University of Maryland Extension


Back to Diseases - Shrubs

lichens on a tree trunk
Closeup of lichens growing on a tree trunk

Key Points

  • 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 are often blamed for killing a tree or shrub but this is not true. They do, however, grow on slow-growing and sometimes declining trees and shrubs.  


  • 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.

Additional Information 

I'm 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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