Garden Q&A: Lichen looks alarming but it’s not

Q: A fungus-like growth is all over trees in the neighborhood. It’s a pastel blue-gray-green color, sometimes flat and sometimes frilly, and it coats much of the bark on some trunks. I worry urban stresses are killing these trees, and I’ve heard mushrooms on trunks are a bad sign. Can we free them from this infection?

A: Urban stresses can definitely hamper tree growth and cause them to live shorter lives (a depressing and lengthy topic for another day), but in this instance, the mystery ailment is fortunately just harmless lichen.


We receive a lot of inquiries about lichen, since they can look like fungus and since unexpected trunk features can be alarming. As it happens, these organisms are partly fungal, just not infectious or disease-causing. Lichen are unique and remarkable life-forms — a combination of fungi and algae living together and dependent on each other. As a group, lichen survive in a very wide range of habitats worldwide. Amazingly, well over 400 species can be found in Maryland.

On trees, they’re just using the bark as a surface to attach to. They don’t have roots and don’t take anything from the tree. While not the cause of tree decline or wood decay, they might increase on an ailing tree due to the greater degree of sun reaching its bark as the canopy becomes sparse. Otherwise, though, the presence of lichen can be a sign of decent air quality, since they are sensitive to certain air pollutants.

Lichen are harmless on a tree,  where they’re just using the bark as a surface to attach to. They don’t have roots and don’t take anything from the tree.

Lichen are valued members of the ecosystem too. Ruby-throated hummingbirds collect lichen to use in nest building. Some lacewing larvae, predators of pest insects, gather bits of lichen into a turtle-like shell for camouflage. Plus, a few caterpillars specialize in eating lichen, and several bird species explore tree bark looking for insect morsels like caterpillars. You can learn more and explore additional resources through our Lichen webpage.

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Q: Several of our evergreens (different kinds) have brown or pale, bleached-looking leaves. Do they have a disease already, and can anything be done? Is it preventable for the future?

A: Most likely it’s winterburn, especially since most infectious diseases won’t cause symptoms this early and seldom impact several unrelated plants to the same degree. Winterburn is an abiotic disorder or injury – abiotic translates to “without” (a-) “life” (biotic) — meaning the condition has a nonliving cause. Abiotic plant disorders are environmental, and causes include wind, water, temperature, and soil pH. In comparison, biotic factors would include insects, mites, fungi, or bacteria.

No treatment is recommended because the damage has been done, but winterburn is rarely a serious threat to a plant’s long-term health. As new growth resumes, the plant will eventually shed the damaged leaves. If it’s too much of an eyesore, you can selectively trim away the worst of it this month. Causes for winterburn typically involve a combination of cold temperatures, wind, and exposure to sun. Any autumn pruning that results in tender regrowth is priming a plant for winterburn, which is one reason it’s not recommended.

No treatment is recommended because the damage has been done, but winterburn is rarely a serious threat to a plant’s long-term health.

Why are cold-hardy evergreens damaged? Leaves “breathe” through tiny pores on their surface, and this gas exchange also allows water vapor to leave the leaf. Moisture leaves our bodies the same way — picture foggy breath on a cold day. Breezy days, especially in winter’s drier air, speeds-up this evaporation, as can the sun’s weak warmth. Meanwhile, during cold snaps, moisture in the surface layers of soil freezes, which prevents roots from absorbing it. Since the plant cannot replenish all of the moisture it’s losing, the leaf tissue starts to essentially freeze-dry. A thaw won’t reverse the damage because the cells have been injured, just like skin with frostbite. (Unlike our skin though, which can heal to an extent, leaf tissue can’t repair itself.)

Broadleaf evergreens are more vulnerable to winterburn than needled evergreens, because the leaf surface area and evaporation potential is so much greater. Younger plants also have greater vulnerability because they are still establishing roots. This is the main reason it’s risky to plant evergreens late in the fall. Cherrylaurel, boxwood, holly, rhododendron, camellia, and southern and sweetbay magnolias are common winterburn victims in our area. Plants kept in containers are also susceptible because their roots dry faster and experience more drastic temperature swings than they would in the ground.

The only action you can take to minimize winterburn risk is to site evergreens out of the brunt of winter winds and to periodically monitor their root zones for moisture, irrigating when dry during a warm spell. Plants overwintering in pots can be sheltered a bit near a wall or windbreak, but don’t bring them inside as the interruption of dormancy may detriment their health.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at Click “Ask Extension” to send questions and photos.