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Extension: What are lichens?
Lichens on a tree trunk. Photo courtesy of Beverly Adams.

By Beverly Adams, Agriculture and Natural Resources Program Assistant

We have had several samples brought into the Extension Office of trees and woody ornamental plants with a “fungus” on them that residents think is killing their tree or plant.  Usually, it is lichen pronounced “liken” and it is not a fungus, and it is not what is killing or damaging your tree or plant.

Lichens are a symbiosis between a fungus and either an alga or a cyanobacteria. The algae or cyanobacteria can survive as an individual organism, but the fungus cannot. It has been said that lichens are fungi that have discovered agriculture. The algae/cyanobacteria produce food through photosynthesis which the fungus feeds on. The fungal component of the lichen provides structure, access to sunlight and protection from drying out for the lichen, enabling lichens to survive in extreme environments that the algae or cyanobacteria could not tolerate on their own.

Beverly Adams
Beverly Adams

Lichens occur in three growth forms: crustose, foliose and fruticose. Crustose lichens look as though they were painted on the tree or other surface they are inhabiting. They are tightly attached, crust-like in appearance and nearly impossible to remove. About 75% of lichen species on earth are the crustose growth form. Foliose lichens are leaf-like in appearance and somewhat loosely attached to the substrate. Fruticose lichens are shrubby and usually elevated from the substrate. Sometimes fruticose lichens may look like little beards hanging in tree branches.

So, what do lichens mean for trees? Lichens may be an indicator of poor plant health, but they are never the cause. Lichens take nothing from trees or other plants that they grow upon. They are simply using this spot as a place to perform photosynthesis. You may find lichens on mailboxes, rocks, soil, plants, tortoise shells and windowpanes.

Lichen on a tree branch. Photo courtesy of Beverly Adams

On trees, they may or may not be a cause for concern. On older trees that are no longer actively growing, they are usually not a warning sign. On young, small trees that should be growing, lichens usually signal environmental stress – especially if you are seeing them in the tree’s canopy. Environmental stress can be moisture stress/poor drainage, drought stress, nutrient stress, storm damage, cold damage, soil compaction – or a combination of those stressors. Remember that lichens need to photosynthesize so if you are seeing a lot of them in the canopy of your tree, that means that the tree is not putting on good leaf cover which is a clear sign of stress. 

Though lichens do not harm trees, they may be confused with fungus which can. If you are seeing fungal growth on your trees – often taking a shelf, mushroom, or globular form – this is a sign of rot. If you have questions about what you are seeing, you can always email a clear picture for diagnosis to the Extension Office.  Lichens play some vital ecological roles that often go unnoticed. Lichens are a food source for caribou, moths, slugs, flying squirrels and mites among other animals. They provide nesting materials for hummingbirds, vireos, and other birds, and are used by humans to monitor air quality and in dyes, deodorant, perfumes and antibiotic salves.

Lichen does not kill plants. An abundant amount of lichen can be an indicator that something else is affecting the plant. You can remove lichen manually by gently scraping it from the bark.

Please visit our website or you can give us a call 770-887-2418, for more information.   We also can be reached via email at