Diagnosing Root and Soil Disorders on Landscape Trees
- Root function and symptoms of root/soil disorders
- Diagnosing root and soil disorders
- 1. Define the root system
- Types of roots
- Depth and spread
- Tree Protection Zone
- 2. Evaluate root spread potential
- 3. Evaluate soil compaction
- Evaluating soil compaction
- Methods to deal with compaction around trees
- 4. Evaluate planting depth
- 5. Evaluate root/shoot hormone balance
On landscape trees, symptoms of root and soil disorders are rather generic making diagnosis difficult. This CMG GardenNotes expands on Step 3, Evaluate Roots and Soil, in the Diagnosing Tree Disorders, (CMG GardenNotes #102).
Roots account for approximately 1/3 of the total biomass of a tree. The functions of tree roots include the following:
- Water and nutrient uptake
- Anchoring the plant
- Production of gibberellins, a hormone that promotes canopy growth
- Storage of photosynthates (along with the woody tissues)
Symptoms of root/soil disorders are extremely generic in nature, including the following:
- Reduction in photosynthesis
- Reduction in root growth
- Reduction in canopy growth
- Reduction in winter survival
- Reduced tolerance to other stress factors (insects, diseases, drought, etc.)
- Poor anchoring of the plant resulting in tree failure
Root, soil and water issues contribute to 80% of landscape plant problems, for example:
- Soil compaction and/or drought are the inciting factor for many contributing insects (borers) and diseases (Cytospora and other cankers).
- Soil compaction and/or hardscape features often limit root spread expressed as reduced growth and leaf scorch.
- Soil compaction reduces a tree’s tolerance to common stress factors, including drought, heat and wind, aphids, mites, and other insects.
- Over watering and drainage problems (soil compaction) are often expressed as iron chlorosis, root rots, leaf scorch, and limited growth.
- Trunk girdling roots, caused by planting too deep, is the most common cause of tree decline and death in the landscape.
Uniform stress through canopy or stress from the top down suggests root, soil, and water related problems. Diagnosis cannot be from the generic symptoms alone, but requires a more complete evaluation of the tree, its rooting system and growth. The following is a systematic approach to diagnosing root and soil disorders, based on common problems.
Root Plate – Zone of Rapid Taper
The root plate or zone of rapid taper is the primary structural roots extending out from the trunk. Roots branch readily, tapering in diameter. It is a continuation of the pipeline carrying water and nutrients from the absorbing and transport roots into the tree trunk.
The root plate is the tree’s primary support in winds up to 40 mph. Thus avoid routine digging or otherwise disturbing the soil and roots in the root plate area. Construction and hardscape features should not encroach into the root plate! When the tree fails by tipping over, often exposing the root plate, it is failure at the edge of the root plate.
As a rule of thumb, the radius of the root plate is three to six times the trunk DSH (diameter at standard height, 4.5 feet). [Figure 1]
Figure 1. Radius of root plate is 3-6 times trunk diameter at 4.5 feet high.
Transport roots serve as a continuation of the pipeline carrying water and nutrients from the absorbing (feeder) roots to the root plate root and trunk. These are the major spreading roots of the tree and follow soil oxygen gradients across the rooting area. In compacted areas (with lower soil oxygen), they will come to the surface. In soils with good structure (higher oxygen), they will be deeper. They also provide additional support to the tree in winds above 40 mph. [Figure 2]
Figure 2. Transport root provide additional support to trees in winds above 40 mph.
Transport roots are typically thumb-size in diameter, long, meandering, and with limited branching. Transport roots do not uniformly spread around the tree. Some areas may be void of roots, other heavily concentrated. In a hole dug in the rooting area, transport roots are readily observed sticking out the side. [Figure 3]
Figure 3. Transport roots are not uniformly spread around the tree. Some areas may have heavy concentrations while other areas may be void of roots.
Absorbing (feeder) roots serve the function of water and nutrient uptake. These tiny roots are found near the soil surface throughout the entire transport root area. As a rule of thumb, they would be found in the top 12 inches on soils with good tilth, and in the top four inches or less in compacted, clayey soils.
Absorbing roots have a short life, being replaced in four to five flushes of growth through Colorado’s growing season. Short-term drought stress (defined as 10 days in a research study) can turn-off growth for 1-5 weeks. While long-term drought stress (defined as 22 day in a research study) can turn-off growth for 1-2 years. With this understanding, it has clarified how to water newly planted trees (refer to CMG GardenNotes #635, Care of Recently Planted Trees).
Sinker roots follow natural openings into deeper soil, as soil oxygen levels allow. It is unknown to what extent trees actually have sinker roots in compacted soils of a landscape setting.
Sinker roots have the ability to extract water from deeper soil depths when the surface soil, with the absorbing roots, is dry. This helps explains how trees have good short-term drought resistance. It also helps explain the severe drought stress observed on trees when we have dry seasons with dry subsoil. Sinker roots also provide additional support in strong winds. [Figure 4]
Figure 4. Sinker roots follow natural cracks in the soil profile.
The tap root develops from the seed radical, being the primary root emerging from the germinating seed. Gardeners are very much aware of the tap root as they try to pull seeding maple or elm germinating as weeds in the garden.
However, beyond the seedling stage, the tap root is nonexistent on most trees. As the root system develops beyond the seedling stage, the roots grow into the root plate system due to low soil oxygen. Studies found less than 2% of landscape trees actually have a tap root. In nursery production, the tap root is cut while tiny, forcing a more branching root system that is tolerant of transplanting. [Figure 5]
Figure 5. Tap roots are rare on landscape trees.
The typical tree rooting system is shallow and wide spreading. Roots only grow with adequate levels of soil oxygen. Rooting depth and spread is a factor of 1) the tree’s genetic tolerance to soil oxygen levels and 2) soil texture and structure (actual soil oxygen levels).
It is difficult to estimate the actual depth and spread of a tree’s root system. Table 1 gives a rule of thumb. Roots will be more sparse and spreading on dryer soils, and more concentrated on moist soils. [Table 1]
Estimated Depth and Spread of a Tree's Root System
With good soil tilth
- 90-95% in top 36 inches
- 50% in top 12 inches (absorbing roots)
- Spread 2-3 times tree height and/or canopy spread
- Modified to by actual soil conditions
With compacted clayey soil
- 90-95% in top 12 inches or less
- 50% in top 4inches or less (absorbing roots)
- Spread 5+ times tree height and/or canopy spread
- Modified to by actual soil conditions
Obviously, not every root is essential for tree health. The Tree Protection Zone, TPZ (Protected Root Zone, PRZ) defines the rooting area with direct influence on tree health and vigor. The TPZ is the area of focus in tree care activities and evaluating root/soil related disorders.
To protect trees in a construction area, there should be NO grading, trenching, parking, or stock piling of materials in the TPZ. Several methods have been used to estimate the TPZ. [Figure 6]
Figure 6. The Tree Protection Zone, TPZ, defines the rooting area with direct influence on tree health and vigor.
The dripline (outer reach of branches) is often used in construction activities and by some city ordinances to define the TPZ. It may be suitable for a young tree with a broad canopy in an open lawn area. But it critically underestimates the critical rooting area for most landscape trees. – It is not recommended for landscape trees. [Figure 7]
Figure 7. TPZ by drip line method understates the rooting zone that needs protection.
In this method, tree height equals the TPZ diameter, or 40% of tree height equals the TPZ diameter. It comes from practices used in conifer forest management, but has little application to landscape trees. – It is not recommended for landscape trees.
Trunk Diameter Method
The trunk diameter is probably the best method for general use on landscape trees. Size of the TPZ is based on the diameter of the trunk, increasing as the tree ages and become less tolerant of stress factors. It may be calculated by measuring the trunk circumference or diameter at DSH (diameter at standard height, 4.5 feet). For trees with a broad canopy in an open lawn, it is approximately 40% larger in area than the drip line method. [Figure 8]
Figure 8. TPZ by trunk diameter method give a good estimation on the rooting area that needs protection.
Trunk Diameter Method by Circumference
TPZ radius = 1 feet per 2 inches of trunk circumference
- Measure the tree’s circumference at DSH (4.5 feet) in inches.
- Divide the number of inches by 2.
- This is the radius, in feet, of the TPZ.
- Circumference = 24 inches
- 24 / 2 = 12
- TPZ radius = 12 feet
Trunk Diameter Method by Diameter
TPZ radius = 1.5 feet per inch of trunk diameter at DSH
- Measure the tree’s diameter at DSH (4.5 feet) in inches.
- Multiply the diameter (in inches) by 1.5
- This is the radius, in feet, of the TPZ
- Diameter = 8 inches
- 8 x 1.5 = 12
- TPZ radius = 12 feet
Area of the TPZ
The area of the TPZ can be calculated by the formula:
[TPZ radius]2 x 3.14
For example - 12 foot radius:
12 feet X 12 feet X 3.14 = 452 square feet
Stress Tolerance and Age Method
Sometimes in construction sites, the professional arborist must very tightly define the TPZ to accommodate the construction and still provide tree protection. In this situation, the Stress Tolerance and Age Method would be used. It takes into account the following items.
- Transplant response (tolerance) of the species
- Drought tolerance of the species
- Root pruning response (tolerance) of the species
- Compartmentalization (decay resistance) of the species
- Native range – tolerance to stress outside the native ecosystem
The potential for the roots to spread is a primary consideration in evaluating a tree’s root system. The mature size, growth rate and longevity of a tree are directly related to the available rooting space. Many trees in the landscape are predisposed at planting to a short life and limited growth potential due to poor soil conditions and limited rooting space.
Figure 9 shows the relationship between root space and ultimate tree size. For example, a tree with a 16 inch diameter requires 1000 cubic feet of soil. On a compacted clayey soil, rooting depth may be restricted to 1 foot or less, requiring an 18-foot or greater radius root spread. Anything less will reduce tree size, growth rates, vigor, and longevity. [Figure 9]
Figure 9. Size of the rooting area determines ultimate size of the canopy.
Tree roots can generally cross under a sidewalk to open lawn areas beyond. The ability of roots to cross under a street depends on the road base properties. A good road base does not typically support root growth due to compaction and low soil oxygen levels.
The rooting area does not need to be rounded, but can be about any shape. Actual rooting areas are not necessarily round. Trees can share rooting space.
When roots fill the available ‘root vault’ area and cannot spread beyond, 1) root growth slows, 2) canopy growth slows, and 3) trees reach an early maturity and go into decline. Routine replacement may be necessary.
With trees, surface roots are an indication of low soil oxygen caused by soil compaction and/or overly wet soil. Soil compaction often expresses as low vigor and dieback. Soil compaction is the most common inciting factor leading to contributing factors in the decline process. (Refer to CMG GardenNotes #101, Plant Health Care, for a discussion of the PIC Cycle.)
Soil compaction is a reduction in large pore space, reducing soil oxygen levels and decreasing soil drainage. As a result, rooting depth is reduced. For additional details, refer to CMG GardenNotes #213, Managing Soil Tilth, and #215, Soil Compaction.
Primary causes of soil compaction include construction activities, foot traffic, and the impact of rain on bare soil. Soils are extremely prone to compaction when wet as the water serves as a lubricant allowing soil particles to slide closer together.
Soil compaction is somewhat difficult to evaluate. Evaluation tools include the following:
- How is the lawn? It shares the same soil conditions as the tree and may be easier to evaluate. Is the lawn thick or thin?
- Screwdriver test – How easy can a screwdriver be pushed into the soil? For this penetration type test, the soil needs to have been watered the day before.
- Soil probe – With a soil probe, evaluate soil type, texture interfaces, and rooting. For this penetration test it is better if the soil was watered the day before.
- Penetrometer – This instrument measures the amount of pressure it takes to push the probe into the soil. The colored dial sections indicate when root growth may be slowed or inhibited. For this penetration type test, the soil must be watered the day before. [Figure 10]
Figure 10. A soil penetrometer measures the compaction levels of the soil. When the dial reads in the red zone, compaction likely limits root growth.
- Shovel – Sometimes the only way to evaluate the soil is with a shovel and some hard work.
Standard methods of dealing with compaction in a garden setting (adding organic matter, cultivating the soil only when dry, and avoiding excessive tilling) do not apply to tree situations, as we do not cultivate the rooting zone.
Practices Worth Considering
- Aeration, with plugs at two inch intervals – Lawn or soil aeration is helpful for tree root oxygen levels if enough passes are made over the area to have plugs at two-inch intervals. [Figure 11]
Figure 11. Aeration helps tree roots if the plugs are two inches apart.
- Managing traffic flow – Established walks help minimize the compaction to other areas. The first time a cultivated soil is stepped on, it could return to 75% maximum compaction. The fourth time a newly cultivated soil is stepped on, it could return to 90% maximum compaction. As a point of clarification, the concern is foot traffic on a non-compacted soil. Foot traffic on a compacted soil causes little additional compaction. Soils are much more prone to compaction when wet, as the soil water acts as a lubricant allowing the soil particles to slide closer together.
- Organic mulch – A wood/bark chip mulch prevents soil compaction from foot traffic if maintained at adequate depths. For medium sized chips, the ideal depth is 3-4 inches. Less does not give the protection from compaction; more reduces soil oxygen levels.
- Soil renovation with an air spade – This is a new method used by arborists on high value trees (due to the expense). Steps include the following:
- Sod in the TPZ is removed with a sod cutter
- Organic matter is spread and mixed into the soil with an air spade. The air spade is a high pressure stream of air that cultivates the soil without cutting the roots.
- The area is covered with organic wood/bark chip mulch. It does not work to replace the sod.
Practices of Questionable Value
- Vertical mulching with an augur – The TPZ is drilled with 2” holes, typically at 12-24 inch intervals. Hole may be filled with coarse sand or organic matter. This method formerly used by arborists is currently out of favor. Long-term research finds that is does not aerate enough soil area for a significant increase in tree vigor. [Figure 12]
Figure 12. Vertical mulching
- Punching holes with a pipe, pick, or bar – This common practice by gardener is not supported by research. It compacts the soil around the punch site, thus not increasing soil oxygen levels. To be effective, the soil cores must be removed. It does not aerate enough soil area for a significant increase in tree vigor.
- Trenching – Trenches (dug between primary rooting paths) are backfilled with improved soil. This method formerly used by arborists is currently out of favor. Long-term research finds that while it improves root growth in the backfilled trenches, it does not support a long-term significant increase in overall tree vigor.
Figure 13. Current research has not found that trenching improves tree health.
- Fracturing – The soil is subjected to a high pressure release of air or water, fracturing the soil profile. It has limited effectiveness in sandy soils. It may actually increase the compaction around the fracture lines on clayey soils.
In summary, there is NO quick, easy fix for compacted soils in tree rooting areas.
Trunk girdling roots are the most common cause of tree decline and death of landscape trees. Trunk girdling roots are caused by planting the tree too deep. It may show up some twelve to twenty plus years after planting, causing decline and death of trees as they have significant growth in the landscape. Thus in evaluating the rooting system of a tree, it makes sense to evaluate the tree planting depth. [Figure 14]
Figure 14. Trunk girdling roots are caused by planting too deep. They may be below grade level.
Circling/girdling roots may also develop as trees are planted up from pot size to pot size in nursery production. They may be hidden inside the root ball.
Recently Planted Trees
Two considerations are important in evaluating the planting depth of trees, 1) depth of tree in the root ball, and 2) depth of root ball in the planting hole. [Figure 15]
Depth of tree in the root ball – Based on research, industry standards include the following:
- Generally, as least two structural roots within the top 1-3 inches of the soil surface, measured 3-4 inches from the trunk.
- For species prone to girdling roots (crabapples, green ash, hackberry, littleleaf linden, red maple, poplars, and possibly others), the top structural root should be within the top one inch of the soil surface.
Depth of root ball in planting hole – To deal with the texture interface between the root ball soil and the back fill soil, the root ball must come to the surface with NO backfill soil over the root ball. The top of the root ball on newly planted trees should rise 1-2 inches above grade (depending on root ball size). When the root ball mellows out, it will be at ground level.
On recently planted trees, the height of the root ball should be slightly above grade or at grade level after the root ball mellows out. The root ball soil should be visible on the surface with the different site soil to the sides. With a small trowel, evaluate the planting depth of the root ball in the planting hole. With a small trowel or screwdriver, evaluate the planting depth of the tree in the root ball.
Figure 15. Tree planting depth. To deal with soil texture interfaces, it is imperative the the root ball rises to the surface with no backfill over the root ball.
Recently Planted Tree, Planted Too Deep
- If the tree is stressed with poor vigor, replace the tree.
- If the tree is currently in good health:
- Live with possible consequences of slower growth and trunk girdling roots. Check for circling/girdling roots.
- Replant the tree. However, this would be very labor intense!
- Dig around the tree exposing the root ball.
- Wrap the root ball in burlap and twine to hold it together.
- Lift the root ball from the hole.
- Replant at correct depth.
Established Trees Planted Too Deep
The lack of a visible root flare is an indication of planting too deep (or that soil has been added over the root system). If the root flare is not visible, check for trunk circling/girdling roots. Circling/girdling roots may be several inches below ground.
Circling roots not embedded into the trunk should be cut and removed. For girdling roots putting pressure on the trunk, cut and remove the root without causing injury to the trunk. The tree will likely recover without any long-term effects.
For girdling roots embedded into the trunk, cut the root without causing injury to the trunk, if possible. However, do not remove the girdling root section if it is embedded into the trunk, as this opens the trunk to decay and the trunk will be structurally weak. The tree may or may not survive; only time will tell.
Auxins (plant hormones produced in the twig’s terminal buds) stimulate root growth. Gibberellins (plant hormones produced in the root tips) stimulate canopy growth. The tree balances root growth versus canopy growth by these hormones. [Figure 16]
Figure 16. A tree balance canopy and root growth with by the concentrations of auxins and gibberelins.
Soil factors that limit root growth will in-turn influence canopy growth.
Storm damage or excessive pruning may reduce auxins, slowing root growth. Following storm damage, trees often put on heavy growth of water sprouts due to a low auxins to high gibberellins ratio, (coupled with unobserved, limited root growth). In the following years, trees decline due to the lack of root regeneration that first years after storm damage. The decline may last several years before the tree rebalances the root/shoot growth.
CMG GardensNotes on Diagnostics
- Diagnosing Soil and Root Disorders of Landscape Trees, #103
- Diagnosing Tree Disorders, #102
- IPM: Plant Health Care, #101
- References and Study Questions: The Diagnostic Process, #100
- Homework: Diagnosing Abiotic Tree Disorders, #107
- Homework: Diagnosing Insects of Trees: Ips and MPB, #110
- Homework: Plant Health Care and the Diagnostic Process, #105
- Worksheet: Diagnosing Insects and Diseases of Trees, #108
- Worksheet: Pest Management Options, #109
- Worksheet: Step 2 in the Diagnostic Process, #104
- Worksheet: Tree Root Spread, #106
- Aspen: A Guide to Common Problems in Colorado. CSU Extension Publication 559A. 1996. $5.00.
- Abiotic Disorders of Landscape Plants: A Diagnostic Guide. University of California Agriculture and Natural Resources Publication 3420. 2004. ISBN: 1-879906-58-9.
- Insects and Diseases of Woody Plants of the Central Rockies. Colorado State University Extension Bulleting 506A. 2004. $40.00
- Plant Health Care for Woody Plants. University of Illinois Cooperative Extension. 1997. ISBN: 1-883097-17-1.
- CMG GardenNotes are available online at www.cmg.colostate.edu
- Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
- Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating
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- No endorsements of products mentioned is intended nor is criticism implied of products not mentioned.
- Copyright. 2003-15. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.
Revised October 2014