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Extension > Garden > Yard and Garden > Trees and shrubs > A practioner's guide to stem girdling roots of trees > Root collar examinations

Root collar examinations

Gary R. Johnson
University of Minnesota

Richard J. Hauer
Minnesota Department of Agriculture

Copyright © 2002 Regents of the University of Minnesota. All rights reserved.

Normal vs. Abnormal Root Systems
Root collar examinations are used to determine if root system abnormalities are impacting a tree. To determine if a root system abnormality exists, one needs to compare against a normal root system. Field observation, along with a review of species-specific root system profiles, will help provide the practitioner with an understanding of a normal root system. An easy way to observe normal root systems is to take a walk in the woods. But in general terms, what is a normal or ideal root system?

Normal root systems are often described as having main (first order) laterals that radiate from all sides of the stem/root interface (Figure 23). The number of main laterals ranges from a few to more than a dozen. Trees in forests with a greater number of main laterals than other trees tend to become dominant survivors in a competitive forest community (Kormanik 1986). The root diameter of main order laterals decreases rapidly through the zone of rapid taper into ropelike roots with approximate diameters of 0.5 to 3 inches. Root spread is usually well beyond the drip line, commonly to about three times the branch spread (Gilman 1997).

Fig. 23 - A normal littleleaf linden's root system, showing the larger, main order roots radiating out from the stem/root interface (root collar).

Most main-order laterals originate from the root collar and parallel the soil surface at depths of a few inches to a foot or more. Many tree species also produce oblique roots, which grow at a sharp angle into the soil and stabilize trees. Sinker roots grow downward from lateral roots on approximately 75% of tree species, function in support and absorption, and usually are located within 6 to 10 feet of the stem.

From main-order laterals arise secondary and tertiary woody and nonwoody roots, which magnify the absorption of water and nutrients. These roots proliferate in zones of favorable moisture and nutrition. Most exist within the top foot of the soil surface.

Stem diameter normally increases from the top downward. Root flares and/or stem tapers are common (except in some conifers) due to a growth pattern in which growth is greater on the top of the root than the bottom. When trees are planted deep or soil fill is placed over the root system, this characteristic pattern might not be visible.

Fig. 25 - Abnormal root systems such as these often develop through propagation and or cultural practices.

In landscape trees, abnormal root systems often develop through cultural practices (Figure 25). Nursery production methods such as the use of containers that induce encircling roots, propagation that encourages stem-origin adventitious roots, or tillage practices such as "hilling-up" plowing to control weeds can create abnormal root systems, as can planting practices such as deep planting, narrow planting holes, and confined planting locations such as small planters. Tree vitality and longevity is not always harmed by production and planting methods. In some forest plantations developed from seedlings with abnormal root systems, seedlings successfully developed into mature trees because of adventitious root development and fusion of the abnormal root system into a central mass (Van Eerden and Kinghorn 1978). In landscape trees a clear picture is also lacking, but the development of a new root system can overcome an existing abnormal root system (Gilman 1997). Regardless of the rules and exceptions, an examination of the root system and its relationship to stem condition is a vital aspect of tree health evaluation, problem diagnosis, and assessment of root system condition.

Performing the Examination
A root collar examination typically takes from fewer than 20 minutes for smaller trees and less invasive examinations, to more than two hours for larger trees or more extensive operations. Equipment can range from simple and basic-trowels, knives, pruners, stiff brushes, saws, and shovels-to elaborate and specialized-tile probes, portable generators, air excavation (Smiley 1999b), wet/dry vacuums, wood gouges, chisels, and water.

Most diagnostic examinations need not be extensive. Begin by probing into the soil near the trunk flare with a 3/8-inch-diameter tile probe or stiff wire (coat hanger gauge) to detect the depth of branch and encircling roots and to determine the soil area around the tree stem you need to remove (Figure 28). For the average size landscape tree (9 to15 inches d.b.h.) with roots 6 to 10 inches from the surface, a 12- to18-inch-wide examination area is usually sufficient. If primary branch roots are deeper than that, you will need to widen the examination area as the examination progresses (Figure 29).

Fig. 28 - Probing the soil area near the root flare can help determine the extent (depth and width) of the examination and if it's warranted.

If sod surrounds the trunk, strip it away to a depth of approximately 2 inches. If the tree is mulched, carefully remove the mulch and any plastic or fabric ground cover beneath the mulch. Loosen the soil gently in the examination area with a trowel, hand cultivator, or knife and remove with a trowel or wet/dry vacuum. Do not use spades or shovels unless absolutely certain that no roots exist in the excavated soil. Gradually loosen and remove deeper layers of soil until the stem/root conflict or the root collar flare is exposed (Figure 30).

Fig. 29 - The examination area for this 7.5-inch d.b.h. sugar maple was 11 inches deep and 30 inches wide. Total time involved with a vacuum was approximately 30 minutes.

You can also use compressed air or water to expose tree roots. Air is blown at the soil through a tool called an Air Spade® to expose the roots. Water under low to high pressure has been used for more than 50 years to expose roots.

Fig. 30 - A small towel and vacuum may be used to loosen and remove soil during a nondestructive examination.

Choose a method based on availability of equipment, available time, and your objectives. Shallow examinations of smaller trees do not warrant elaborate equipment, such as vacuums and generators. However, with larger trees and examination areas, vacuums and portable generators are much more efficient and safe. Vacuuming loosened or water-saturated soil is less destructive to roots, reduces the chances of cutting through utility cables, and offers a clearer view of the root/stem conflict area.

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