A girdling root is one that circles around the trunk of a tree rather than growing out away from it. The structural damage is twofold: Every root that grows around a tree’s trunk is one fewer root that offers the tree lateral support; additionally, as girdling roots grow, they press against the tree’s trunk, cutting into the tree like a self-imposed tourniquet.
Girdling roots are wholly a result of human involvement and only appear in the urban forest. Usually, a girdling root problem is established while a tree is being planted – and it gets progressively harder to correct from then on. In nurseries, trees begin their lives in small pots with little room for their roots to spread outward, so they begin to circle. By the time saplings are ready to plant, they often have many jumbled, circling roots. It is important to prune and rearrange these roots as part of the planting process, otherwise a girdling pattern is established. Girdling roots also appear when the natural growth pattern of a tree’s root flare is obstructed. Just like in the small nursery pots, if a tree root hits a barrier (such as a sidewalk curb) it will alter its course and potentially circle back around the tree trunk.
One more major cause of girdling roots is the absence of a root flare. The root flare is where a tree trunk transitions into its roots, and should be located just above ground level. However, trees are often planted inappropriately deep, burying their root flares in soil (or excessive volcano-shaped piles of mulch). Roots then have the opportunity to grow up around the buried base of the trunk, invisible to observers above ground.