TREE CARE & PREVENTION
Stop Mower Damage to Trees: The 3-3-3 Mulch Ring
If a young tree in your yard died for no clear reason, look at the base of the trunk. Mowers and string trimmers strip bark, and that cuts off the food going down to the roots. NDSU Extension horticulturist Tom Kalb puts it bluntly: “The mower is one of the leading killers of trees in yards, parks and orchards.”
The fix is cheap and simple. NDSU calls it the 3-3-3 rule: mulch 3 inches deep, keep it 3 inches away from the trunk, and make the ring at least 3 feet wide.
TREE HEALTH
How a Mower Kills a Tree
Kalb calls bark “the armor of a tree.” Right under it is the phloem, a thin layer that carries food made in the leaves down to the roots. Here’s what happens when a mower deck or trimmer line hits it:
- The bark tears or gets scraped off. Purdue Extension says bark on young, smooth-barked trees is “less than 1/16 of an inch” thick. It doesn’t take much.
- Less food reaches the roots. NDSU says damage to the phloem “reduces the amount of nutrients going to the roots.”
- Weak roots can’t feed the tree. The roots then struggle to send water and minerals back up.
- Decay gets in. Purdue says wounds let insects, disease and decay fungi in.
How bad is it? NDSU says trees generally recover when “damage is limited to 25% or less of the bark around the tree.” And “if the bark is stripped around the entire tree, it will die.” Iowa State Extension adds that “repeated collisions with a tree may girdle the trunk,” so one small nick every week adds up.
SPOT THE DAMAGE
Signs Your Tree Was Hit
- Scars or torn bark right at mower-deck height
- A ring of stripped bark low on the trunk from trimmer line
- Bruised, sunken or cracked bark near the base
Iowa State says young, thin-barked trees are hit hardest.
NDSU MULCH GUIDANCE
The 3-3-3 Mulch Ring
| Rule | What it means | Why |
|---|---|---|
| 3 inches deep | Shredded bark about 3 inches deep in the ring | Holds moisture and keeps grass from coming back |
| 3 inches from the trunk | Leave a gap, like a donut | NDSU warns mulch heaped on the trunk causes stem rot and draws bark-biting voles |
| 3 feet wide, at least | The ring is at least 3 feet across | Gives the mower and trimmer a reason to stay away |
Wider is better. NDSU’s mulch guidance says “at least three feet wide; the wider the better,” building to 3 to 4 inches deep at the outer edge. Casey Trees, a tree group in Washington, D.C., uses the same 3-3-3 idea but measures the 3 feet from the trunk out, which makes a ring about 6 feet across.
Use shredded bark, not rock. NDSU warns that “the jagged edges of rock mulch can cause harm on tender bark,” and calls rock “a heat trap.”
How to Make One
- Mark a circle at least 3 feet across around the trunk. A stake and a string make a clean circle.
- Remove the grass inside it. Cut it out with a flat spade, and don’t dig deep near the trunk.
- Spread shredded bark about 3 inches deep, thinner near the trunk.
- Pull it back so there’s about a 3-inch gap around the trunk.
- Edge it so the circle stays neat and the mower has a clear line to follow.
- Top it up as it breaks down, and widen the ring as the tree grows.
Iowa State says mowers should “stay several inches away from the trunk,” grass shears should handle grass close in, and string trimmers “should not be used around trees and shrubs.”
BISMARCK TREE CARE
Bismarck Extras for Tree Rings
- Get chips at no charge. The City of Bismarck offers single-grind wood chips at no charge, on a trial basis, at Cottonwood Park and the Ash Coulee Water Tower. They work well in tree rings. The city says these chips can’t leave Burleigh County because of emerald ash borer.
- Pull mulch back in fall. Iowa State notes that mulch against bark hides voles while they chew. Before snow, pull mulch at least 6 inches back from the trunk. See snow mold and vole damage.
- Guard against rabbits. University of Minnesota guidance suggests guards about 24 inches tall for rabbits, mowers and trimmers. Plan for snow depth, since rabbits stand on drifts. See protecting trees from rabbits and deer in winter.
WHAT TO DO NEXT
Already Hit? What to Do
- Look at how far around the damage goes. Under about 25% of the trunk, NDSU says trees generally recover. All the way around, the tree will die.
- Don’t paint or seal the wound. Illinois Extension says that in most cases the answer is “No.” Sealers can slow wound tissue and “may be sealing rot organisms against the open wound.”
- Let a pro trim ragged bark. Illinois Extension suggests using sterilized tools to clean up torn edges, and calls this work “best suited to a certified arborist.”
- Water it in dry spells. NDSU says young trees should be first in line for water in a drought.
If the damage is bad or the tree is big, see tree services in Bismarck.
LONG-TERM TREE HEALTH
Other Reasons Young Trees Die
- Volcano mulch piled against the trunk, which NDSU links to trunk rot and mice
- Voles and rabbits chewing bark under the snow
- Drought in the first few summers, before roots spread out
- Trunk guards left on too long. The University of Minnesota warns guards that stay on as the stem grows can “girdle and kill a tree.”
Questions
Mulch Ring FAQ
Answers about tree bark injuries, mower damage, and safe mulch rings.
Need help protecting your trees?
Call us to plan safer rings and tree care.
What is the 3-3-3 rule for mulch?
It’s NDSU Extension’s simple rule for mulching around trees: mulch 3 inches deep, keep it 3 inches away from the trunk, and make the ring at least 3 feet wide. Wider is better. It keeps mowers and trimmers away from the bark.
Can a tree survive lawn mower damage?
Often, if the damage is small. NDSU Extension says trees generally recover when damage covers 25% or less of the bark around the trunk. If the bark is stripped all the way around, the tree will die.
Should I paint or seal a tree wound?
In most cases, no. Illinois Extension says wound sealers can slow the growth of wound tissue and may trap rot organisms against the wound. Leave it open to the air, and let an arborist clean up ragged bark.
How wide should a mulch ring be?
At least 3 feet across, according to NDSU, and wider is better. Some tree groups use a 3-foot radius, which makes a ring about 6 feet across. Widen it as the tree grows.
Do trunk guards stop mower damage?
They help. The University of Minnesota lists flexible plastic protectors for mowers and string trimmers, about 18 to 24 inches tall. Don’t leave a guard on as the trunk grows, because it can girdle the tree. A mulch ring is still the best everyday fix.
Can I use rock instead of mulch around a tree?
It’s better not to. NDSU Extension says the jagged edges of rock mulch can harm tender bark, and calls rock mulch a heat trap. Use shredded bark instead.
Bismarck Landscape
Want Your Trees Ringed?
We’ll cut clean mulch rings around your trees and check for old damage. Call Bismarck Landscape at (701) 299-0740.
See our contact options. For beds and edging, see landscape rock, mulch and curbing.