University of Maryland Extension

Lichens - Trees and Shrubs

lichens on a tree trunk

Back to Common Problems - Trees and Shrubs


Lichens are living organisms composed of a fungus and algae living in a symbiotic relationship. In a symbiotic relationship, two organisms function in a way that is mutually beneficial. Because the algae derive nutrients through photosynthesis, and the fungi protect the algae from drying out, lichens can live and grow in extremely barren areas. They grow in colonies on tree trunks, rocks, and fences, even in Antarctica. Lichens grow in many interesting forms. Those with a flattened and crusty appearance are called crustose lichens. Foliose lichens have raised, leaf-like lobes. Fruticose lichens have branched growths with finger or thread-like projections. Colors range from gray-green to bright orange-red.

Lichens grow harmlessly on tree trunks and no control is necessary. They are considered an indicator of good air quality. They are extremely sensitive to sulfur dioxide and are not usually found in industrial areas. Lichens rarely develop on rapidly growing trees, probably because the bark is shed before the lichens have time to spread. There may be more lichens on a mature, declining, or less vigorous tree, due to slower growth rate. Lichens themselves do not cause the decline.

For more information read:

I'am Lichen It! - Peak Season for Lichen Peeping

Did you hear the story of a fungus meeting algae, and they took a “Lichen” to each other?

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