Girdling Roots Kill Trees

By Paul Hetzler

“My girdle is killing me” was an obnoxious slogan from a TV ad that ran in the late 1960s and early 1970s in the US. The widely-mocked catchphrase was meant to inspire women to rush out and buy a certain brand of non-murderous undergarment. I doubt the ad’s plaintive tone helped boost sales, but hey – I’m no marketing expert.

And yet, underclothes can be dangerous. In 2009, the so-called “underwear bomber” stuffed his shorts with explosives and boarded a plane. Luckily, he couldn’t ignite his stuff, and his plot fell flat. In 2020, Alexey Navalny, a fierce critic of the Kremlin, nearly died when a Russian agent smeared nerve toxin on his boxers (because nothing says “strong, confident world leader” like poisoning one’s critics, right?).

Girdles may not be homicidal, but they kill countless trees in yards, parks, and along roadsides, putting us in danger in the process. If you know what to look for, it’s often possible to arrest this type of “crime” and reverse the damage. Better yet, trees can readily be girdle-proofed when they’re planted.

The sort of girdles that kill trees are girdling roots, and we’re to blame for these arboricidal miscreants. They begin when roots curve inside the planting hole rather than pointing straight out away from the trunk. If a container-grown tree is root-bound and the twisted roots are not cut and straightened, this is the outcome.

Girdling roots also result when a planting hole is too deep and/or not wide enough. Failing to remove burlap, wire, or other impediments to normal root development will cause this problem as well. Banking mulch against a tree in a “mulch volcano” is yet another cause. Although girdling roots sometimes form on the surface, they usually develop out of sight in the soil.

These dizzy roots that go around and around in a planting hole don’t cause trouble at first. But as the trunk widens, at some point it begins to press against the ring of roots. From then on, the tree’s girdle is killing it, and it’s too late to switch to a more comfortable brand.

Over time, the expanding trunk is garroted ever tighter until the tree’s vascular system can no longer get water and nutrients to the leaves. The tree will begin to show evidence of acute stress like undersize and/or slightly pale leaves, a sparse canopy in general, or early fall colour. As the process advances, the tree declines and dies.

Girdling roots frequently choke a portion of the trunk and not the whole circumference. This leads to the decline and death of a corresponding part of the canopy. While it sounds better than losing the entire tree, it presents another risk: dead wood. The problem can be exacerbated when the remaining live foliage in the canopy hides the danger from view and corrective action is delayed until large dead wood begins to fall.

Given that all trees perish sooner or later, death by girdling may not seem calamitous. One heartbreak of girdling roots is that before a tree can reach maturity, it is choked to death; typically this occurs within 20 to 30 years. Two or three decades of life is better than nothing, to be sure, but compared to the 300- to 400-year potential lifespan of sugar maples, red oaks, and white pines, or 500+ years in the case of bur oaks and white oaks, it’s not much.

Nurturing a tree to the point that it gives us shade, pollution control, ambient cooling, carbon storage and other “ecosystem services” takes money, time, and energy, whether that tree lives 25 years or 250 years. A truncated tree lifespan is a major impact of girdling roots.

Not only do girdling roots lead to an early grave for an afflicted tree, they create life-threatening hazards. The girdle choke-point soon becomes mechanically unstable, and gets weaker with time. A brilliant report out of the University of Minnesota states that girdling roots are to blame for at least one-third of all trees toppled in storms. Because these situations usually take place below ground, the condition tends to go unnoticed until the tree fails at the base. When you learn to spot the signs of girdling roots, they can be detected in the majority of cases.

The best clue that girdling roots are present is when a tree goes straight into the ground like a utility pole. Sometimes a trunk actually narrows at the soil line. Tree trunks normally widen at their base; this is called a trunk flare. Incidentally, the trunk flare should always be visible after you’ve planted a tree. It is nature’s depth gauge. When one side of the trunk has a flare and the other is straight, only half the tree is being choked.

Any kind of tree can develop girdling roots if it is not planted properly, but some species are especially prone to them. By far, maples and lindens lead the pack in this regard. Pines, oaks, beech and elms regularly fall victim as well. Norway maples, a tough, nearly bullet-proof species widely planted in urban areas, appear to be genetically disposed to girdling roots. Given that the Norway maple is now considered an invasive species, very few arborists are shedding tears over its habit of strangling itself.

Although treating a root-girdled tree is not always feasible, the only way to find out is to examine the upper roots. For this task it’s best to hire an arborist, who may use an “air spade” to gently remove soil and expose the root crown. An arborist can also tell if excising an embedded girdling root will do more harm than good. Girdling-root removal is a bit like surgery, and may involve hand saws, chisels and gouges. A successful procedure gives the patient a longer, healthier life. There are cases, though, where the safest course is to remove a badly girdled tree.

The time to prevent girdling roots is when a tree is planted. A planting hole should be 2 to 3 times as wide as the root ball, but never any deeper. Locate the trunk flare to guide you as to proper depth. Carefully cut and straighten all circling roots on container-grown trees. If the root ball is wrapped in burlap, all fabric must be removed, along with the wire cage, once the tree is situated in the hole. When the planting area is mulched, be sure to pull it away from the trunk.

Girdles are out of fashion, but girdling roots are never in vogue. Help save trees: kill those girdles.