Here’s a statistic that surprises most homeowners: the single most common cause of death in young trees isn’t a pest, a disease, or drought. It’s lawn mower damage to the tree trunk, or the string trimmer version of the same injury. Arborists have a name for it, “mower blight” or “lawnmower disease,” and once you know what to look for, you’ll start spotting it on trees all over your neighborhood, usually on trees that were doing everything else right.
The frustrating part is how avoidable it is. This isn’t a hidden problem underground or a fungal infection that snuck in unnoticed. It’s repeated, visible physical damage from equipment most people run every single week.
Worried a tree in your yard already has trunk damage from mowing or trimming? Get a free assessment from our arborists before it gets worse.
Why a Few Nicks Add Up to a Dead Tree
A tree’s trunk has a thin layer just under the bark called the cambium, and it’s doing all the real work. The cambium is the living tissue responsible for transporting water and nutrients between roots and canopy, and it’s where new growth rings form each year. Damage the bark deeply enough to hit the cambium, and you’ve cut off part of that transport system permanently in that spot.
One nick from a string trimmer rarely kills a tree outright. The problem is that it never happens once. It happens every mowing cycle, week after week, all growing season, for years. Each pass adds another wound in roughly the same band around the base of the trunk. Eventually those wounds connect all the way around the circumference, a process called girdling, and at that point the tree can no longer move water and nutrients past that ring. Everything above the girdled section starts to die, sometimes gradually over a season or two, sometimes faster if the girdling is severe.
Mower damage tends to be blunter and more concentrated: a scrape or gouge from a deck edge or wheel hitting the trunk. String trimmer damage is thinner but often worse over time, because the nylon line can whip repeatedly around the same low band of bark in seconds, and it’s easy to not even notice you’ve done it until the bark starts peeling back later.
Which Trees Are Most at Risk of Mower and Trimmer Trunk Damage
Young trees are far more vulnerable than mature ones, for a simple reason: their bark is thin and hasn’t developed the thicker, more protective outer layer that older trees grow over time. A tree in its first five to ten years in the ground is exactly the size and height where a trimmer line naturally makes contact during routine mowing, right at ground level around the base.
Species with naturally thin or smooth bark carry extra risk even as they age, including:
- Maples, especially young Japanese and red maples
- Birches
- Fruit trees (apple, cherry, plum) commonly planted in home orchards
- Newly planted ornamental trees of almost any species during their establishment years
Mature trees with thick, deeply furrowed bark (oak, for instance) are more resistant to this kind of damage but not immune, especially if the same equipment repeatedly strikes the exact same spot.
Signs of Mower and Trimmer Damage
Catching this early matters, because the fix is dramatically easier before the damage has gone all the way around the trunk.
- Bark that’s scraped, gouged, or missing in a band around the base, usually within the bottom 6 to 12 inches of the trunk
- Thin, frayed vertical stripping in the bark, characteristic of repeated string trimmer contact rather than a single impact
- Sap or discoloration seeping from the wound area, especially on species like fruit trees
- New sprouts (suckers) emerging from the base of the trunk below the damage, which is often the tree’s stress response to disrupted nutrient flow above the injury
- Dieback in the canopy that seems to correspond to one side of the tree, if the girdling is partial rather than complete around the trunk
- A trunk that looks pinched or narrowed at ground level compared to the rest of the trunk diameter, a sign of repeated old wounding that never fully healed
If a tree shows canopy dieback with no obvious pest or disease symptoms elsewhere, checking the base of the trunk for old mower or trimmer scarring is one of the first things we do on a site visit, and it’s an easy thing for homeowners to check themselves too. It’s also worth comparing what you find against our guide on tree bark peeling off, since equipment damage and natural bark shedding can look similar to an untrained eye but call for very different responses.
The Fix: A Mulch Ring, Not a Sealant
There’s really one prevention method that works consistently, and it’s the same one we recommend for several other trunk protection issues: a mulch ring around the base of the tree, extending at least 2 to 3 feet out from the trunk, wider if you can manage it. This physically keeps mowers and trimmers away from the trunk entirely, since there’s no grass to cut right up against the bark anymore.
Keep the mulch itself pulled back 3 to 4 inches from direct contact with the trunk. Piling mulch against the bark creates its own problems (trapped moisture, rot, pest habitat), so the goal is a donut shape around the tree, not a volcano against it. Our guide on how to mulch around trees covers the right depth and technique in more detail.
If mulch isn’t your style, a physical trunk guard, a plastic spiral wrap or rigid mesh tube sized for the trunk diameter, works as a mechanical barrier, especially useful for young trees in their first few seasons. Just remember to check and loosen these periodically as the trunk grows; a guard left too tight for too long can cause its own girdling problem, which is a common mistake people make with staking new trees too.
Can a Girdled Trunk Be Repaired?
This is where we have to give an honest answer people don’t always want to hear: there’s no sealant, wound paint, or treatment that repairs cambium damage. Wound dressings and tar-based sealants, once commonly sold for exactly this purpose, are now understood to trap moisture against damaged tissue and can actually slow the tree’s own natural healing response rather than help it. Modern arborist guidance is to leave wounds open to air and let the tree wall off the damage on its own, the same compartmentalization process it uses for any other injury.
What determines the outcome is how much of the trunk’s circumference got damaged:
- Damage covering less than roughly a quarter of the circumference: the tree can usually compartmentalize the wound and continue growing normally, sometimes with a visible callus scar that remains for the tree’s life
- Damage covering a third to half the circumference: the tree may survive but often shows reduced vigor, some canopy dieback, and increased vulnerability to pests and decay entering through the wound
- Damage that’s gone most or all the way around (true girdling): this is usually fatal, sometimes quickly, sometimes over a slow one-to-two-year decline as stored energy reserves run out
If you catch partial girdling early, stopping further damage immediately (mulch ring, moving the mower pattern, hand-trimming with scissors instead of a string trimmer near that tree) gives the tree its best shot at compartmentalizing what’s already happened. Trunk wounds that go unmanaged can also become an entry point for the kind of slow decline we cover in our girdling roots guide, since both problems ultimately choke off the same water and nutrient pathways, just from different directions.
A Local Example
A homeowner in Harris Ranch planted a row of five ornamental cherry trees along a fence line in 2021, all doing fine for the first two seasons. By year three, two of the five had started showing dieback on one side of the canopy, and on inspection we found trimmer scarring around roughly 40 percent of the trunk circumference on both, clearly built up over repeated mowing seasons rather than one incident. The other three trees, mulched from the start by a neighbor who’d done landscaping for a living, showed zero trunk damage. We installed mulch rings on all five immediately. The two damaged trees survived but never fully recovered their symmetry, one still shows a visible lean in growth toward the undamaged side four years later. Replacing those two trees at that point would have run roughly $600 to $900 combined for comparable ornamental cherries plus installation, money that a $40 bag of mulch per tree back in year one would have avoided entirely.
A different case in Caldwell involved a young maple in a front yard where the homeowner did his own mowing and trimming weekly. He’d noticed “some bark missing” at the base for about a year but assumed it would heal on its own. By the time he called us, the girdling was close to complete around the trunk, and the tree was already showing significant canopy dieback. We recommended removal and replanting rather than trying to save it, since the damage had progressed too far. Total cost for removal, stump grinding, and a new tree: around $850, compared to what would have been essentially free prevention with a mulch ring from day one.
Prevention Comparison
| Method | Cost | Effectiveness | Notes |
|---|---|---|---|
| Mulch ring (2-3 ft radius) | $20-60 in materials per tree, DIY | Highest; removes equipment access entirely | Best long-term option, also helps soil moisture |
| Plastic trunk guard | $5-15 per tree | Good for young trees | Must be loosened/removed as trunk grows |
| Hand-trimming with scissors near trunk | Free, just time | Good if consistently followed | Relies on remembering every single mowing |
| Wound sealant after damage | $10-20 | Not effective, sometimes counterproductive | No longer recommended by arborists |
| Doing nothing | Free upfront | Poor; damage compounds over seasons | Often leads to tree loss and replacement cost |
Frequently Asked Questions
How long does it take for a tree to die from mower damage? It varies widely. A single significant wound covering much of the trunk can show canopy decline within one growing season. Gradual, repeated smaller nicks over several years can take two to three years before the tree finally succumbs once girdling completes.
Can I use tree wound paint or sealant to help it heal faster? No. Current arborist guidance recommends against sealants and wound paints for trunk injuries. They can trap moisture against the wound and interfere with the tree’s own natural compartmentalization process. Leave the wound exposed to air.
Is it too late to protect a tree that already has some trunk scarring? Not necessarily. If the damage covers less than about a quarter of the trunk’s circumference, installing a mulch ring or guard now to stop further damage gives the tree a good chance to compartmentalize what’s already happened and keep growing.
Do trunk guards need to stay on year-round? For young trees in their first few years, guards help most during the growing season when mowing is frequent. Check periodically that they aren’t binding tight against the growing trunk, and remove or size up as needed.
Does this only happen with gas-powered trimmers, or electric ones too? Any string trimmer or mower blade can cause this damage regardless of power source. The risk comes from the equipment contacting bark, not the type of motor running it.
My tree has suckers growing from the base. Is that related to mower damage? It can be. Suckering at the base is sometimes a stress response to trunk damage disrupting normal nutrient flow, though it also happens for other reasons like root stress or certain species’ natural tendencies. Worth checking the base for scarring if you’re seeing unexplained sucker growth.
Should I hire a landscaper if I know they’ll mow right up to my trees? Ask specifically about their approach around tree trunks before hiring, or install mulch rings yourself so equipment access to the trunk simply isn’t possible regardless of who’s mowing.
The Bottom Line
Mower and trimmer damage is quiet, cumulative, and completely preventable, which makes it one of the more frustrating tree losses to see, because there’s no pest to blame and no disease to treat. A simple mulch ring solves the problem permanently for the cost of a bag of wood chips. If you’re already seeing bark damage at the base of a tree, the sooner you stop further contact and get eyes on how much of the trunk is affected, the better the tree’s odds.
Call (509) 351-8404 or request your free consultation online.