Gray crusty organisms growing on trees and shrubs can be a concern for home gardeners. Luckily, lichens are harmless to plants. A lichen is not a single organism, but the result of a partnership between a fungus and an alga (or cyanobacteria). The lichen fungus provides its partner a home and gains nutrients in return.

Lichens are harmless to plants. Lichens grow on objects that are not moving such as rocks. They will also grow on plants that are not growing. Growing plants will shed bark as the stems get larger in diameter, much like we shed skin cells constantly. If we did not shed skin we might have lichens growing on us too! Trees and shrubs that are not growing are a sign of unhealthy plants.

How do you get rid of lichens? I suggest concerned gardeners fertilize their plants with a low rate of their preferred fertilizer 2 or 3 times per growing season. Or, one application of Osmocote slow release fertilizer would be a good choice to get plants growing. Fertilizer will get the plants growing, their stems will increase in diameter, and the lichens will eventually fall off as bark is shed.

How does pruning affect lichen occurrence? A plant will put its energy into growing new growth in the spring. This is because new leaves are more photosynthetically productive, making more food than older leaves. If you remove this new growth year after year by pruning or shearing in the early summer, then the plant never has the energy to grow wood in the older parts of the plants, thus the lichens. Again, lichens are a sign of weakness in the plant, not a cause. Lichens are harmless to plants. Fertilizer will help your plant grow.