In the early seventies a man was going through a car wash in Houston Texas. He was watching spinning car wash brushes when an idea hit him, like a diamond right between the eyes. Why not use rapidly spinning plastic brushes to cut grass!
George Ballas is credited for inventing the string trimmer in 1971. Who knew that a tin can, some fishing line and an electric motor could produce such a useful and potentially deadly tool. From that point in time forward no tree has been safe from its ravages!
Trees are living organisms. There bark is designed to keep moisture in and keep insects and diseases out. Bark is no match for a string trimmer designed to cut tough fibrous grass. Trees and shrubs are susceptible to string trimmer injury year round but the most devastating damage occurs in late spring. Once trees leaf out the trunk start to grow in diameter. The vascular cambium, a slippery layer of plant cells just under the bark begins to divide and produce new living cells. Some cells differentiate in to phloem tissue immediately below the bark. Other cells differentiate into xylem closer to the hard woody tissue in the center of the tree. Xylem transports water and nutrients upwards in the tree and phloem brings carbohydrates back down the tree for storage and fuel to power plant processes. While this growing and differentiating is taking place the bark is “slippery” meaning it can release from the tree exposing the living wood of the tree. This is in stark contrast to later in the year when these cells have hardened and removing the bark is almost impossible.
You can test this on a twig or small branch, just dig your fingernail under the bark and see how easily the bark releases from the living wood underneath.
Impact from a trimmer string at the slippery stage can remove whole sections of bark.
The damaged area dies and the top part of the tree becomes disconnected from the bottom. If the damage completely circles or girdles the tree the tree will die. If the damage is less extensive the tree will struggle and eventually die.
While I don't advocate returning to the dark ages and cutting each blade with hand snips, I do have a few tips that can eliminate the damage caused by string trimmers.
Remove grass and unwanted plants from the area directly around the tree. Correctly munching under trees can eliminate the need for string trimming close in. There are commercial guards available that will protect the base of the tree. I don't recommend using them unless they are the only solution as they may restrict the growth of the trees by binding the root flare. Porous 6 inch plastic drainage tile, cut into a 6 inch tube and sliced on one side will allow the tree to breath. If you have to use these guards make sure they are loose and remove them frequently to check for damage. I have had great success with this on boulevard trees where the city was maintaining the lawn.
If you have an idea to protect your trees from string trimmer damage check with your certified arborist, they are experts in tree preservation.