What looks like green mould on the trunk or branches of your cherry tree is probably lichen. These grey or grey-green plants grow on rocks, trees and other surfaces in damp climates. Lichens will not harm your cherry tree, but they can be unattractive. The presence or absence of lichens can also indicate environmental problems that could be hurting the tree.
Lichens come in several different types. They may be flat, scaly and mould-like, crusty, powdery or even shaped like small shrubs. Technically, lichens are fungus contain green or blue-green algae, which photosynthesises, producing food for the fungus from sunlight. Lichens do not feed on trees or other surfaces on which they grow.
Some people mistakenly believe that lichens are parasitic and damage their trees. While lichens do slowly degrade rock and dead wood, they don't hurt trees. The only downside to lichens on your cherry tree is their appearance. Some gardeners don't consider lichens to be unattractive, and actually deliberately introduce them on trees to create a more interesting look.
High humidity levels encourage lichens to grow. They're more common in wet areas, such as the Pacific Northwest, than they are in drier climates. One way to reduce lichen growth is to increase airflow around the trunk of your tree. Remove plants that may be crowding the tree and prune away excess branches to open the canopy. This admits more air and sunlight, two things that don't encourage lichens. Washington State University also recommends fertilising the tree to increase its vitality and encourage better growth conditions.
The presence of lichens on your cherry tree indicates good air quality. Lichens are an effective way of tracking air pollution, since they die off in areas with high-pollution levels. Areas that suffer from pollutants, such as sulphur dioxide, tend to be devoid of lichens. These poor air conditions can also affect tree health. Lichen growth on an older cherry tree actually means that local pollution levels are relatively low.