Young healthy trees in our landscapes and along our streets are dying overnight from an unusual cause.

Some might think it resulted from a new disease infecting the root systems. Others might think another invasive insect has bored into the wood or chewed the leaves that make food for the plant. The real culprit, however, is us and the wonderful power tools we use to cut grass.

If the grass grows close to a young tree, then that tree potentially could become infected with "moweritis." Whenever we use string trimmers or weed whackers to cut grass around trees, those flailing blades or plastic strings can repeatedly wound the trunks of young trees, tearing away bark that protects the important vascular, or water and food-conducting tissues, found just under the bark.

I know we all think of tree trunks as wood, a tough material, but trees actually are vulnerable to wounds because their most important growth tissue, called cambium, is just under the young, thin bark. A thin layer of growth cells, cambium creates new wood.

As the string trimmer nicks and beats off the bark in a circle around the circumference of the trunk, the tree will become girdled, and the vascular tissue in the stem will die. Once that dies, the tree can no longer bring water to its leaves, where it is needed to make food, or transport food to its roots, where it is needed to grow new roots. In essence, the tree dies because cutting the grass wounded its trunk.

Now don't think this only happens when homeowners forget their fourth-grade science class, where they learned how plants grow. Much of the damage inflicted on young trees in the landscape comes from lawn-care professionals and landscapers who have not trained their workers and who often are in a hurry to mow the grass and move on.

If you have a young tree in your landscape that you want to grow into maturity and provide shade and beauty to your home, then you should take the following steps to protect the trunk from mowers and string trimmers:

n Apply a thin layer, around 2 to 4 inches deep, of mulch (wood chips) around your trees. Don't pile it up on the trunks, which can hurt the tree by causing decay and rot to enter the trunks.

n Place a thick, plastic protective sleeve or tube around the base of your tree to a height of 12 to 18 inches. Flexible, corrugated drainage pipes that are split open on one side work well as protective devices because string trimmers then hit them instead of the trees.

n Talk with your landscapers about this issue and check the bases of your trees for damage.

For more information on how trees grow, wounds and mulching, visit www.exten or

VINCENT COTRONE is urban forester for Penn State Extension in Lackawanna County.