Growing Together: Do trees scarred by mowers, trimmers recover?

Don Kinzler
Forum News Service

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.

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.

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.

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.

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.

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 5 feet in diameter, 5 inches thick, and kept 5 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.

Fielding questions

Question: My husband and I are in our mid-80s and live in our own home. We are limited in what we can do in our yard, but we look forward to your gardening articles in The Forum to see if any of your ideas are things we could do.

About a year ago, you wrote an article about Easter lilies and said they could be planted once they are finished blooming and maybe another would grow. It bloomed this week and I am sending you a picture. I wanted you to know that it actually worked! Amazing! — Delayne and Ron Thorson, Fargo.

Answer: I am so grateful that we have this venue for sharing our successes or challenges, and I appreciate the feedback.

Delayne continues, “I took my dried-up potted Easter lily and planted it outdoors and surprisingly enough it grew, providing me with a lot of joy as I watched it grow. Thank you for your weekly articles. My husband took your advice and mows our grass at 2 7/8 inches, which is as close to 3 inches as our mower sets it.”

Thanks for a wonderful story, Delayne and Ron.

Q: I’m having an issue with weed trees, mostly seedling elms. I have a 2-acre plot of natural grasses and wildflowers, and I’m finding a lot of weed trees that I did not plant.

For the past several years I’ve been cutting them off at the base and using a brush to paint the cut with Roundup. When I’ve cut some of them in recent days, I can tell they’ve been cut before, so what I’m doing isn’t killing the roots. Since this is very labor intensive, I’m checking to see if you have a better solution. — Judy Peterson.

A: I learned a solution for killing unwanted tree seedlings about 45 years ago while I was a summer laborer at North Dakota State University, working under horticulture professor Neal Holland.

Sometimes weedy seedling trees are too well-rooted to simply pull out, and just cutting them off causes them to resprout with vigor. The method we were taught is highly effective, and I’ve used it many times since to kill seedling trees.

Two things are needed: a squirt bottle, and lawn weed herbicide whose active ingredient is 2,4-D. Don’t use products with other additives like dicamba and triclopyr. Roundup certainly does kill trees, but this method using 2,4-D is faster-acting on woody tissue.

With pruning shears, loppers or a saw, cut the seedling tree as close to ground level as possible. Immediately saturate the cut surface with 2,4-D herbicide, undiluted, from the squirt bottle. Then cover the cut surface with several handfuls of soil to seal in the fumes. Don’t use a spray that produces mist that can drift to your “good” plants, but instead use a squirt-type bottle, such as a dish detergent bottle, that can apply the herbicide directly where desired.

Q: What causes cucumber fruits to curl? I always plant the same slicing variety, but this year some of the fruit are curled in a shape like the letter “C” instead of being straight like normal. — Sam N., Harwood, N.D.

A: Although some unusual cucumber types always curl as one of their characteristics, when this happens to varieties that are normally straight, pollination problems are usually to blame.

Cucumber flowers are pollinated by insects, mostly bees, and inadequate pollination happens when there are too few bees, or when weather conditions are too hot, cool, dry, wet or cloudy.

If only part of a flower’s ovaries is fully pollinated, the enlarging ovary, which is the little cucumber fruit, becomes lopsided. One side lengthens, while the other side stays shorter, causing curved fruit.

We can’t control adverse weather, but we can encourage bees and other pollinators to visit our yards by planting marigold, snapdragon, dianthus, poppy, clematis, monarda, borage, echinacea and many more types of flowers.

Bark wounds from string trimmers and lawn mowers is cumulative and can slowly kill trees.
After blooming, this Easter lily was planted in Fargo, and another one grew.