Don't do this to your tree; from blog.caseytrees.org
It’s time to go out and mow the lawn. Almost done, there are just a few tufts of taller grass right up against a tree trunk. It only takes a second with a weed whacker (or string trimmer) to remove the offending grass, but the tree might never recover. Hitting a tree with a weed whacker results in an ugly gash and can lead to disease and eventual death of the tree. The tree’s circulatory system is just under the bark; injuries to the area interfere with water, nutrient and starch transport between the roots and leaves. As a result, the tree may decline, leaving it more susceptible to pests and diseases. The injury opens the door to fungus, which can cause wood decay and tree death. If the wound completely encircles the trunk, death is inevitable.
Arborists call this damage “weed whacker blight,” and a recent survey of an SDOT corridor of young trees found that close to 60% of trees were victims. So what can you do to protect your trees (besides going easy on the weed whacker or lawn mower)? Simply prevent turf grass from growing at the base of the tree. Gently remove turf and create a ring of mulch 2-3” thick around the tree roots. Don’t pile up mulch around the base of the tree, which encourages decay. Instead of lawn, you can plant groundcovers or perennials which don’t need to be mowed near the base of the tree. If nothing else works, a flexible plastic sleeve around the base of the tree can prevent the worst of the damage.
 Planting and mulching around base to prevent weed whacker blight |
 A plastic sleeve protects the trunk. From http://www.rainbowtreecare.com |