A girdling root is one that circles the base of a tree at or just below the surface. Girdling roots can also girdle other roots, but without harm. The most commonly affected species are maples, lindens, and ashes.
The threat of a girdling root depends on two major factors, the size of the root and the amount of circumference affected on the tree. The expansion of the root and the stem squeezes them against each other, interfering with the transport of water, nutrients, sugars, and other necessary compounds. Eventually, there may be damage which reduces vigor, and often leads eventually to whole stem failure.
The only sure sign is to see a root circling the main stem at or crossing other roots at or just below the soil surface. Typically, roots should grow laterally, away from the trunk. Poor root systems grow horizontally, across each other and the main stem at the root flare. Look for a crossing network of roots near the trunk or larger, woody roots growing horizontally. It may be helpful to gently displace the soil at the tree base to see if a circling root lies just under the soil surface.
Common symptoms include one side of the trunk flattened, marginal and/or tip scorching on the leaves, crown discoloration or dieback. However, many of these symptoms can be caused by other factors such as root damage, flooding, or wilt diseases.
Treatment should be conducted by a qualified, International Society of Arboriculture Certified Arborist who understands the proper protocol for root surgery. This is done by the use of a saw or chisel to sever the girdling root. Arborists who remove stem girdling roots as part of their regular tree care program report that roots can be removed until their total cross sectional area is 25 percent of the trunk cross sectional area at 4.5 feet from the ground. Root severance is very serious work that requires knowledge and experience but can be done successfully in the right situations, and especially prior to worsening conditions.
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When Tree Roots Surface, Purdue Yard & Garden
Lindsey Purcell, Urban Forestry Specialist
Department of Forestry and Natural Resources, Purdue University