Lichens are unique creatures in the plant world. However, the appearance is that of a single plant. Lichens manufacture their own food and grow on soil, on rocks, and trunks and branches of trees and shrubs. They occur in several forms such as crusty gray, green, yellow, or white growths. Some are leaf-like, others resemble a tuft of horse hair hanging from branches.
Lichens are amazing creatures for other reasons as well.
They frequently serve as a source of food for snails, mites, and insects. In subarctic regions, lichens are a food source for reindeer and caribou. In hard times, lichens have served as a source of food for man, such as when Washington's troops at Valley Forge used lichens as a soup thickener. Most often lichens appear on plants that for some reason are in a poor or declining condition. But for whatever reason, such plants often support profuse lichen growth, and it's easy to understand why home landscapers think the lichens are responsible for poor plant growth.