, while the other back-fills the trench. The product must be installed within 12 hours after opening the sealed wrapper, and the cooler the area, the longer that the product will last.
Another barrier is made of copper screen. The small holes stunt the roots but might allow some roots to penetrate. The barrier can be applied vertically, horizontally, or around the perimeter of the planting pit. Other barriers coming on the market are painted with copper hydroxide, a root growth regulator that kills root hairs. Care is required when working with the copper products to be sure the installation follows the manufacturer's instructions because the products are considered toxic.
A controlled-release chemical treatment works best because the proper dosage is released over a long period. A single application is too strong when installed and is then depleted over time. Irrigation is essential to encourage a higher density of feeder roots in exchange for an enlarged root system.
Barrier Tips
Recent discussions with educators and city arborists generated the following points:
The Pros of Root Barriers
Root barriers are used to deflect tree roots deeper into the soil where they will cause less damage to walks and curbs. In ideal soil conditions, these barriers work very well. In poor soils, the results are not as good. Critics of root barriers insist that tree stability is jeopardized if horizontal roots are not established within 8 feet (2½ m) of the trunk. Furthermore, the buttress roots will be contorted and this can cause a premature collapse of the tree's health and vigor. Proponents of root barriers say that while this is true, it may take 20 – 30 years for the problem to become a concern. Researchers have found that barriers with vertical ribs do in fact direct root growth downward and protect hardscapes but then the roots will grow toward the surface once the barrier is cleared. Root barrier installations are most effective when installed as a preventative measure prior to the need to cut a root that has outgrown its limited soil area. The barriers are quite an improvement over costly repairs because it allows the tree roots to grow naturally, but in a specific direction.
The use of a barrier will be different for each location based on soil type and depth, species, slope, moisture, and other soil environmental factors. Each time a root is cut, a barrier should be placed to deflect the growth of new regenerated roots from the cut end. Otherwise, the roots will grow right back to the areas that they were cut from.
There are some basic types of barriers used to direct roots away from curbs and sidewalks and to encourage the roots to grow down and away from surface hardscape.
Physical Barriers
Years ago, there were many types of barriers in use such as concrete panels, sheets of plastic, tar paper, bottomless containers and chemicals. Then came the development of the 90º, raised rib molded plastic panels. They are generally made of semi-rigid plastic 60 – 80 mm thick. One material is polyethylene, which is considered to have superior durability. Another plastic material is polypropylene, which is harder, and still another is polystyrene, which is an older product that will break down when exposed to the ultraviolet light. All of these plastic barriers are built as panels and are connected together with interlocking couplings, locking strips or are held together with bonding glue. These barriers are placed into planting holes around the tree or in a linear fashion along one side of a planting between 12 – 24 inches (30 – 60 cm) deep. The vertical ribbed panel is the most commonly purchased barrier.
Geo-textile Barriers
Another type of barrier is made of nylon and may require support to stand vertically. The nylon mesh is heat fused and may droop without proper backfilling and support. The geo-textile fabrics may work, but many species' roots commonly engage or escape in the felt fabric rather than being deflected. When inserted vertically in the soil, the material acts as a root barrier and a root conductor. A thick plastic mat covered with filter cloth offers an ideal rooting surface. This product may encourage deeper rooting by creating corridors that increase oxygen availability deeper into the soil.
Chemical Barriers
Flexible fabric barriers impregnated with a chemical are an alternative to rigid barriers. These barriers typically contain herbicide that burn off the root hairs. The herbicide is Trifluralin, also called Treflan, and the fabric is Typar. Trifluralin is released as a vapor around each nodule in time-release form.
The fabric root barrier is a non-woven filter fabric with a coating of herbicide heat-bonded to the fabric. It is non-toxic and doesn’t leach into the soil. Roots are stopped at the barrier, while the fabric allows water and nutrients to pass through. It is very thin and it is glued to the hard surface by a silicon seal. Installation of the product usually requires two people. One will hold the fabric in place, while the other back-fills the trench. The product must be installed within 12 hours after opening the sealed wrapper, and the cooler the area, the longer that the product will last.
Another barrier is made of copper screen. The small holes stunt the roots but might allow some roots to penetrate. The barrier can be applied vertically, horizontally, or around the perimeter of the planting pit. Other barriers coming on the market are painted with copper hydroxide, a root growth regulator that kills root hairs. Care is required when working with the copper products to be sure the installation follows the manufacturer's instructions because the products are considered toxic.
A controlled-release chemical treatment works best because the proper dosage is released over a long period. A single application is too strong when installed and is then depleted over time. Irrigation is essential to encourage a higher density of feeder roots in exchange for an enlarged root system.
Barrier Tips
Recent discussions with educators and city arborists generated the following points:
- Root barriers must extend above the soil and mulch line to prevent surface roots growing over the top. The soil should not allow the barrier to settle.
- After trenching and/or barrier installation, tree care and maintenance should be monitored to quickly aid in the adjustment.
- Water and liquid fertilizer should be applied on the inside of the disturbance area 6 – 12 months in advance if possible. This will stimulate root growth prior to the barrier installation. Root barriers can be effective in deterring and reducing pavement damage for certain species in certain soils.
- Vertical ribs are effective in deflecting roots downward.
- Often, the only suitable soil for root growth is the 3–8 foot (1–2 m) wide tree lawn next to the street. Efforts should be made to make this area as long as possible toward the next tree location so the roots can share the limited amount of soil in the tree lawn.
- Impervious barriers may block horizontal moisture movement in the soil. This may or may not be detrimental, depending on the existing soil texture, structure, compaction level, etc.
- Do not expect barriers to solve specific urban tree root problems.
- Root excavations analyzing the effectiveness of barriers in poor and shallow soils need to be conducted to determine if and when a condition of risk may develop.
- Deeper root corridors or channels may be needed for roots to be able to grow under a sidewalk.
- A long-term solution is to avoid planting potentially large trees in narrow tree lawns.
- Avoid installing fast growing species with invasive roots in shallow or poorly drained soil.
- Root barriers have been shown to be effective in deterring damage to curbs and sidewalks but in compacted clay soil, roots may return to the surface jeopardizing anchorage and stability.
- For root barriers to be most effective, proper preparation of the planting site may be critical to their long-term effectiveness, especially subsurface soil preparation. On sites where soils are suitable for root growth well below the depth of the root barrier, root barriers can reduce the damage done to urban
hardscapes, saving money and prolonging the life of the tree. - In new urban developments, educate all parties and make a case for wider tree lawns if larger trees are desired.
- To lessen hardscape conflicts, some city arborists have secured easements to install trees on private property (outside the ROW) instead of in the public tree lawn. Maintenance of the tree depends on the agreement with the homeowner and local regulations.
- The barrier must also be ultraviolet resistant as well as resistant to vandals or vehicular damage.
- Root control is easiest when the tree and the pavement are installed at the same time.
- Proper planning for the largest available root space will contribute to success.
The Pros of Root Barriers
- Root barriers have allowed cities a wider variety of street trees.
- Any of the barrier panels can be used in either linear or surround installations. The panels are installed against the hardscape, on each side of the area where the tree will be planted.
- Root barriers control the direction of root growth.
- Circling roots may be less of a problem with vertical ribbed barriers.
Root Hardiness
This section is about how root hardiness differs from tree hardiness for containerized trees. Proper acclimation is critical for plants to survive over winter. Plants develop the ability to survive winter by exposure to shortening days and lower temperatures. Plus, a plant is only as hardy as its roots and roots are much less cold hardy than the shoots. If exposed to consistently lower temperatures, without sudden damaging ups or downs, many plants are able to tolerate very cold temperatures. Cultural practices such as fertilizing, watering, and pruning stimulate late season growth and should be avoided.
Continuing attempts by nurseries to meet the need for readily available plants try to lengthen the growing season. This has led to plants being dug earlier and later than normal and more container and B & B stock is being stored outdoors over winter. These plants left exposed to the elements for even short times can suffer root injury from cold temperatures.
Although using USDA hardiness zones for evaluating the survival potential of a mature plant in a given region has been well used, it is not a very useful approach for container grown stock. The reason is that root hardiness, a key factor for over wintering container plants, does not consistently correspond to shoot hardiness. Plants with similar USDA hardiness ratings often have widely divergent root hardiness. Therefore the USDA ratings may be of little use for predicting their success in containers. For example, flowering dogwood, Cornus florida, is hardy to Zone 5, but its roots die in the range of 11° to 21° F, depending on root maturity. Juniperus conferta, with roughly similar shoot hardiness, will not start to lose roots until soil temperatures drop to around 12° F, and larger roots can survive down to about -10° F. Further, plant material derived from a warmer region may have less cold tolerance than specimens of the same species from a colder region.
All the known research done to measure root hardiness has been done with container stock and may not give exact temperatures when considering B&B stock dug and left in the field, but observations have indicated that the temperatures reported below are relatively accurate when applied as a hardiness rating of B&B stock.
Water Requirements
Desiccation is the number one stress related to freezing. Plants stored outdoors with their root balls protected but with their tops exposed can suffer wind desiccation. Reduced watering schedules can result in a failure to provide adequate water to the plants in mid-winter. Cold or frozen soils inhibit water movement to plant tops that need water on a warm winter day. Therefore, plants should be watered when put into storage and checked for water need throughout the storage period.
Desiccation is also a threat to container plants during winter. Ground temperatures below the top few inches of soil generally do not drop below the 20° to 30° F range, which many plants easily tolerate. However, in containers the soil more closely reflects air temperatures. Therefore, it gets much colder and may freeze completely. Freezing, per se, does not necessarily kill roots, but it does prevent them from taking up water.
Many evergreen conifers may tolerate winter water stress better than most broadleaf evergreens. Deciduous plants are less vulnerable, though not immune, to winter water stress. Container soil should be checked periodically and water added if necessary. Watering is important because water is being used as a source of heat. Liquid water has a temperature above 32º F and when it freezes it gives off heat. Because of this, dry soil freezes much more quickly than wet soil.
Other Hardiness Factors
Larger containers are less prone to temperature swings than smaller containers. Larger mature roots tolerate much lower temperatures than small roots. Thus, older and larger specimens should survive better than younger, smaller plants. Furthermore, containerized plants cannot take advantage of the temperature buffering effect of the soil. In a nursery, containers can be stored close together and protection can be provided on the perimeter by covering plants with mulch.
Winters vary in temperature, precipitation, snow cover, and wind, all of which can affect a plant's survival. Therefore, do not be surprised if a container plant that has made it through a few winters finally succumbs in an especially harsh one.
The key to overwintering is keeping the plants cold and alive but not actively growing. The degree of winter protection should be based on the expected minimum temperatures at a particular site and the root hardiness of the species being grown. Unfortunately, root and crown hardiness in most herbaceous perennial plants has not been clearly identified by researchers. Much of the current information is based on limited grower experience. As a result, most growers provide more protection to plants than is necessary. Until root hardiness is identified, growers must take steps to avoid losses of plants.
Preparing plants for successful overwintering begins when plants are potted. Most plants should be potted up by early October and allowed to establish themselves for several weeks (early November) prior to the ground freezing. The better the root system, the better the chance the plant will survive the winter.
Soil temperatures should be in a range of 30° to 34º F for most plants. Temperatures cooler than 30º F may kill some sensitive species. Use a soil thermometer to verify that plants are in this range. Fluctuations always occur during the winter. If the temperatures warm up for several days above 40º F then ventilation should be provided. The more that plants are allowed to freeze and then thaw, the greater the risk for plant loss.
A plant is capable of surviving even if losing most of its immature roots to winter injury, but it cannot lose its mature roots. Delayed spring growth, stunted spring growth, and death of branch tips are all indications that plants have suffered the loss of immature roots. Repeated loss of primary roots from winter injury also means that the plant's ability to recover and produce normal growth can be significantly reduced.
This chart applies to container-grown ornamentals only.
Root-killing Temperatures
Botanical Name ° F ° C
Acer palmatum 'Atropurpureum' 15 -9.4
Buxus sempervirens 27 -2.7
Cornus florida 22 -5.6
Cotoneaster adpressus var. praecox 12 -11.1
Cotoneaster congestus 25 -3.8
Cotoneaster dammeri 23 -5
Cotoneaster horizontalis 17 -8.3
Cryptomeria japonica 17 -8.3
Cytisus x praecox 16 -8.9
Daphne cneorum 23 -5
Euonymus alata 19 -7.2
Euonymus fortunei 'Carrierei' 15 -9.4
Euonymus fortunei 'Colorata' 5 -15
Euonymus fortunei var. vegeta 23 -5
Hedera helix 'Baltica' 15 -9.4
Hypericum species 23 -5
Ilex crenata 'Convexa' 23 -5
Ilex crenata ' Dazzler' 25 -4
Ilex crenata 'Helleri' 23 -5
Ilex crenata 'Hetzii' 23 -5
Ilex crenata 'Stokesii' 23 -5
Ilex glabra 16 -8.9
Ilex opaca 23 -5
Ilex 'Nellie R. Stevens' 23 -5
Ilex merserve 23 -5
Juniperus conferta 12 -11.1
Juniperus horizontalis 'Douglasii' 0 -17.8
Juniperus horizontalis 'Plumosa' 12 -11.1
Juniperus squamata 12 -11.1
Kalmia latifolia 15 -9.4
Koelreuteria paniculata 16 -8.9
Mahonia aquifolium 10 -12.2
Mahonia bealei 23 -5
Magnolia x soulangeana 23 -5
Magnolia stellata 22 -5.6
Pachysandra terminalis 15 -9.4
Picea glauca -10
P. omorika -10
Pyracantha coccinea 'Lalandei' 23 -5
Rhododendron carolinianum 0 -17.8
Rhododendron catawbiense 0 -17.8
Rhododendron prunifolium 20 -6.7
Rhododendron schlippenbachii 15 -9.4
Rhododendron "Exbury Hybrid" 17 -8.3
Rhododendron 'Gibraltar' 10 -12.2
Rhododendron 'Hinodegiri' 10 -12.2
Rhododendron 'Hino Crimson' 19 -7.2
Rhododendron 'P.J.M.' hybrids -9 -23.3
Rhododendron 'Purple Gem' 15 -9.4
Taxus x media 'Hicksii' -4
Taxus x media 'Nigra' 10 -12.2
Thuja occidentalis 10 -12.2
Viburnum carlesii 15 -9.4
Viburnum plicatum f. tomentosum 20 -6.7
Vinca minor 15 -9.4
Sources
This section is about how root hardiness differs from tree hardiness for containerized trees. Proper acclimation is critical for plants to survive over winter. Plants develop the ability to survive winter by exposure to shortening days and lower temperatures. Plus, a plant is only as hardy as its roots and roots are much less cold hardy than the shoots. If exposed to consistently lower temperatures, without sudden damaging ups or downs, many plants are able to tolerate very cold temperatures. Cultural practices such as fertilizing, watering, and pruning stimulate late season growth and should be avoided.
Continuing attempts by nurseries to meet the need for readily available plants try to lengthen the growing season. This has led to plants being dug earlier and later than normal and more container and B & B stock is being stored outdoors over winter. These plants left exposed to the elements for even short times can suffer root injury from cold temperatures.
Although using USDA hardiness zones for evaluating the survival potential of a mature plant in a given region has been well used, it is not a very useful approach for container grown stock. The reason is that root hardiness, a key factor for over wintering container plants, does not consistently correspond to shoot hardiness. Plants with similar USDA hardiness ratings often have widely divergent root hardiness. Therefore the USDA ratings may be of little use for predicting their success in containers. For example, flowering dogwood, Cornus florida, is hardy to Zone 5, but its roots die in the range of 11° to 21° F, depending on root maturity. Juniperus conferta, with roughly similar shoot hardiness, will not start to lose roots until soil temperatures drop to around 12° F, and larger roots can survive down to about -10° F. Further, plant material derived from a warmer region may have less cold tolerance than specimens of the same species from a colder region.
All the known research done to measure root hardiness has been done with container stock and may not give exact temperatures when considering B&B stock dug and left in the field, but observations have indicated that the temperatures reported below are relatively accurate when applied as a hardiness rating of B&B stock.
Water Requirements
Desiccation is the number one stress related to freezing. Plants stored outdoors with their root balls protected but with their tops exposed can suffer wind desiccation. Reduced watering schedules can result in a failure to provide adequate water to the plants in mid-winter. Cold or frozen soils inhibit water movement to plant tops that need water on a warm winter day. Therefore, plants should be watered when put into storage and checked for water need throughout the storage period.
Desiccation is also a threat to container plants during winter. Ground temperatures below the top few inches of soil generally do not drop below the 20° to 30° F range, which many plants easily tolerate. However, in containers the soil more closely reflects air temperatures. Therefore, it gets much colder and may freeze completely. Freezing, per se, does not necessarily kill roots, but it does prevent them from taking up water.
Many evergreen conifers may tolerate winter water stress better than most broadleaf evergreens. Deciduous plants are less vulnerable, though not immune, to winter water stress. Container soil should be checked periodically and water added if necessary. Watering is important because water is being used as a source of heat. Liquid water has a temperature above 32º F and when it freezes it gives off heat. Because of this, dry soil freezes much more quickly than wet soil.
Other Hardiness Factors
Larger containers are less prone to temperature swings than smaller containers. Larger mature roots tolerate much lower temperatures than small roots. Thus, older and larger specimens should survive better than younger, smaller plants. Furthermore, containerized plants cannot take advantage of the temperature buffering effect of the soil. In a nursery, containers can be stored close together and protection can be provided on the perimeter by covering plants with mulch.
Winters vary in temperature, precipitation, snow cover, and wind, all of which can affect a plant's survival. Therefore, do not be surprised if a container plant that has made it through a few winters finally succumbs in an especially harsh one.
The key to overwintering is keeping the plants cold and alive but not actively growing. The degree of winter protection should be based on the expected minimum temperatures at a particular site and the root hardiness of the species being grown. Unfortunately, root and crown hardiness in most herbaceous perennial plants has not been clearly identified by researchers. Much of the current information is based on limited grower experience. As a result, most growers provide more protection to plants than is necessary. Until root hardiness is identified, growers must take steps to avoid losses of plants.
Preparing plants for successful overwintering begins when plants are potted. Most plants should be potted up by early October and allowed to establish themselves for several weeks (early November) prior to the ground freezing. The better the root system, the better the chance the plant will survive the winter.
Soil temperatures should be in a range of 30° to 34º F for most plants. Temperatures cooler than 30º F may kill some sensitive species. Use a soil thermometer to verify that plants are in this range. Fluctuations always occur during the winter. If the temperatures warm up for several days above 40º F then ventilation should be provided. The more that plants are allowed to freeze and then thaw, the greater the risk for plant loss.
A plant is capable of surviving even if losing most of its immature roots to winter injury, but it cannot lose its mature roots. Delayed spring growth, stunted spring growth, and death of branch tips are all indications that plants have suffered the loss of immature roots. Repeated loss of primary roots from winter injury also means that the plant's ability to recover and produce normal growth can be significantly reduced.
This chart applies to container-grown ornamentals only.
Root-killing Temperatures
Botanical Name ° F ° C
Acer palmatum 'Atropurpureum' 15 -9.4
Buxus sempervirens 27 -2.7
Cornus florida 22 -5.6
Cotoneaster adpressus var. praecox 12 -11.1
Cotoneaster congestus 25 -3.8
Cotoneaster dammeri 23 -5
Cotoneaster horizontalis 17 -8.3
Cryptomeria japonica 17 -8.3
Cytisus x praecox 16 -8.9
Daphne cneorum 23 -5
Euonymus alata 19 -7.2
Euonymus fortunei 'Carrierei' 15 -9.4
Euonymus fortunei 'Colorata' 5 -15
Euonymus fortunei var. vegeta 23 -5
Hedera helix 'Baltica' 15 -9.4
Hypericum species 23 -5
Ilex crenata 'Convexa' 23 -5
Ilex crenata ' Dazzler' 25 -4
Ilex crenata 'Helleri' 23 -5
Ilex crenata 'Hetzii' 23 -5
Ilex crenata 'Stokesii' 23 -5
Ilex glabra 16 -8.9
Ilex opaca 23 -5
Ilex 'Nellie R. Stevens' 23 -5
Ilex merserve 23 -5
Juniperus conferta 12 -11.1
Juniperus horizontalis 'Douglasii' 0 -17.8
Juniperus horizontalis 'Plumosa' 12 -11.1
Juniperus squamata 12 -11.1
Kalmia latifolia 15 -9.4
Koelreuteria paniculata 16 -8.9
Mahonia aquifolium 10 -12.2
Mahonia bealei 23 -5
Magnolia x soulangeana 23 -5
Magnolia stellata 22 -5.6
Pachysandra terminalis 15 -9.4
Picea glauca -10
P. omorika -10
Pyracantha coccinea 'Lalandei' 23 -5
Rhododendron carolinianum 0 -17.8
Rhododendron catawbiense 0 -17.8
Rhododendron prunifolium 20 -6.7
Rhododendron schlippenbachii 15 -9.4
Rhododendron "Exbury Hybrid" 17 -8.3
Rhododendron 'Gibraltar' 10 -12.2
Rhododendron 'Hinodegiri' 10 -12.2
Rhododendron 'Hino Crimson' 19 -7.2
Rhododendron 'P.J.M.' hybrids -9 -23.3
Rhododendron 'Purple Gem' 15 -9.4
Taxus x media 'Hicksii' -4
Taxus x media 'Nigra' 10 -12.2
Thuja occidentalis 10 -12.2
Viburnum carlesii 15 -9.4
Viburnum plicatum f. tomentosum 20 -6.7
Vinca minor 15 -9.4
Sources
- E-mail conversations between Dave Dockter, Ed Milhous, Bruce Roberts, and Ray Morneau, 2007.
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Urban Horticulture, University of Washington, Autumn 2001. - Carlson, Chris, "Root Barriers", City Trees, Mar/Apr 1995.
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Sidewalks", Journal of Arboriculture, Vol. 28, No. 6, November 2002. - D'Amato, Nicholas E. T., Davis Sydnor, Robin Hunt, and Bert Bishop, "The Root Vs
The Crack", Journal of Arboriculture, Vol. 28, No. 6, November 2002. - Dodge, Linda and Jim Geiger, "Tree Roots and Sidewalk Damage", City Trees, The Journal of The Society of Municipal Arborists Vol. 37, No. 4, July/August, 2001.
- Dunwell, Winston C., and Robert E. McNiel, "Overwintering Nursery Crops" University of Kentucky, College of Agriculture, 2005
- Gouin, Dr. Francis R., "Better Growing" University of Maryland, College Park, August, 2005
- Havis, J. R. 1976. “Root Hardiness of Woody Ornamentals”. HortScience 11 (4):385-39.
- Jacobs, Karel, et al, "Effect of Biobarrier on Mycorrhizae", Journal of Arboriculture 26(2) 92 – 96.
- Kane, Brian & H. Dennis P. Ryan "Tree Roots and Hazard Tree Evaluation", TCI Magazine, December 2002.
- Krause, Charles, "Root Hardiness and Influence of DNA Herbicides in Overwintered Containers" USDSA Research project, Funded FY 2004 Research 2003 - 2008
- Liskey, Eric, "Researching Maintenance", Grounds Maintenance Magazine, July 1, 1998
- Mann, Gordon, "Root Pruning And Root Barriers" City Trees, May/June 1995.
- Mann, Gordon, "Sidewalks and Roots", City Trees, The Journal of The Society of Municipal Arborists Vol. 46, No. 3, May/June 2010.
- McPherson, E Gregory, "Conflicts With Street Tree Roots", Journal of Arboriculture 26(6): 289-297.
- Neely, Dan, and Gary Watson, "The Landscape Below Ground II", International Society of Arboriculture, 1998.
- Nicoll, B. C. & A. Armstrong, "Tree Roots & Pavement Damage", Arboricultural Research & Information Note 138/9 71SII N.
- Nordin, A., C. Uggla, and T. Näsholm, "Nitrogen In Pinus sylvestris", Tree Physiology, 21:5964, 2001.
- Pataky, Nancy, "Tree Root Problems", University of Illinois Extension, October 28, 1998
- Phillips, Len, Editor, "Rubbersidewalks", Online Seminars # 19, March/April 2008.
- Phillips, Len, Editor, "Structural Soils", Online Seminars # 19, March/April 2008.
- Phillips, Leonard, "Tree Nutrition", City Trees, The Journal of The Society of Municipal Arborists Vol 37, Number 6 November/December 2001.
- Randrup, Thomas B. E., Gregory McPherson, and Laurence R. Costello, "Tree Root Intrusion In Sewer Systems", Journal of Infrastructure Systems, 7:1, 2001.
- Rao, Bal, Brian Jeffers, Tom Hamlett, "Barriers To Inhibit Tree Roots", The Davey Tree Expert Company.
- Rose, Mary Ann, “Tree Roots and the Hardscape” Landscape And Nursery Dialog, The Ohio State University, December, 1996.
- Schutt, Jim, "Effect of Biobarrier by Soil & Depth", Texas Transportation Institute Study 405141.
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