It’s not difficult to miss the crusty, gray colored growths that are now seen on many of our deciduous trees and shrubs. These growths are actually primitive plants called lichens. Some even contain chlorophyll and make their own food.
Lichens consist of both a fungus and an algae living in association with one another to give the appearance of a single plant. They grow on soil, trunks and branches of trees and shrubs, and on rocks. They are found nearly everywhere, particularly under the most extreme environmental conditions. The Arctic tundra, for example, is dominated by lichens. They serve as an important food source for reindeer. Worldwide, some 20,000 species can be found, however they are rarely found in water.
Like algae, lichens manufacture their own food. They occur in several forms, such as crusty gray, green, yellow and white growths. Some are leaf-like, others resemble a tuft of horse hair hanging from the branches. Some people like these plants for the color they add to trees and shrubs during the winter. Since they make their own food, they do not directly injure the plant on which they grow. Despite this, there are still good reasons why they should be controlled. They can have a “suffocating” effect on plants as well as 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.
One of the easiest things home gardeners can do to eliminate lichens is to simply prune infested trees and shrubs to allow better air circulation and light penetration. Pruning both shrubs and trees may be necessary. Reduce plant numbers to open up the landscape. Algae and lichens, as well as moss, often become established on slow-growing plants. Therefore, improving plant vigor by proper fertilization, watering and cultural management will help limit infestations.
In our coastal climate, these practices probably will not eliminate these epiphytic plants, but they will reduce their number and vigor. A number of fungicides will kill algae and lichens in addition to moss. In order to use a particular fungicide, you must find the plant that you want to spray listed on the label. Since the dormant season is a good time to apply sprays for algae, moss and lichen control, follow label directions for disease control during the dormant season. During the dormant season, these fungicides are used in higher concentrations, making them more effective. They should not be used at these concentrations during the growing season because they can injure the leaves. One dormant spray should give adequate control. However, although the spray may have effectively killed these growths, they will remain attached and visible for some time. Weathering and plant growth eventually will slough them off. The extent of the original problem, the density of the foliage and the vigor of the plant will determine whether treatments are required each year.
