Symptoms of root and soil disorders on landscape trees are often non-specific, making diagnosis difficult. This CMG GardenNotes expands on CMG GardenNotes #112 Systemic Plant Evaluation, step 3 – Soil and Rooting Area.
Root Function and Symptoms of Root/Soil Disorders
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 non-specific 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 a large portion 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, which is expressed as reduced growth and leaf scorch.
- Soil compaction reduces trees’ tolerance to common stress factors, including drought, heat and wind, aphids, mites, and other insects.
- Overwatering 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.
Diagnosing Root and Soil Disorders
Uniform stress throughout the canopy or stress from the top down suggests related root, soil, and water problems. Diagnosis cannot be from these 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.
1. Define the Root System
Root Plate – Zone of Rapid Taper
The root plate or zone of rapid taper consists of the primary structural roots extending outward from the trunk. Roots branch readily, tapering in diameter. It is a continuation of the pipeline carrying water and nutrients from the absorbing and transporting roots into the tree trunk. [Figure 1]
The root plate is the tree’s primary support in winds up to 40 mph. Avoid 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 DBH (Diameter at Breast Height, 4 ½ feet).
Transport Roots
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]
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, others heavily concentrated. In a hole dug in the rooting area, transport roots are readily observed sticking out the side. [Figure 3]
Absorbing Roots
Absorbing (feeder) roots serves the function of water and nutrient uptake. These tiny roots are found near the soil surface throughout the entire transport rooting area. As a rule of thumb, they are found in the top 12 inches on soils with good tilth, and in the top four inches or less in compacted, clayey soils. [Figure 2]
Absorbing roots have a short life, being replaced in four to five flushes of growth through Colorado’s growing season. A short-term drought stress (defined as ten days) can shut down growth for one to five weeks. Long-term drought stress (defined as twenty-two days) can shut down growth for one to two years. Refer to CMG GardenNotes #635, Care of Recently Planted Trees.
Sinker Roots
Sinker roots follow natural openings into deeper soil as soil oxygen levels allow. It is unknown to what extent trees have sinker roots in the compacted soils of a landscape setting.
Sinker roots can extract water from deeper soil depths when the surface soil is dry. This ability helps explain how trees have good short-term drought resistance. It also helps explain the severe drought stress observed on trees when there are dry seasons with dry subsoil. Sinker roots also provide additional support in strong winds. [Figure 4]
Tap Root
The tap root develops from the seed radicle, being the primary root emerging from the germinating seed. Gardeners become aware of the tap root when they try to pull seeding maples or elms 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 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.
Depth and Spread
The typical tree rooting system is shallow and wide spreading because roots only grow with adequate levels of soil oxygen. Rooting depth and spread are a factor related to 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 shows a ‘rule of thumb’ on root spread. Roots will be sparser and spreading in dry soils, and more concentrated in moist soils. [Table 1]
Table 1. Estimated Depth and Spread of a Tree’s Root System
| Good Soil Tilth | Compacted or Clayey Soil |
|---|---|
| 90-95% in top 36 inches 50% in top 12 inches (absorbing roots) Spread 2-3 time tree height and/or canopy spread Modified by actual soil conditions | 90-95% in top 12 inches or less 50% in top 4 inches or less (absorbing roots) Spread five or more times tree height and/or canopy spread |
Tree Protection Zone/Protected Root Zone
The Tree Protection Zone, TPZ (Protected Root Zone, PRZ) defines the rooting area with direct influence on tree health and vigor. Not every root is essential for tree health. 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.
Dripline Method
The drip line is the rooting area defined by the outer reach
of the branches. The drip line 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 significantly underestimates the critical rooting area for most landscape trees.
Trunk Diameter Method
The trunk diameter is probably the better method for use on landscape trees. The size of the TPZ is based on the diameter of the trunk, increasing as the tree ages. It may be calculated by measuring the trunk circumference or diameter at DBH (Diameter at Breast Height, 4 ½ feet). For trees with a broad canopy in an open lawn, it is approximately 40% larger in area than the dripline method. [Figure 5]
| Trunk Diameter Method by Circumference TPZ radius = 1 feet per 2 inches of trunk circumference 1. Measure the tree’s circumference at DBH (4 ½ feet) in inches. 2. Divide the number of inches by 2. For example: 1. Circumference = 24 inches 2. 24 / 2 = 12 3. TPZ radius = 12 feet |
| Area of the TPZ The area of the TPZ can be calculated by the formula: [TPZ radius] 2 × π For example – 12-foot radius: 12 feet × 12 feet × 3.14 = 452 square feet |
2. Evaluate Root Spread Potential
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. [Table 2]
Table 2 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. In a compacted clayey soil, rooting depth may be restricted to one foot or less, requiring an 18-foot or greater radius root spread. Anything less will reduce tree size, growth rates, vigor, and longevity.
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. 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.
Table 2. The relationship between tree size and the required soil volume to accommodate its root system.
| Ultimate Tree Size (DBH) | Soil Volume Required (cubic feet) |
|---|---|
| 4” | 200 |
| 8” | 400 |
| 12” | 580 |
| 16” | 1000 |
| 20” | 1200 |
3. Evaluating Soil Compaction
Surface roots of trees are an indication of low soil oxygen caused by soil compaction and/or overly wet soil. Soil compaction is often expressed as low vigor and dieback symptoms. Soil compaction is the most common inciting factor leading to contributing factors in the decline process. Refer to CMG GardenNotes #101, IPM and Plant Health Care, for a discussion on Predisposing, Inciting, and Contributing factors, known as 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 GN #213, Managing Soil Tilth: Texture, Structure, and Pore Space, and GN #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 difficult to evaluate. Evaluation tools include the following:
- Look at 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 easily can a screwdriver be pushed into the soil? For this test, the soil needs to have been watered the day before.
- Soil probe – With a soil probe, evaluate soil type, texture interfaces, and rooting. It is best if the soil was watered the day before performing this test.
- Shovel – Sometimes the only way to evaluate the soil is with a shovel and some hard work.
- Penetrometer – This instrument measures the amount of pressure it takes to push the probe into the soil. Typically only used by professional landscaping companies.
Methods to Deal with Compaction Around Trees
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.
- Managing traffic flow – Established walks help minimize the compaction to other areas. The first time a cultivated soil is stepped on, it can return to 75% maximum compaction. The fourth time a newly cultivated soil is stepped on it could return to 90% maximum compaction. Foot traffic on 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. When using medium sized chips, the ideal depth is three to four inches. Less does not give protection from compaction; more reduces soil oxygen levels.
- Soil renovation with an air spade – This method is 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.
Practices of Questionable Value
- Vertical mulching with an auger – The TPZ is drilled with two-inch holes, typically at twelve-to-twenty-four-inch intervals. The hole may be filled with coarse sand or organic matter. Research has found that this practice 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. Research has found that while it improves root growth in the backfilled trenches, it does not support a long-term significant increase in overall tree vigor.
- Punching holes with a pipe, pick, or bar – This practice compacts the soil around the punch site and does not increase soil oxygen levels. It does not aerate enough soil area for a significant increase in tree vigor. To be effective, the soil cores must be removed.
- 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 increase the compaction around the fracture lines in clay soils.
In summary, there is NO quick, easy fix for compacted soils in tree rooting areas.
4. Evaluate Planting Depth
Trunk girdling roots are a 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 after they have significant growth. In evaluating the rooting system of a tree, it makes sense to evaluate the tree planting depth. [Figure 6]
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.
For additional information on tree planting, refer to CMG GardenNotes #633, The Science of Planting Trees.
Recently Planted Trees
On recently planted trees, the height of the root ball should be slightly above grade or at grade level after the root ball settles. The root ball soil should be visible on the surface with the 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.
Two considerations are important in evaluating the planting depth of trees: the depth of tree in the root ball, and the depth of root ball in the planting hole.
Depth of tree in the root ball – Industry standards include the following:
- Generally, at least two structural roots should be within the top one to three inches of the soil surface, measured three to four inches from the trunk.
- On 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 one to two inches above grade (depending on root ball size). When the root ball settles, it will be at ground level.
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 and watch for circling/girdling roots.
- Replant the tree:
- 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. This will be difficult to do!
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.
When girdling roots are 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.
5. Evaluate Root/Shoot Hormone Balance
Auxins (plant hormones) produced in the twig’s terminal buds stimulate root growth. Gibberellins (plant hormones) produced in the root tip stimulate canopy growth. The tree balances root growth versus canopy growth by these hormones.
Soil factors that limit root growth will influence canopy growth.
Storm damage or excessive pruning may reduce auxins, slowing root growth. Following storm damage, trees often develop a large amount of water sprout growth due to a low auxin/high gibberellin ratio (coupled with unobserved, limited root growth). This is followed by a decline in the canopy caused by the reduced root growth.
This publication, reference GardenNotes #113, is developed as part of the Colorado State University Extension Master Gardener Program.