Lichens: Harmless Hitchhikers, Not Tree Killers

Every kid (and most adults) remembers being falsely blamed simply because they were standing nearby when an accident happened. Although accidents seem to happen more often when kids are present, it's erroneous to assume they are the sole cause.

R. M. may be guilty of making a similar false assumption. He wrote: "I have heard conflicting views on whether lichens harm the trees they grow on. I live in Highlands, NC and have lichens on several firs and hardwoods. The trees look sickly and have died over time. If lichens are harmful, how can I treat my trees?"

The Poster Children for "I Didn't Do It"

Lichens are the poster children for "I didn't do it—I was only standing there!"

Symbiotic Partnership

Lichens are a symbiotic relationship between an algae and a fungus. The algae photosynthesizes in bright light and feeds the fungus, which wraps around the algae to protect it from drying out.

Diverse Forms

Lichens grow in many shapes, depending on which fungi and algae associate. Foliose lichens resemble gray tree leaves. Fruticose lichens are composed of many small threads, giving a shrubby appearance. Crustose lichens are crusty, adhering tightly to bark.

Not Harmful—Just Indicators

Since lichens are almost always found on weak plants, do they cause the sickness? In fact, lichens are totally harmless and in no way responsible for poor tree health. They are simply indicators of infirm health. As a woody plant loses vigor, leaf number and size decrease, allowing more sunlight on the trunk and branches. When enough light reaches the bark, lichens colonize it.

Improving Plant Health Reduces Lichens

If lichens are unsightly, the best way to reduce them is to increase plant health. Old azaleas can be severely pruned in February to stimulate new limbs and dense leaf cover. Examine soil and surroundings for environmental problems: adequate summer water? Poor drainage? Nearby trees casting too much shade? There might be little light for tree leaves but plenty for lichens.

Lichens can be brushed off with a stiff brush, but unless the true cause of decline is corrected, they will reappear. If a plant becomes healthier, less sunlight strikes limbs and trunk, and lichens gradually diminish.

Broken lamps, stepped-on flowers, and sofa spills might initially look like a child's handiwork. Any parent knows the damage might not be the child's fault. Unlikely as it appears, lichens are blameless when it comes to plant decline.