Although we like most things green, particularly on our plants, we don’t like lichens. Lichens (pronounced likens
) are combinations of either an alga and a fungi, or an alga and a bacterium in a mutually beneficial relationship. They are often found growing on rocks, plants and other non-mobile objects. Although harmless, not taking nutrients from plants, they do indicate some stress on the plants.
Lichens are not pathogens, meaning they don’t cause disease in plants. They use the plants as a surface to grow on. When a tree or shrub begins to decline due to some sort of environmental stress or other disorder, its leaf canopy will thin and allow sunlight to enter and support lichen growth. If overall plant health is improved, a dense, vibrant leaf canopy should inhibit any sunlight available for lichen.
The two most common causes of stress on trees and shrubs are injury to the plant by string trimmers and poorly drained soil. Other reasons include herbicide damage, poor plant quality and planting technique, such as failure to remove plant ties and tags that girdle branches.
Lichens are a clue that something is wrong causing the plants to grow slowly and decline in health. This could be a combination of factors like plant competition, draught stress, root stress, overwatering, soil compaction, poor nutrition or improper pH. If you remove whatever is stressing your plants, the lichen will go away.