Take Care When Mowing Around Trees

By Laurie Casey
The Morton Arboretum

This weekend, homeowners across Chicagoland will spend hours mowing lawns or wacking at weeds. It’s a chore that some relish; but most complain about.

And if they could, our trees might complain, too.

String trimmers and lawn mowers, while indispensible to weekend warriors, can be weapons of mass destruction if not used with care. Mowing over roots or gouging the wood just under the bark stresses a tree, making it more vulnerable to insects and disease.

“String trimmers are one of the worst hazards,” says Doris Taylor, Morton Arboretum plant information specialist. “People are trying to get as close as possible to the tree to get every blade of grass, and so they damage the tree. And it’s a repetitive attack every week.”

Often accidents occur when people mow too fast around a tree, especially with the newer, self-propelled mowers, says David Lane, lawn crew supervisor at the Arboretum. “Slow down and don’t get super close with the mower. I tell my crew you can always come back and trim around the tree later.”  

Taylor advises making a no-mow area, usually a five-to-six-foot ring of mulch around the tree. Other options include planting a groundcover in that space, or hand cutting the grass near the tree with grass shears.

If you must use a string trimmer right near the base of the tree, “use almost no throttle.  It will be enough to cut the grass, but not damage the tree bark,” says Lane.

If you’ve never used a string trimmer before, or you take it out for the first time after the long winter, practice around a sign post, fence, or sidewalk edge first, says Lane.

Some homeowners, desiring a clean expanse of lawn, intentionally cut exposed roots. Not a good idea, says Taylor. Our heavy clay soils encourage roots to come above ground to look for oxygen. “And some trees, like maples and elms, have shallow roots to begin with,” she adds.

The majority of a tree’s roots are in the top 6-18 inches of soil, “It’s a myth that most trees have deep tap roots,” says Taylor.

While it looks nice, edging a deep circle in the soil around the tree may injure the very important, fine, hair-like roots.

These roots feed another important part of the tree, right under the bark. The vascular system works like our arteries, pulling water and nutrients up from the roots and distributing them throughout the tree. Bang a lawnmower into the tree trunk, however, and you can damage the vascular system.

A tree’s way of crying “Ouch!” is not through sound, but through scent.

Freshly cut or bruised wood emits a chemical signal as it dries, according to Taylor. “The signal attracts insects that like to feed on trees. Some insects may carry fungus, such as Dutch elm disease or oak wilt,” she explains. A fungus-carrying bug may land on a tree’s open wound, spreading disease.

A single tree gives many more benefits than a thousand blades of grass ever could: we get shade, clean air, a place for the birds to sing, among other things. So tread lightly around your trees and they will pay you back for decades.

Glad You Asked

Q. I accidentally nicked my tree’s trunk with the lawn mower. Should I paint the damaged wood?
A. It’s best to do nothing. The tree will heal itself starting from the outer edge of the injured area and working its way in. You can carefully cut off the damaged, loose bark, being sure not to rip off surrounding healthy bark.

Q. Is there a way to protect my tree trunk from weed whacker damage?
A. You can apply a plastic field tile, a cylindrical, perforated “shield,” as a trunk guard. It also can protect against small animals. The guard should be wide enough so that it does not “girdle” or strangle the trunk.  

At the Arboretum
Hang out after hours at the monthly Gardeners’ Night Out series. Register now for the first program on “Liberty Gardens,” Thursday, September 4. Tickets are $60, which includes dinner; discount for Arboretum members. Call 630-719-2468.

Laurie Casey is a staff writer of The Morton Arboretum.