Scaly squamulose lichen clings to a tree branch.

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During gray, wet periods, Virginia Cooperative Extension offices often receive phone calls from residents concerned about previously unseen growths on tree trunks and branches.

Although the growths could be something else, many of them are harmless (but profoundly interesting) lichen (pronounced liken). Covering 8 percent of the earth’s land, lichen are often seen encrusted on rocks and fallen tree branches in winter.

The good news is that you do not need to treat lichen; they are neither parasitic nor harmful. Lichen are a composite of a fungus and an organism that performs photosynthesis: either algae or cyanobacteria.

The fungus and photosynthetic organism are called symbionts because each provides a benefit to the other, and together they behave as one organism.

The main body of the lichen, known as the thallus, is formed by fungal filaments that surround the photosynthetic cells. The algae or bacteria provide energy via carbon dioxide. The fungus obtains water and minerals, and protects the algal cells from drying out.

And so I can’t resist sharing this joke: Did you hear the story of the fungus who met the algae, and they took a “lichen” to each other?

Where they grow

Lichen can survive in harsh environments, such as on exposed rock. Many lichen grow rapidly when exposed to full sunlight, which explains their common occurrence on leafless trees.

Lichen can be found on dead wood, rocks, soil, tombstones, sidewalks, and other sunny places as well. During dry weather, lichen shrivel and become faded to protect the algae.

When remoistened by rain, lichen expand, their fungal surfaces become transparent, light reaches the internal algae or bacteria, and photosynthesis resumes. Then the algal colors become noticeably brighter.

A rainbow of colors

Classification of lichen is through their amazing colors and variety of shapes. Common lichen on trees are typically a gray-green color, but other species may be orange, yellow, blue or black. You may notice three-dimensional shapes similar to beards, ears or cups.

There are four major growth forms of lichen: foliose, fruticose, crustose, and squamulose. Foliose lichen have leaflike lobes. These are two-dimensional gray-green structures that can often be seen growing on tree trunks. They are slightly raised and can grow rapidly, covering several inches of bark or more. If moistened, they become somewhat rubbery.

Less common, fruticose lichen have hair-like or stringy thalli and look shrubby. Crustose lichen are crust-like, often found tightly embedded on rocks or lower tree trunks. Powdery crusty colors such as bright orange almost look like spray paint.

Squamulose lichen can be described as a mix between foliose and crustose forms. They are scale-like and attach like tiny shingles. There are several other types as well, including a slimy gelatinous type.

Reasons to like lichen

Living creatures have developed with lichen over the years. In the Richmond area, we can see ruby-throated hummingbirds lining their nests with strips of lichen. Over 50 other species of birds use lichen as nesting material as well.

Native painted lichen moth caterpillars eat lichen exclusively.

Humans use lichen as well. Because lichen are extremely sensitive to air pollution, they serve as a bellwether for air quality. Lichen also contribute nitrogen and minerals to their ecosystems.

Rocks with lichen add interest to rock gardens. Historically, people have used lichen to make dyes and some medicine.

Scientists are currently investigating lichen because they produce unique biochemicals to fend off herbivores, prevent freezing, and stop seeds from germinating in their soft tissue.

Chemists think lichen hold promise for the development of new medicines and agricultural chemicals.

Lela Martin is a Master Gardener with the Chesterfield County office of the Virginia Cooperative Extension. She was recently recognized for over 1,000 hours of volunteer service.