Girdling roots

also called Circling roots, Stem-girdling roots

A tree planted from a pot with its roots still going round in a circle, slowly strangling its own trunk. It shows ten years later, and by then it is late.

Disorder No cure Doesn't spread

What you'll see

A trunk that goes straight into the ground like a telegraph pole, with no visible flare at the base.
A flat side or a distinct indentation on one side of the trunk at ground level.
A root visible at the surface crossing and pressing against the trunk.
Thin, sparse, early-coloring leaves on one side of the crown — the side above the girdling root.
Slow decline over years, poor drought tolerance, and eventually a tree that snaps at the base in a storm.

Why it happens

Roots in a container hit the wall and turn, and go on circling. Planted out without correction, they keep that shape and thicken. As the trunk and the root both grow, the root presses harder on the trunk each year until it crushes the tissue carrying food down to the roots. The tree effectively strangles itself, and the process takes ten to twenty years.

Container-grown trees above all, and any tree planted too deep so the root flare is buried. Common on maples, lindens, magnolias and pines. Piling mulch against the trunk encourages new roots to grow into it and does the same thing from the other direction.

Treating it

Excavate carefully around the base to find the root flare and expose it.
Cut a girdling root cleanly on both sides of the trunk with a chisel or saw and remove the section, if it is under about a third of the trunk diameter.
Do it in early spring and only one or two roots in a year; removing several at once can kill the tree.
Pull the mulch back so it forms a doughnut, not a volcano, with bare soil at the trunk.
Accept that a badly girdled mature tree cannot be saved, and plan its replacement before it fails.

Keeping it away

At planting, wash or tease the outside of the rootball and cut every circling root, however drastic it feels. This is the whole prevention.
Find the root flare in the pot and plant so it sits at or just above the finished soil level, even if that means removing compost from the top of the rootball.
Prefer bare-root or field-grown stock, or a pot-in-pot or air-pruned container tree.
Dig a wide shallow hole rather than a deep narrow one, and do not amend the backfill.
Keep mulch clear of the trunk for the life of the tree.

Who gets it

Container-grown trees and shrubs of all kinds — maples, lindens, magnolias, pines and cherries most often.