Guilt by association: This often is the plight of lichens in the landscape.

Lichens are crusty growths that can be found on just about any outdoor surface, from tombstones and rocks to the bark of woody plants. Lichens are often unnoticed or ignored until a tree or shrub begins to decline. The truth is that lichens are harmless to plants.

Lichens actually are formed by the marriage of green or blue-green algae and a fungus. The two organisms partner together and live as one, each depending on the other for survival.

The roles of the partnership are complex, but in general, the fungus provides protection and moisture, allowing algae to survive in drier places than they could on their own.

While lichens themselves do not harm plants, they may indicate that a plant is under stress. Trees that are declining generally have smaller and fewer leaves than normal, allowing more sunlight to reach the trunk and branches.

Lichens take advantage of the increased light and become more abundant on the bark. For this reason, many people mistakenly believe that lichens are killing the plant.

Actually, lichens are just using the bark as a place to sit and take advantage of the sunlight that they use to make their own food.

Lichens can act as a “canary in a coal mine.” Heavy lichen growth on a tree or shrub may indicate that the plant is under stress from things like disease, insects, environmental conditions, or cultural problems.

Chemical control of lichens on plants is not typically recommended, but you should take notice of your “canary.” Lichen on older plant material is normal and usually not a concern. Lichen forming on younger plant material is a good indicator of a problem. 

Rebecca Bates is an MSU Extension-Lincoln County agent, and can be reached at 601-835-3460 or by e-mail at rebecca.bates@msstate.edu.