Lichen a gray-green growth on bark

Q. I have a maple tree that has a gray-green growth on the bark. What is it?

A. Chances are the gray-green growth is lichen. A lichen may look like a single organism, but it actually consists of a fungus and a green or blue-green alga growing together in a mutually beneficial relationship. Lichens can be flat, leafy, or branched and hair-like.

All three forms occur on tree bark as well as on rocks, and soil. Colors may range from white to gray, red, green, yellow, and black.

Although lichens grow on tree bark, they are not parasitic (disease-causing organisms), and do not harm trees.

That's because it takes its nourishment from the air rather than from its host. Since most lichens will not grow in a polluted atmosphere, the air nearby is relatively pure

Lichens are often blamed for the decline and death of trees because they are commonly found on dead branches. The truth is that exposed limbs on damaged plants simply give lichens access to the sun they need for growth with little competition.

Lichen grows well in winter after the leaves have fallen from deciduous trees and no longer block the light. Lichen also grows actively during wet, humid conditions.

• Provided by Mary Boldan. Master Gardener Answer Desk, Friendship Park Conservatory, Des Plaines, open 9 a.m. to noon on Monday, Wednesday, Friday and Saturday. Call (847) 298-3502 or email Cookcountymg.com@gmail.com.