Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Watch out for "mower and line trimmer blight" damage to your trees. This dastardly disease is the result of tissue damage to the trunks of trees from careless mowing techniques and overenthusiastic weed whacking. The bark of young trees is especially vulnerable to attack because its thin and tender, but older trees with thin, smooth bark are also susceptible.
Lawnmower blight damages the cambium tissue layer that lies right underneath the bark. It's only a few cells thick. The cambium is important because it produces the cells, which develop into the conductive tissue in the trunk. Damage to the cambium results in decreased flow of water and nutrients upward and sugars downward in a tree. A tree is "girdled" if damage occurs to the tree all the way around the trunk. Eventual death is the result of girdling.
Too often we carefully plant and tend our trees, but fail to protect them from "lawnmower blight." Once a tree experiences this type of damage to its trunk, there isn't much you can do. Cambium tissue cannot be replaced... it doesn’t regenerate and the tree can't heal itself. The best control is prevention!
The very best way to prevent damage to the trees is to eliminate grass and weeds growing close to the trunk. Create at least a 12-inch “weed-free-zone” around the trunk of the tree. This eliminates the need to mow and potentially damage the tree. (If you use glyphosate or Roundup to eliminate weeds around the trunk of your tree, be sure to keep the chemical off the bark. Young bark can absorb the chemical and result in damage.) Edge the “weed-free-zone” with vinyl edging, bricks, or cedar bender board. Mulch the area with bark or compost.
Keeping the “zone” weed and grass free isn't just aesthetically pleasing; it's also good for your tree. Research has shown that allowing grass to grow over a young tree's root zone will stunt the tree's growth. This happens because the grass is a good competitor for water and nutrients. Another reason is that grass exudes chemicals that retard root growth.
If maintaining a “zone” around the trunk of your trees just isn't going to work, you can cage your trees with wire fencing or heavy plastic netting that is about 12 inches high. However, you should have at least a three-inch, weed-free-zone right at the trunk base and keep the caging material from direct contact with the trunk. This three-inch zone is important because otherwise the grass and weeds will grow up inside the cage resulting in an overgrown, unkempt mess.
Some folks use 6-to12-inch long pieces of large, plastic irrigation pipe to protect the trunk of their trees. They slit the pipe on one side and then they pry it open to slip it over the trunk. While this might not be the most attractive way to protect the tree, it does the trick of preventing mower blight.