Lichens growing on a mailbox.

I get quite a few calls about lichens. Most clients bring in a sample or send me a picture of a frilly something growing on their tree or shrub and say, “This is killing my plant!”  Well, actually, its not.  Lichens (pronounced “liken”) are not parasitic. Lichens are made up of symbiotic organisms, a fungi and usually an algae.  The alga converts sunlight and carbon dioxide to food for the lichen fungus and in return the lichen fungus protects the alga from drying out.  Together the fungus and the alga make up what is known as the lichen thallus.

The color and growth form of the thallus is used to group and classify the lichens. The most common species of lichens on trees tend to be a gray-green color, but other species may be orange, yellow, slate blue, or black. There are three major growth forms of lichens: foliose, fructicose, and crustose. Foliose lichens have leaf-like lobes. These are the gray-green structures that can often be seen growing on tree trunks or branches.  If moistened, they become somewhat rubbery and can be removed. Fructicose lichens have hair- like or stringy thalli and are less common. Finally, as the name implies, crustose lichens have crust-like thalli. Cruostose lichens can often be found tightly embedded on rocks or lower tree trunks.

Lichens are found on rocks, tortoise shells, window panes, and plants. As plants are stressed and begin to decline, the reduced canopy allows sunlight to enter and support photosynthesis for the lichen. The presence of lichens are often an indicator of poor plant health but it is never the cause. Lichens are harmless to plants and, if overall plant health is improved, the vibrant canopy should inhibit any sunlight available for lichen photosynthesis.  I even found lichens growing on a mailbox.