In old, overgrown landscapes, mosses and lichens are a common sight on trunks and branches of trees and shrubs. In sunny areas, algae and fungus combine to produce lichens. The fungus creates a protective structure for the algae, which produces food through photosynthesis. Mosses are plants which form a green layer on trees and shrubs in heavy shade. Chemical and natural methods are available for killing mosses and lichens, which are more unsightly than harmful.
Copper sulphate spray kills moss by disrupting photosynthesis. Lime sulphur is a fungicide spray or powder that also controls moss. To avoid damaging an actively growing tree or shrub, apply these chemicals during the plant's winter dormant period. Although chemical sprays will kill lichen, there are no chemicals specifically approved for lichen control on tree and shrubs. Copper sulphate and lime sulphur are toxic to humans, and lime sulphur is corrosive to painted surfaces. Do not use copper sulphate near natural bodies of water. The Oregon State University Department of Biology recommends hiring a professional to apply these chemicals.
According to the University of Georgia at Auburn Center for Urban Agriculture, cultural improvements will eliminate moss and lichen. Lichens thrive in sunny areas on slow-growing, bare, drought- or heat-stressed plants. Vigorous, healthy trees and shrubs with dense foliage are less susceptible to infestations. Follow the preferred watering, fertilisation, soil maintenance and pruning practices for your specific tree or shrub species. Mosses grow in shaded, crowded areas. To control moss, prune back excess tree or shrub growth to increase direct sunlight and air circulation.
You can kill lichen and moss by physically removing them from a tree or shrub. The University of California Ventura County Cooperative Extension recommends removing moistened lichen with a scrub brush after rainfall. During the winter dormant period, remove moss by hand from small shrubs or use a power washer to remove moss from large branches and trunks. Use a power washer carefully to avoid damaging small branches or buds.
Mosses and lichens generally do not directly harm trees or shrubs. Lichens contribute to soil formation and provide food for animals. According to the Oregon State University Department of Biology, mosses and lichens can look attractive on some trees and shrubs. Both organisms are good indicators of air quality because they rarely grow in polluted areas. Linda McMahon, a faculty member of the Oregon State University Extension, recommends killing lichens only if it interferes with fruit production.