“One of my shade trees appears to be losing vigor, and I am worried about its future. A neighbor thinks it is planted too deeply and may have girdling roots. Is there something I can do to help this tree?”
— Ed Smith, Palos Hills
Trees are often planted too deep, which could be one of the issues affecting your tree’s vigor. You should be able to see the trunk flare of your tree. The trunk flare is the area at the base of a tree where the stem or trunk broadens to form roots. This zone is often also referred to as the root collar, root crown and root flare.
Trees planted too deep look like a telephone pole coming out of the ground and are easy to identify in the landscape. They often develop girdling roots.
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Girdling roots are a common problem and can restrict the flow of water and nutrients over time, leading to a tree’s decline and, occasionally, death. Symptoms include a reduced growth rate, undersized leaves and sparse foliage, chlorotic leaves, early fall color and leaf drop, late leaf emergence in spring and branch dieback in the tree’s crown.
Symptoms can take several years to appear, especially if mature trees are mulched too deeply or have had soil filled in around them. Decline caused by girdling roots can take 10 years or more to show up.
Young trees installed within a period of two or three years are worth replanting at the proper depth with the trunk flare positioned 2 or 3 inches above grade in typical Chicago clay soil. It is best to plant just above grade if your garden’s soil is sandy and well-drained. Carefully remove excess soil above the tree’s trunk flare once it is repositioned in the planting hole.
For older, established trees, the soil around the trunk flare should be carefully removed to expose the root collar. Mulch should not be piled up around the base of a tree — it’s best to keep the mulch a couple of inches away from the trunk.
An air spade typically is used to remove the excess soil so the roots are not damaged by digging with a shovel. It is best to work with an arborist to properly expose the root collar on mature trees and to implement a program to correct any girdling roots, if present.
The arborist may also recommend using the air spade to work the soil in the root zone of the tree and then apply amendments. The Chicago Botanic Garden has achieved good results with this practice.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.