When landscape trees are stressed and declining, folks commonly notice a greenish/grey, frilly growth on the bark. This growth is an organism known as a lichen and is frequently blamed for the failing health of the tree, but is it the real problem?
Lichens are made up of two different types of organisms: fungi (mycobiont) and cyanobacteria or algae (photobiont). The mycobiont and photobiont form a symbiotic relationship, meaning they both benefit from their interaction with one another. The photobiont photosynthesizes to produce food for the mycobiont. In return, the mycobiont provides the structure for the photobiont and protection from the elements.
Since the photobiont photosynthesizes like a plant, lichens do not obtain their nutrition from their host. This sets lichens apart from plant parasites like mistletoe, which need a living host for nutrients. Therefore, lichens are not actually harming the trees on which they are growing. If that is hard to believe, consider that lichens also grow on rocks, from which there is no nutrition to be gained.
The wind disseminates the reproductive structures of lichens. Even a slight breeze can disperse them for miles. Once they are blown to a suitable environment like a tree, large rock, brick wall, or anything else that isn’t frequently moved and receives ample sunlight, they begin to grow. Lichens prefer a surface that does not change much over time, so we tend to see lichens more often on slower-growing tree species like oaks or pecans, as opposed to faster-growing species like pines.
Lichens are more noticeable on stressed trees that are growing slower than normal or have a canopy that is declining, allowing more sunlight to nourish the organism. The cause of the stress could be due to too much or not enough light or water, improper planting depth, or the tree is being attacked by an insect or disease. The lichen itself is not causing the tree any stress. Lichens often go unnoticed until a tree starts looking bad. Lichens have a relatively slow growth rate, so by the time it’s noticed covering a tree branch, it’s been there a while.
There are no chemicals labeled for use on lichens. If lichens are found on a tree where they have never been seen before, investigate possible stress sources to the tree. Correcting the stress is the best way to keep lichens from further developing or returning.
For more information on lichens, see HGIC 2354, Spanish Moss, Lichens, and Slime Mold.