Feature

Preventing Girdling Roots

How a Healthy Root System Serves as the Heart and Backbone for a Well-Established Plant

By Amy Dismukes, Tennessee State University Extension Area Specialist

Growers and landscapers are existentially aware of plant roots, but many may not fully appreciate the importance of these vital resources. Plant root functions are comparable to the services provided by our own arteries and veins. The consequences of root rot and vessel blockage are just as significant to plants as they are to each of us. Roots absorb and transfer moisture and minerals throughout the plant, as well as provide support for the above ground portion of the plant or tree, hence their comparable role as heart and backbone. There is a fine balance between the root system and the top of the tree; if part of the root system dies, a part of the crown may also die (Perry 1982). If roots are not present or cannot function as intended, the tree will eventually die.

Not All Roots are Created Equal

There are two general categories of tree roots, woody structural roots and non-woody feeder roots. The health and well-being of both are imperative for maintaining a healthy canopy and plant. In fact, unimpeded root function is as important to photosynthesis and plant performance as is health of the leaves (Perry 1982). Woody structural roots are large lateral roots that form near the root collar at the base of the trunk, contribute to the structural support framework, and help anchor the tree. The abundance and arrangement of these roots will be different for each tree species (Perry 1982). Woody structural roots also serve as a carbohydrate sink and provide a location to store the carbohydrates generated from photosynthesis for future use by the tree. Woody roots generally can be found 8 to 12 inches below the soil surface and these roots can extend well past the drip line of the tree. In mature landscape trees, woody roots may become visible above the soil line. In trees that form taproots, which are thick, long roots that penetrate straight down into the ground, the taproot may be among the longest woody roots. For oaks growing in optimal soil conditions, the taproot may extend nearly as deep as the tree is tall. Not all tree species form a taproot, which may make these species less stable in heavy winds, light soils, or if a root rot fungus becomes established.

Non-woody roots, also known as feeder roots, are fibrous to hairlike roots that extend away from the sides of larger, woody roots. Feeder roots penetrate into and spread mostly through the upper soil surface and may emerge into decomposing leaf litter, or humus, when this organic layer is present. Feeder roots make up the largest surface area of the tree root system. These fine roots are critical for tree growth and development. Fine roots connect to the leaves through an elaborate plumbing system that extends across larger transport roots, trunk(s), branches and twigs. Nonwoody roots are much smaller in diameter than woody structural roots, yet they perform critical functions for the tree. Feeder roots absorb water and nutrients from the soil and then transport those resources to the tree; similar to the way that your heart pumps oxygen, nutrients, and blood, feeding your body. Most trees also have root hairs that extend away from the feeder roots, further increasing the root surface area and vastly enhancing the nutrient and water uptake capability of the tree. If the soil conditions are healthy, the surface area covered by feeder roots can grow far beyond the drip line, often extending out as far as the tree is tall.

Healthy Roots are Happy Roots

All roots require oxygen, water, space, and food to survive. Changes in soil depth around established trees can cause injury to root systems by inhibiting access to these resources. By understanding root-system basics, we can also understand why the addition of soil over a root system drastically reduces the amount of oxygen, nutrients and water that are available to the roots, thereby inhibiting root growth. When root growth is inhibited, damage will become visible in the tissue above ground. Root inhibition is also commonly caused by roots that girdle or encircle each other. Roots that girdle the main trunk can eventually expand laterally to kill the tree.

When Roots Go Wrong… What are Girdling Roots?

Girdling roots are lateral woody roots that emerge at or slightly below the soil surface and impose upon at least one side of the main trunk and impede root functioning. As the tree grows, the increase of secondary wood production increases both the root and stem diameters. As these two portions squeeze against each other, movement of water and nutrients to the leaves is restricted (PHOTO 1). Affected trunks and stems will eventually become weakened. Bark above girdled roots may split or crack (PHOTO 2). The tree may die from the girdling roots alone, or in conjunction with environmental stressors or insect pest and plant disease activity. Dieback due to circling or girdling roots may first become evident after a severe drought. Girdling roots may grow around or constrain other roots as well, but trees may survive loss of a lateral root if others nearby continue to provide anchorage. A girdled taproot may result in loss of tree stability across time. Cultural practices like fertilization, irrigation, and pruning will not offset the slow growth and premature tree death caused by girdling roots. The level of threat imposed by a girdling root depends on two major factors: the size (diameter) of the root, and the corresponding amount of trunk or stem circumference and crown being impacted.

PHOTO 1: Root girdled trunk

PHOTO 2: Pressure ridges and cracks in bark can result from forces imposed by girdling roots below ground

Symptoms and Diagnosis of Girdling Roots

One of the most obvious symptoms of root girdling is a visibly flattened or depressed trunk, which may occur on one or multiple sides of the shrub or tree (PHOTO 3). Non-girdled trees rarely show this abnormal development. The appearance of this condition indicates that something is pressing on the trunk, the cause of which may be hidden below ground. When properly planted, trunks of trees flare out where the main stem enters the ground (PHOTO 4). Girdling roots will prevent the collar flare. There are many other symptoms of girdling roots that may be observed in affected trees (TABLE 1). Many of these symptoms can also be characteristic of other factors including drought, excessive water, and nutrient imbalances. The only sure way to determine if a girdling root is the cause of a problem is to examine the root system and its relationship to the tree trunk. Sometimes this requires excavation or use of an air spade. But the best practical option is to closely examine the tree roots at the time of transplanting, before the tree is ever installed.

PHOTO 3: The flattened left side of this tulip poplar trunk is due to root girdling.

PHOTO 4: Oak crown flare

TABLE 1:

Trees and shrubs may demonstrate that there are problems related to girdling roots in several different ways. Many of these symptoms also are induced by other problems, so excavation and examination of the area around the main trunk just beneath the soil may be necessary for accurate diagnosis. Symptoms of circling / girdling roots can include:

• leaf scorch

• premature fall coloration

• early leaf drop

• foliar decline or discoloration that may be restricted to one or a few branches

• abnormally small leaf size

• excessive twig dieback

• an appearance of large, dead, or leafless branches (flagging) in the crown

• a thinning crown

• overall tree stunting

• loss of structural integrity / tree leaning

• increased susceptibility to environmental extremes and other biotic problems

NATURE vs NURTURE? Balancing Genetic Tendencies with Cultural Best Practices

Many tree roots, like those of pine, magnolia, and sugar, red and Norway maples (TABLE 2) tend to circle naturally. For other plant species, cultural management and poor growing practices can lead to situations that enhance the likelihood of girdling roots. Their formation can be triggered by delays in nursery and transplanting practices. If held too long in liner pots and containers, roots may begin to circle, eventually leading to girdling and death of the tree (PHOTO 5). Poor planting techniques may also cause roots to girdle. For example, burying root collar tissue by planting too deeply, or by placing too much soil or mulch on top of the root can lead to girdling roots. Rather than providing a protective blanket of soil, when root systems are buried, less oxygen is available and less water may be able to reach the roots. Soon after planting too deeply, or mulching excessively (e.g., volcano mulching), roots will begin to grow towards the surface of the soil and may begin to encircle the trunk (PHOTO 6). In this way, the more deeply buried the roots are, the fewer healthy roots will be available to feed and anchor the tree, leading to slow growth and failure to establish. Additional practices also can adversely affect natural growth. These include planting trees and shrubs in a hole that is too small so that roots cannot easily spread out, planting container-grown trees that have roots growing in a circular pattern, and planting a bare root tree by improperly twisting roots to squeeze them into a small hole.

TABLE 2:

Trees and Shrubs that are recognized 1 to frequently develop girdling roots, particularly when liners and plants are started or grown in containers without timely transplanting.

Maples (Acer spp.)

Hackberry and sugarberry (Celtis spp.)

Dogwood (Cornus spp.)

Gingko (Gingko biloba)

Honeylocust (Gleditsia triacanthos)

Holly (Ilex spp.)

Juniper (Juniperus spp.)

Sweetgum (Liquidambar styraciflua)

Crapemyrtle (Lagerstroemia indica)

Magnolia (Magnolia spp.)

Crabapple (Malus spp.)

Mulberry (Morus spp.)

Black Gum / Tupelo (Nyssa sylvatica)

Spruce (Picea spp.)

Pines (Pinus spp.)

Poplar (Populus spp.)

Cherry (Prunus spp.)

Callery pear (Pyrus calleryana)

Oaks (Quercus spp.)

Common Baldcypress (Taxodium distichum)

Linden (Tilia spp.)

Hemlock (Tsuga canadensis)

Elm (Ulmus spp.)

Zelkova (Zelkova spp.)

1 This list is largely adapted from “A Practitioner’s Guide to Stem Girdling Roots of Trees”, Johnson & Hauer, 2000.

PHOTO 5: Girdling roots above ground

PHOTO 6: Root system of river birch held in container too long

Proper Prior Planning and Problem Prevention

To prevent girdling, it is easiest and far less expensive to closely inspect a tree or shrub’s root system at time of planting. Bare-root plants are a great option to eliminate girdling roots at time of planting. If a planting hole is not dug widely or deeply enough, installers may twist plant roots into the hole in order to make the plant fit, rather than fixing the planting hole size. This behavior can cause otherwise well-distributed roots to encircle the trunk, eventually growing into girdling roots. Another major cause of girdling roots is planting trees and shrubs into very compacted soil, from which the new roots have difficulty growing away from the planting hole and into the surrounding hard soil. Roots can circle at the bottom of a hole dug in compacted soils in a manner that is similar to being grown in an undersized container. Eventually, several of these roots can begin girdling the trunk. The root system of bare-root plants must be protected and kept moist if not installed immediately. Soak roots in water, for up to 24 hours for large root balls, before planting.

Container-grown nursery plants may require special handling to ensure proper growth and survival. Plants should be watered before attempting to remove from the container around the roots. For container grown red maple, live oak, and other tropical trees, using a sharp digging spade to shave an inch of roots from the bottom and around outer edge of the root/substrate mix improved root quality (Gilman et al. 2010). For roots that may not shave easily, look for and cut any visible girdling roots using a sharp, sanitized knife. Make an additional three or four vertical, ¼-inch deep cuts down the outside of the root system to stimulate new growth (Klingeman, et al. 2008). Loosen the root ball to open newly exposed roots to the soil. Circling roots that are two or more years old will be woody and may have to be cut and removed from the root system.

Another major cause of girdling roots is planting deep (PHOTO 7) or in compacted soil in which the new roots have difficulty growing. Roots can circle at the bottom of the planting hole, not unlike those growing in an undersized container. Eventually, several of these roots can begin girdling the trunk. Deep planting and volcano mulch have also played their own role in the development of circling roots on many different tree species in landscapes.

PHOTO 7: Braided and twisted roots of tulip poplar, resulting in swollen trunk (planted too deep)

There is a standing debate about whether to remove B&B materials (i.e., the wire basket, burlap, pinning nails, ties, etc.), or if the wire basket and burlap can be left in place while setting the plant. A good recommendation at time of transplanting is to: 1. Untie and remove any nylon or webbing ties from around the trunk of the tree; 2. Remove pinning nails; 3. Fold burlap down around the root ball; 4. Remove wire basket if the ball is small. Remove old burlap down around the root ball; 4. Remove wire basket if the ball is small. It may not be necessary to remove the basket from larger plants, however, it is good practice to use wire cutters to remove and discard the top section of the basket ring (Klingeman, et al. 2008).

In sum, girdling roots are for the most part preventable. Landscape installers can become familiar with the natural tendencies of the tree and shrub species that they manage. Professionals will also benefit by looking for challenging soils conditions, and closely inspecting (and fixing) girdled, challenged, or compromised root systems at the time of transplanting and installing plants.

References & Additional Reading:

Gilman, E.F., M. Paz, and C. Harchik. 2010. J. Envir. Hort. 28(1):13-18. https://hort.ifas.ufl.edu/woody/documents/articles/EFG0901.pdf

Johnson, Gary R. and Hauer, Richard J. (2000). A Practitioner’s Guide to Stem Girdling Roots of Trees. Univ. MN Ext. Serv. https://hdl.handle. net/11299/49810 .

Perry, T.O. (1982). The Ecology of Tree Roots and the Practical Significance Thereof. Journal of Arboriculture 8(8): 197-211. https:// joa.isa-arbor.com/articles.asp?JournalID=1&VolumeID=8&IssueID=8

Klingeman, W.E., A. Campbell, F. Priestly and L. Knapp. 2008. Best Management Practices for Planting Ornamental Plants. UT Extension Pub. PB 1621 & 1621-S (in Spanish), https://extension.tennessee.edu/ publications/Pages/default.aspx.