The year was 2013, and I came upon the crime scene accidentally on a hot July day.

The victim was visible as I drove by, a flowering crab apple with a fresh, gaping wound. The tree’s bark was gashed near ground level, and the defenseless victim was standing there, left to fend for itself.

You don’t quickly forget such a vision. I devoted a weekly gardening column to the crime when it happened seven years ago, and although I titled it “The mystery of the murdered tree,” the crime was really no mystery. I’ve thought about the victim often, having last visited the tree two years ago.

Don Kinzler looks at a typical tree damage by a lawn mower at a tree in July 2013. David Samson / The Forum
Don Kinzler looks at a typical tree damage by a lawn mower at a tree in July 2013. David Samson / The Forum

It’s time to reopen the case, not because we need to find the perpetrator, but because we need to publicize and prevent a practice that’s killing our trees. The tree wasn’t hit by a car, girdled by rabbits, gnawed by a beaver or chewed by voles — it was injured by a human with a lawn mower and a string trimmer, in repetitive abuse under the guise of trimming the lawn.

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How is the victim today? Do trees whose bark is nicked or scarred by lawnmowers and trimmers recover from their wounds? I paid a recent visit to the tree.

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In 2013, although the trunk was injured, the tree’s leafy canopy was full and normal looking. When I visited in 2018, I noted the tree was in a sad state of decline, with a high percentage of the branches dead and bare.

Today, there is little left. Branch stubs are all that remain on much of the tree where death was pruned away, leaving a lopsided scarecrow of a tree with a tiny smattering of weak leaves trying their best to hold on. Even an optimist would soon sound the death knell.

The tree wounded by mowing and trimming in 2013 has steadily declined and is nearly dead. David Samson / The Forum
The tree wounded by mowing and trimming in 2013 has steadily declined and is nearly dead. David Samson / The Forum

Let’s talk about what happened. The biology of a tree trunk explains why such damage is so serious.

The outer visible bark layer is the tree’s protective armor, safeguarding the lifeblood of the tree, the cambium layer. This thin, greenish-white layer, immediately inside the bark, is where the tree’s growth occurs. Tissues around this thin layer conduct water and nutrients up and down the tree. Farther inside the tree trunk are the rings of wood that structurally support the tree.

If the outer protective bark is damaged and the cambium layer injured, the tree is figuratively left to bleed to death. Whether death comes slowly or quickly depends upon the depth and circumference of the injury.

Bark wounds from string trimmers and lawn mowers is cumulative and can slowly kill trees. David Samson / The Forum
Bark wounds from string trimmers and lawn mowers is cumulative and can slowly kill trees. David Samson / The Forum

Years of tree growth can be ruined in seconds by damage from a mower or trimmer. The damage is also cumulative. A little nick this week, a little scrape next, and soon the damage is irreversibly compounded. The damage that began as bark injury can cause deep bark cracks, dead branches, overall decline in vigor and possible death over time. A weakened tree is also more susceptible to attacks by insects, disease and winter injury.

Purdue University describes it well: “One of the most dangerous pests of trees is humans, especially humans with equipment. The tree trunk is protected by bark, which guards a very important plant transport system that moves nutrients and water between the roots and leaves to keep the tree alive. Damage to the bark and to this transport system can affect tree health and the tree could die. No matter what size the wound is, the damage done is irreversible.”

How can we prevent this from happening while mowing and trimming? Add shredded bark, or other wood product mulch, following the 5-5-5 rule: a circle five feet in diameter, five inches thick, and kept five inches away from the trunk. Rock mulch is not as tree-friendly as wood mulch. Rocks retain and transmit heat, compact the soil from their weight, and don’t always move aside when the tree trunk needs to expand in diameter. Few forest floors are covered in rock mulch.

ARCHIVE: Read more of Don Kinzler's Growing Together columns

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at kinzlerd@casscountynd.gov or call 701-241-5707.