Walk the young trees on almost any lawn in Monticello and check the bark at ground level. On a surprising number you will find a ring of scarred, chewed or missing bark a few inches high. That is not disease or insects. It is the string trimmer, a little every week, and it is one of the most common ways a landscape tree dies before it ever gets big.
How trimmer damage kills a tree
Just under the bark of a tree is a thin layer of living tissue that carries water up and sugars down. On a young tree with thin bark, a trimmer line moving at full speed strips that layer in one pass. The tree can heal a small wound. It cannot heal a wound that gets reopened every Saturday.
Over a season the damage goes around the trunk and the tree is girdled. Water still goes up through the wood, so the top looks fine for a while, but the roots stop getting fed. The tree declines slowly, drops leaves early, pushes weak growth, and dies over two or three years. By then nobody connects it to the mowing.
The same thing happens with mower decks bumping trunks and with the plastic edge of a mower rubbing the bark on every pass around a tree.
What it looks like
- Bark missing or shredded in a band at ground level, usually on the sides that face the open lawn.
- Pale, exposed wood or a raised callus ridge around old damage.
- On smooth-barked trees like crape myrtle, young maple or magnolia, a cleaned-off area that looks like someone sanded it.
- Thinning canopy and small, pale leaves with no other obvious cause.
- Suckers sprouting from the base, which is the tree trying to grow around the wound.
Pecans, live oaks and pines with thick bark tolerate it better once mature, but every tree is vulnerable in its first five to ten years.
The fix is a mulch ring
The solution is not to trim more carefully. It is to make trimming unnecessary. A mulch ring around the trunk, three feet across at minimum and wider as the tree grows, means the mower and the trimmer never need to get near the bark. It also keeps turf from competing with the tree roots for water, which is a separate benefit we cover in tree roots and turf.
Do it right:
- Kill or strip the grass in the ring rather than piling mulch on top of live turf.
- Spread two to three inches of pine straw or shredded bark, no more. Our mulch depth guide covers why.
- Pull the mulch back from the trunk so there is a few inches of bare ground around the bark. A mulch volcano piled against the trunk rots the bark and invites borers.
- Edge the ring so it has a clean boundary. A crisp circle reads as intentional; a ragged one reads as neglect.
Trunk guards: a second-best option
Plastic trunk guards and corrugated pipe collars stop trimmer line from reaching the bark, and they are better than nothing on a tree that has to sit in open lawn. They have drawbacks. They trap moisture against the bark, hide insect damage and get forgotten as the tree grows into them. If you use them, choose a loose-fitting, ventilated type, check under it twice a year, and remove it once a mulch ring goes in.
Shrubs and beds get it too
The same damage happens at the base of shrubs along a bed edge, especially azaleas and boxwoods with multiple thin stems. It happens to irrigation heads, downspouts, and the paint on the bottom foot of a fence. Every one of those is a sign that the trimmer is being used to do a job that a proper edge or a bed line should be doing. Our post on bed edging covers how to lay out a yard so it can be maintained without collateral damage.
If a tree is already damaged
Do not paint or seal the wound; that traps moisture and slows healing. Clean up any loose, hanging bark with a sharp knife back to where it is firmly attached. Put a mulch ring in immediately so the wound is never touched again. Water the tree through the dry spring. Then wait. A tree with less than a third of its circumference damaged usually recovers. One girdled more than halfway around may not, and you will know within two seasons.
FAQ
How big should the mulch ring be on a mature tree?
As far out as you are willing to give up lawn. Out to the dripline is ideal for the tree; three to four feet is the practical minimum.
Can trimmer damage cause the tree to fall?
Not directly. It kills the tree slowly, and a dead tree eventually fails. Remove any tree that has died before it does.
Is it safe to spray the grass around a trunk instead of trimming?
Glyphosate on green suckers or thin bark can damage a young tree. Kill the grass once to make the ring, then mulch, rather than spraying repeatedly.
Do your crews use trimmers around trees?
We put in mulch rings wherever a customer will let us, precisely so we do not have to. It is part of how we set up a maintenance plan.
If your young trees have scars at the base and you'd like them protected, reach out and we'll set the rings and the edges up properly. We handle mowing and bed work across Jefferson County, from Waukeenah up to Ashville.