Lawn mower and string trimmer injury to trees: what extension services describe

This page describes what the named extension services say. It does not diagnose a tree, say whether any tree is safe or unsafe, or tell anyone what to do about a wound.

Where the sources place the wound

Purdue Extension (publication FNR-492-W, Lindsey Purcell, August 2014) says the site of injury is usually the root flare, where the tree meets the turf and sits in the path of a mower or trimmer. The Union County Center of North Carolina Cooperative Extension (June 4, 2021) calls the root buttress the most common site, and adds that wounds may be found anywhere from the roots to several feet above the ground. Iowa State University Extension (Richard Jauron, May 10, 2006) says young, thin-barked trees are particularly prone to injury, and that stems of shrubs can also be girdled.

What the sources say the machine does

Source	Mower	String trimmer
Iowa State (2006)	Bumping a young tree bruises it and often tears away a section of bark. Repeated collisions may girdle the trunk and kill it.	The nylon string beating on the trunk strips away bits of bark and may girdle the tree.
Purdue (2014)	Any damage to or removal of the bark and the transport system can cause problems. Repeated wounding adds up, and a wound that runs all the way round (girdling) ultimately kills the tree.	Covered together with the mower: both are called a path to the same bark wound.
Caldwell County, North Carolina (2019, updated December 18, 2024)	Repeated bumping of the trunk with a mower spells almost certain death for dogwoods and other trees.	The damage is in the cambium layer, the conducting tissue, and at first there is no obvious sign of it.

Why the sources say it shows up late

Caldwell County says damage from mowers and trimmers often takes a few years to kill a tree. Wounds near ground level usually go unnoticed for a few years, grass or weeds at the base camouflage them, and fungi and bacteria invade the vascular system silently. After a few years, growth declines and the tree looks weak, and weak trees are easy prey for secondary insects such as the ambrosia beetle. Purdue says the bark on young, smooth-barked trees such as maples and birches can be less than 1/16 of an inch thick, and more than an inch thick on other species, so a young, newly planted tree has little protection.

Union County says the worst mower injuries come when the bark slips, in early spring during leaf emergence and in early fall during leaf drop, because even minor injuries then make large wounds.

What the sources say follows the wound

Purdue says that however small the wound, the damage is irreversible, and the tree has to spend energy and resources sealing it off. The wound is an opening for wood-rotting organisms and decay fungi, which can attack the injured bark and the healthy tissue next to it and enlarge the wound. Union County says it is not unusual for a tree to be completely girdled by microbial attack after a mower or weed eater wound, and that internal decay can make the tree break at the stem or root collar. Purdue puts it as a tree that may eventually die and become a hazard to people around it. Iowa State says the damaged areas may become entry points for wood decay fungi.

What the sources list for prevention

Iowa State lists a 2 to 4 inch mulch layer of shredded bark or wood chips at planting, staying several inches from the trunk when mowing, cutting the grass near the trunk with grass shears, and keeping string trimmers away from trees and shrubs.
Purdue lists removing turf at the base, mulch rings, a 2 to 3 inch layer of mulch over the root zone as a visual cue to keep equipment away, and white expanding trunk guards, which it says can also reduce winter injury. It also lists spraying herbicide around the base and says to follow label directions.
Caldwell County describes smothering the vegetation from trunk to drip line with cardboard or layered newspaper, then 2 to 4 inches of hardwood or pine mulch, and does not recommend plastic weed barriers.
Union County lists replacing turf around trees with mulch and trimming the grass by hand next to the trunk.

Look-alikes the sources name

The overview of weather, soil and chemical damage lists other injuries with a non-living cause. The sibling pages on sunscald and frost cracks cover bark damage from temperature, and the page on vole damage covers gnawing at the soil line, another way bark is lost at the base of a trunk.