GARDEN TALK

There are three types of lichens: crustose forms, which are flattened against the limb; folicose forms, which produce leaflike folds above the limb; and fruiticose lichens, which produce highly branched structures with hair or fingerlike projections. Most lichens seen on trees and shrubs are gray green; however, the color of other species varies from yellow and orange to dark brown. When wet, most lichens are firm, but some black or brown forms are gelatinous.

Reproduction of most lichens occurs when small pieces break off or the entire thallus (body) fragments. Lichen fragments are spread by wind, splashing water and other means to suitable substrates. The fungal partner of many lichens produces spores. A lichen may appear if a spore of the fungal component germinates near compatible algae. Some spores capture algal cells as they are ejected from the fungal fruiting body. Most lichens will not thrive on heavily shaded twigs and branches of healthy woody plants. Few lichens are found in areas with high levels of ozone, sulfur dioxide, acid rain and other common air pollutants. In fact, lichens are believed to be good indicators of air quality.

Three of a Kind

Clockwise from Left: Fruiticose lichens produce hairlike or fingerlike projections; Crustose lichen forms are flattened against the host's surface; Folicose forms look similar to folded leaves on the host stem or branch.

Heavy infestations of lichens are most common on shrubs and trees in declining or poor health. Drought, old age, diseases, insects and poor nutrition can all put stress on plants. When they are unhealthy, plants often drop leaves, resulting in thin canopies with exposed surface areas. Lichens can move in and grow on them. Over time as the plants continue to decline in health, portions of the plants die. Lichen in turn continues growing on the sick or dead portion and helps in decomposition of the plant. Thus, lichens are often already present when plants die and are frequently assumed to be the cause of the death of plants. Note dead and fallen twigs are usually covered in lichen.

The remedy of preventing and limiting the number of lichens growing on your plants is to correct what is causing your plants to be under stress. Following recommended establishment, watering and fertility practices would promote the development of a thick leaf canopy that could inhibit lichen growth on twigs and limbs. Better growing conditions and soil fertility could stimulate new plant growth and ultimately suppress the lichens.

Light pruning of affected limbs will remove some lichens and stimulate new shoot growth in the plant, which could help shade out the remaining lichens. Fruit trees, in particular, need annual pruning, including the removal of dead wood with lichen. Landscape trees and shrubs in extremely poor condition often do not respond to better care and should be replaced.

Lastly, in some specific situations, a pesticide might be considered for control of unwanted lichen growth. According to a recent Mississippi State University Extension publication, chemical control products containing potassium salts of fatty acids may be applied according to label directions. Examples of labeled products registered for use include Scotts 3-in-1 Moss Control, Bonide MossMax Ready to Spray, and Bayer Advanced 2-in-1 Moss and Algae Killer Ready to Spray. These products control lichens on bark, as well as hard surfaces in the landscape, like sidewalks or structures.

Lichens are just another interesting, yet misunderstood, creature is this amazing natural world.

For help on other home and garden questions, contact your local county Extension office or visit us online at aces.edu.