Our trees are under attack and the problem is getting worse. It’s frightening because young trees and those just approaching nice size are the most susceptible. Once affected, the trees can begin a downward spiral with little chance of recovery. The predator attacks all tree species. None are immune.
I’ve joked about my slow-moving-vehicle driving habit because I enjoy looking around our neighborhoods to observe landscapes and gardens. But I also keep my eyes peeled for problems we can tackle together, and the tree trouble I’ve observed for several years is serious.
Is a new insect sweeping the country or a different tree disease heading this way? No, the problem is already here, and it’s us. We’re the killers.
The culprit? Lawnmowers and string trimmers. They’ve caused more death and destruction of trees planted in our region than insects and diseases combined.
It’s hard to take the problem seriously because we take lawn mowing for granted. I can’t repeat this strongly enough: lawnmowers and string trimmers are killing our trees. We all need to pitch in to stop the problem.
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Mower damage When mowing lawn, we like to sidle up closely to fences, dog houses and backyard tool sheds to trim the adjacent grass. But when we hug the mower next to a tree, it’s easy to rub against the bark as we circle around the trunk with both push-type and riding mowers.
What’s wrong with rubbing against a tree trunk? Each time a little bark can be scarred in a spot or two. Think about the number of times we mow our lawns each growing season. Little scars can add up through the summer. Multiply that over 10 years and the lower part of a tree trunk can be scuffed up hundreds of times.
String trimmers Whirring around so rapidly we can hardly see the cutting edge, line trimmers are very dangerous to operate next to tree trunks. They cause even greater damage than mowers because they can quickly whip against tree bark and scar the trunk. There isn’t a safe way to trim grass next to a tree trunk using a string trimmer.
How trees are damaged Is a little scuffing of a tree trunk really so bad? Yes, it can kill a tree. Understanding how is the key to educating all of us so the problem stops.
A tree trunk is not the same as a wooden fence post or telephone pole. A tree trunk is like a human leg. It’s alive. Both a tree trunk and a leg can be injured if you whir against them with a string trimmer, even a little. It’s vital to understand that a tree trunk is not an inanimate wooden pole.
Botany helps explain why tree-trunk scuffing is serious. The outer visible bark of a tree is the armor that protects the interior from damage. Very close under the outer bark is a thin layer of tissue called the cambium layer. It’s the lifeblood of the tree from which all growth occurs, and two types of tubes run up and down around it. Outside the cambium is phloem that carries food manufactured during leaf photosynthesis down to the roots. Inside the cambium, the xylem pipes water and nutrients upward to branches and leaves.
These three vital layers are very thin and easily injured, which is why nature protects them with bark. But bark takes years to develop thick, corky ridges. During the first 10 years, a tree’s bark is smooth, thin and less protective. Scarring the bark usually damages the cambium, xylem and phloem, interfering with the tree’s pumping water and materials up and down. If the damage extends all around the trunk, it’s said to be “girdled.”
Symptoms If a tree trunk is completely girdled, the tree usually dies. If a portion of the trunk is damaged, part of the tree suffers or dies. Trunk damage is cumulative, meaning small scars accumulate over time, continuing to depress the tree’s vigor.
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Trunk damage often results in slow death. Rot organisms invade the wounds. The tree becomes less vigorous and loses its ability to fight off attacks by disease and insects. A downward spiral begins that is nearly impossible to reverse. A tree that’s accumulating trunk damage may take years to die, leaving the real culprit undetected as secondary invaders are blamed.
Prevention The best way to eliminate trunk damage is to add a 3-inch thickness of wood mulch in a 3-foot diameter circle, kept 3 inches away from the trunk. Plastic tree guards are not as effective because they can be shredded by string trimmers, and are best removed during summer to allow the trunk to breathe.
If we all become tree-watchers, we can stop the destruction.
Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler’s Greenhouse in Fargo. Tune in to his weekly radio segment at noon Wednesdays on WDAY Radio 970. Readers can reach him at forumgrowingtogether@hotmail.com .
He also blogs at http://growingtogether.areavoices.com .
