When you look up at your favorite cedar tree and notice a gray-green scaly mat covering its branches, you’re likely to think, “Well, that tree’s a goner.”
Not so fast. Lichen (which can be gray-green, white, orange, or yellow) isn’t hurting your tree—it’s only using it as a place to perch while it hums along on its own power. All lichens are symbiotic organisms: a combination of fungus with either algae or cyanobacteria. The algae make their own food from sunlight (photosynthesis) and the fungi in lichen help retain water and minerals.
Lichens are actually beneficial. They provide nesting material for many birds, including hummingbirds; food for caribou, mountain goats, frogs, and other creatures; and shelter for insects.
Though lichens look like they might be smothering the trees they grow on, they aren’t parasites and don’t do any damage. Lichens can tell you something about the health of the tree, however: If a tree has very thick lichen, it can be a sign that the tree is dying or at least not growing properly. Lichens are also reliable indicators of pollution. They’re practically nonexistent in areas with bad air pollution, and they grow best where the air is clean.
There’s no need to remove lichens, since they don’t cause damage, and many people consider them beautiful. But if you can’t stand how they look on your trees (or walls, or stones), remove them with a soft scrubbing brush or a wood chip and warm water. Be careful not to damage the bark. To help prevent lichens from growing back, keep the branches dry (don’t let the sprinkler spray that high) and expose them to as much sunlight as possible.
Photo of lichens growing on Fraser fir (Abies fraseri) courtesy of the Kemper Center for Home Gardening at the Missouri Botanical Garden.