Tree Killers in Our Yards

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Published on: May. 2, 2007

Last revision: Nov. 30, 2010

Would you let someone come in to your yard with a chain saw and cut down all your trees? Would you do it yourself? How about the trees in your city park?

Trees provide many benefits, including increased real estate values, reduced heating and cooling bills, reduced storm water flows, shady places to play and park and noise buffering. They are usually considered assets around homes and cities. Yet, many people are gradually killing their trees with their lawn care equipment.

Recently, an acquaintance of mine stopped me in the grocery store. She knew I was the urban forester for her region, and she wanted me to contact a local city government about some lawn maintenance contractors who were hitting “her” tree—one which had been planted in her honor at a nearby city park.

She knows what many people do not know: that trees can be seriously injured or killed when they are repeatedly hit with lawn mowers and string trimmers. I see this all the time in yards, city parks, commercial developments and other places that have trees and lawn.

Over time, the lawn care equipment gradually rubs or chips away the lower bark of a tree. This disrupts the tree’s vascular system, which is located just beneath the bark.

Trees need to have their leaves and branches attached to their root system so water and nutrients can go up the tree and products of photosynthesis—sugars and starches that are needed for tree growth and survival—can go down to the roots.

The path up for the water is called the xylem and is located in the wood. The path down for the sugars and starches is called the phloem and is located in the bark. The growth layer of the tree, the cambium, is located between the xylem and phloem at the divide between bark and wood.

Cambium cells divide every year, making a new row each of phloem and xylem. As every school child knows, you can count the rings—xylem rows—to tell how old a tree is. Only the new portions of xylem and phloem are used for transporting food and water.

Because these vital pipelines run through the outer part of the tree, they are easily injured by the careless use of equipment.

Young trees are more susceptible to lawn equipment damage because their bark is thinner, but even older, thick-bark trees can be killed. It is the equivalent of cutting the tree down with a chain saw, only slower.

Look for Damage

The most obvious sign of tree injury is missing bark at or near the ground line, where the string flays the tree or the lawn mower deck bumps it. Sometimes you can see one or several spots where the wood is worn bare.

Damaged trees usually show signs of stress, including leaves turning color too early in the year, leaves drying up in the summer and areas barren of foliage. The degree of stress is proportional to the number of xylem, phloem and cambium cells that have been destroyed. In most cases, highly stressed trees will die.

Preventing Damage

This type of injury is 100 percent preventable by simply keeping lawn mowers and string trimmers away from trees. Also, do not use trees as pivot points for mowers or allow pet chains to rub against them.

If a strip of taller grass and weeds around the tree trunk bothers you, either hand-trim it carefully or keep a weed-free zone around the tree with mulch or herbicide. Keeping the area clear of grass and weeds has the added benefit of removing competition for nutrients and moisture.

Use care when applying herbicides, especially around thin-barked trees. Don’t allow the herbicide to get on the bark. Use a shield if necessary.

Mulching is more effective if you kill grass and weeds below it with herbicide first. Place 2–3 inches depth of mulch around each tree as far out as you want to go. Mulch should not touch the tree trunk. Think doughnuts—with a hole in the middle for the tree trunk. A publication on proper mulching is available from Conservation offices and online at www.missouriconservation.org/317.

You can also use ground cover barriers to keep grass and weeds from growing next to the tree. Covers that let oxygen and water penetrate through to the root system are better than those that do not, such as black plastic. Make a slit in the cover to place it around the tree. Don’t let it touch the tree trunk. Put mulch on top of the cover to make it look nicer and to help hold the cover in place.

You can further protect a small tree with a 6- to 12-inch-long piece of plastic irrigation pipe. Slit it along its length to allow you to fit it around the trunk. Keep it loose enough that it doesn’t affect growth. Remove it or replace it with a larger size as the tree gets bigger.

If Your Tree is Injured

Stop further damage! Protect the tree, and make sure whoever cares for your lawn understands how easy it is to damage bark.

Nothing you can put on the tree’s wounds will fix the damage. However, trees are not defenseless. They grow over wounds, compartmentalizing them. The tree then attempts to restore or replace connections between the roots and the branches.

If the bark is gone from more than 50 percent of the circumference of the tree, it’s usually best to remove the tree. Chances of recovery are slim, and the tree will be susceptible to falling, especially during storms.

If less than 25 percent of the circumference of the tree is affected, it will likely recover. If the bark is missing from 25–50 percent of the tree’s circumference, watch the tree for health issues like rot or branch dieback. A little time may help you to make the decision whether to keep or remove the tree.

Be sure to water damaged trees. Aim for a thorough soaking of the ground below the tree, whether by natural rainfall or supplemental watering, every 7–10 days during the growing season. Hopefully, there will be enough xylem left to transport sufficient water from the roots to the leaves.

Trees are living organisms. They look strong and invincible, but they are more easily killed than many people believe. Don’t be a tree killer. Be careful with your lawn mower and string trimmer. Make it a policy to avoid hitting trees with them.

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