Now that the majority of our deciduous plants have dropped their leaves, you may notice gray growths on the trunks of some trees and shrubs in your landscape.

Most likely, these are lichens, which, while not harmful themselves, can be a sign a plant is being stressed.

Lichens are rarely found on healthy, vigorous trees. But remember that lichens do not cause the problem; they just benefit from unfortunate situations.

Because lichens photosynthesize, they prefer sunlight and moisture provided by trees that have suddenly lost leaves or branches. More light can reach the trunk surface where lichens have set up camp, encouraging them to grow.

You can lightly prune damaged branches to stimulate new branch growth. This helps establish a fuller canopy.

Try to identify and address stressors such as drought, poor drainage, plant competition, root stress, soil compaction, poor nutrition and improper soil pH. Insects and diseases as well as injury from trimmers, poor planting techniques and chemical injury from herbicides also can cause plants to decline.

If you remove what’s stressing your trees and shrubs, the lichens will go away, and the plant’s overall health will improve. A fungicide containing copper can also be applied to aid in control.

There are three forms of lichens, classified by the way they look.

Foliose lichens can be flat, produce wavy folds like leaves of lettuce or can be full of ridges.

Branchy fruticose lichens appear hairlike and stand upright, sometimes displaying a cup.

Crustose lichens, as the name indicates, form a thin crust on the surfaces they adhere to.

Lichens are a unique combination of fungi and algae, or sometimes cyanobacteria, that live in a symbiotic relationship on trees, rocks and other surfaces,

The fungi needs the blue-green algae or cyanobacteria for food; the algae needs the fungi for protection. The algae and cyanobacteria supply food via photosynthesis, while the fungi gather water and other needed nutrients from the air and surrounding surfaces.

Lichens are found in every ecosystem on Earth, from deserts and urban landscapes to tropical rainforests and tundras.

According to the U.S. Forest Service, there are approximately 3,600 species in North America alone, and those are just the ones that have been classified. Worldwide, there are approximately 17,000 species.

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