Using a string trimmer right up against a tree is a serious no-no. Once the cambium layer is severed all the way around a tree, the flow of sap is interrupted and the tree will die without fail. The cambium layer is between the old, hard wood (the inner core of the tree) and the bark. It is the living tissue of the tree, the most important part. If you scrape your fingernail along a woody stem, the cambium is the green skin you see. It's responsible for growth and healing and without an intact system, your tree is lost.
In fact, cutting all the way around the trunk of a tree, or girdling, is an ancient land-clearing method. In most cases, it will kill a tree in one growing season. In between mowings and trimmings, your tree is going to try to repair damage to its trunk, but over time, the string trimmer will win. The tree below, in the common area of our neighborhood, was killed by girdling and is now marked with paint for removal.
I see some of you are trying to protect your young trees with collars like these, which I guess is, at least temporarily, better than being whipped to death by a string trimmer. But please don't forget to remove the collar. If the tree grows over it, the collar will girdle the tree as surely as cutting the cambium will.
A better idea? Clear the grass from around the trunk, all the way out to the dripline, if possible. This will keep the lawnmowers and string trimmers away from the trunk, and reduce competition for water. Cover the bare soil with a light layer of mulch, not a volcano. You should still be able to see the flare where the trunk meets the ground.
By the way, trunk damage is even more serious in palm trees. Palm trees are anatomically more like grasses than hardwood trees, and cannot heal damage to the trunks.
I just forwarded this to our city park committee member. I'm so disgusted that no one seems to care that landscapers are girdling our palm trees.
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