Root Collar
AnatomyThe transitional zone at the base of a tree where the trunk flares out and meets the major structural roots, also called the root flare or trunk flare.
The root collar (also called root flare or trunk flare) is the area at the base of a tree where the trunk widens and transitions into the major buttress roots. In a properly planted tree, this flare should be visible at or slightly above the soil surface, giving the base of the trunk a characteristic flared appearance rather than a telephone pole shape.
A buried root collar is one of the most common and serious problems affecting urban trees. When the root flare is covered by excessive soil, mulch, or raised grade, the bark of the lower trunk is held in constant contact with moisture. Trunk bark is not adapted to subterranean conditions; prolonged moisture contact promotes decay, encourages girdling roots that encircle and strangle the trunk, and creates favorable conditions for Phytophthora and other pathogens.
Exposing a buried root collar, sometimes called root collar excavation, is an important remedial treatment. This typically involves carefully removing soil, mulch, and circling roots from around the base of the trunk using hand tools or a pneumatic air tool (Air Spade) that blows away soil without damaging roots. Many trees showing decline symptoms like canopy thinning, small leaves, and dieback respond positively once the root collar is properly exposed and girdling roots are removed.