Key takeaways
- The ISA lists trees badly damaged by lawn mowers or string trimmers as a conflict between trees and turf.
- Bark protects the living tissue under it. Repeated wounds near the soil line weaken the trunk and let decay in.
- A mulch ring 2 to 4 inches deep, kept off the trunk, reduces the chance of trimmer and mower damage.
- There are no cover-ups: wound dressings are not recommended.
Nobody sets out to hurt a tree. A trimmer touches the bark for a second on each pass, and over a summer those touches add up.
What does a trimmer do to a tree?
The ISA's trees and turf brochure lists trees badly damaged by lawn mowers or string trimmers when turf is too close to the trunk as one of the conflicts between trees and grass. Bark is the tree's skin. The living tissue just beneath it moves water and sugars, and a wound there interrupts the flow. Our glossary entry on the cambium explains the layer.
The ISA's risk brochure lists wounds from other human activity among the defects that cause trees to fail, and says they may be the result of construction, grade changes, compaction or poor pruning. Equipment strikes at the base fall in the same category.
Why is it so easy to miss?
Damage is low, often hidden under grass and rarely noticed until the leaves look wrong or the bark opens. A tree wounded on one side may seem fine for years. Look at the base of each young tree this week.
How does a mulch ring help?
The ISA lists reduced likelihood of tree damage from string trimmers and mowers among the benefits of mulch. It recommends 2 to 4 inches of organic mulch over the soil beneath the drip line as its preferred alternative to turf around a tree, and says turf out-competes young trees for water and nutrients. The ring also holds soil moisture and reduces weeds.
The ISA warns against piling mulch against the trunk. Pull it back a few inches so the root flare stays visible. In fire areas, the Living With Fire guide asks for no wood mulch in the first 5 feet next to a house and thin layers in the next zone, so use rock or bare ground there.
What if the tree is already wounded?
Stop the cause first: keep equipment away. The ISA says most experts recommend not using wound dressings. Have an arborist look at the tree. A tree wounded around most of its circumference may not recover, and one with a partial wound may be fine. The ISA's mature care brochure says to report abnormalities to a local tree care professional.
We prune young trees for structure and remove trees that cannot be saved. See young trees in new subdivisions and call 775.232.8243 for a free estimate.
What else protects young trees?
Keep sprinklers from soaking the trunk, avoid staking too tight, and teach whoever mows where the ring is. If an HOA or landscaper handles your lawn, send them a note. Use hand tools or a trimmer guard near the trunk.
Sources
- International Society of Arboriculture, Trees Are Good: Trees and Turf (brochure) (read October 17, 2026)
- International Society of Arboriculture, Trees Are Good: Proper Mulching Techniques (brochure) (read October 17, 2026)
- International Society of Arboriculture, Trees Are Good: Recognizing Tree Risk (brochure) (read October 17, 2026)
- International Society of Arboriculture, Trees Are Good: Pruning Young Trees (brochure) (read October 17, 2026)