Now that the majority of deciduous trees and shrubs have lost their leaves, gray, powdery, mossy, crusty growths have become highly visible. The combination of these growths often gives heavily infested plants a ghostly appearance. Although most noticeable on deciduous plants, conifers may also become infested. The growths are a mixture of lichens, algae and moss that thrive in our cool, wet coastal climate. Home gardeners often wonder if these growths might kill their plants and if there is a way to prevent the growths from invading their plants.
Moss, algae and lichen growth on trees and shrubs in the home landscape may be unattractive, but in most cases it isn’t harmful to plants. In fact, some people have remarked they like the aesthetics of these growths by giving landscape plants a “mature” look. These primitive plants contain chlorophyll and make their own food so they do not directly injure the plants on which they grow. Despite this, there are still good reasons why they should be controlled. They can harbor insects and also hold extra water on the plant. During a freeze, the water turns to ice and adds weight to the plant, making it more susceptible to wind damage.
Lichens have two components, a fungus and an algae living in association with one another to give the appearance of a single plant. Lichens grow on soil, trunks and branches of trees and shrubs, and on rock. They are mainly gray to green in color. They may form as crusty patches, leafy mats, or upright branching or hanging growths that resemble a tuft of horsehair hanging from the branches. Lichens are found nearly everywhere, particularly under the most extreme environmental conditions. Lichens for example, dominate the Arctic tundra. They serve as an important food source for reindeer. Worldwide, some 20,000 species can be found. They are rarely found in water.
Our wet winters favor the development of both moss and lichens. They are most prevalent where landscape plantings are crowded and in trees that have been pruned poorly or not at all. Opening up the plant canopy to allow better air circulation and light penetration will help prevent moss, algae and lichen growth.
Dormant sprays applied now, according to labeled directions should give adequate control. Be careful not to get dormant sprays on evergreen plants as leaves may be injured. Although dormant sprays will effectively kill these growths, they will remain attached and visible for some time. Weathering and plant growth eventually will slough them off.