James A. Chatfield, Ohio State University Extension, Northeast District/Horticulture and Crop Science; Nancy A. Taylor, Plant Pathology, C. Wayne Ellett Plant and Pest Diagnostic Clinic, Ohio State University; Erik A. Draper, Ohio State University Extension, Geauga County; and Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District.
Horticulturists receive lots of calls about green stuff growing on tree trunks and branches. The questions are usually about whether or not the green growth is harmful to the tree. The answer is no. The green growth is sometimes moss, a type of plant which grows on all sides of trees. Or the strange growth on trunks or twigs of trees (or rocks, tombstones, etc.) is a lichen.
Lichens are mutualistic symbionts in a neat companionship between fungi and algae (or cyanobacteria). The fungi help this dual organism adhere to the surface of the stem or stone, while the algae or cyanobacteria photosynthesize, providing food. They do not damage the tree or shrub at all, though people occasionally think they do because the lichen sometimes seems to get healthier on declining plants. This occurs because in such cases, leaf drop and branch dieback let more light in for photosynthesis by the alga or cyanobacterium. Nevertheless, many perfectly healthy plants have copious lichen growth — and they are no better or worse for it.
If you want to know more about lichens, check out a fabulous new book titled, Lichens of North America by Irwin W. Brodi, Sylvia Duran Sharnoff, and Stephen Sharnoff.