Ask the master gardeners
Q: I have lichen and moss growing on my trees. Are they harmful?
A: Lichen (a combination of fungi and algae working symbiotically) and moss (a nonvascular plant classified as Bryophyta) form crusty growths on stems, branches and trunks of trees and shrubs. They are green or gray and powdery or mossy.
Lichen and moss are also found on other surfaces such as rocks, paved surfaces, concrete and soil. Lichen growing on the bark of trees can be worrisome to inexperienced gardeners, as it is often mistaken for a fungal disease. However, neither moss nor lichen growing on trees is harmful because they do not extract nourishment by entering the inner bark where food is transported.
Conditions that favor moss growth include the overcrowding of branches on trees and shrubs (often found on unpruned plants), high humidity and shade. In addition to humidity needed to reproduce, lichen requires sunlight for photosynthesis, so it is less likely to colonize trees where the canopy is dense.
The presence of either moss or lichen does not affect the health of plants. Lichen is especially sensitive to pollutants and prefers areas with clean air, so its presence is always a good sign!
There is no need to remove moss or lichen from trees. There are no registered chemicals for management of lichens and moss on plants, but they may be somewhat minimized by improving air circulation through pruning to create conditions where plants dry rapidly.
In conclusion, moss and lichen should be viewed positively; their presence tells us that we live in a healthy environment and they give any garden an established look.
Ursula Woolley, New City, master gardener with Cornell Cooperative Extension of Rockland