
Many of our customers are saying, "I have moss in my trees...is it hurting them?" Because the trees aren't fully leafed out right now, people notice the pale green growths on the bark and branches of their woody plants. This is actually lichen, not moss, and it does no harm to the trees.
Lichens produce their own food and are epiphytes, not parasites. This means that they attach onto the surface of trees (and decks, rocks or roofs) but don't take nutrients from the plants they grow on.
Lichens are a good indicator of a tree that is either older, or not very healthy, because they grow best on trees with a thin canopy. Because lichens photosynthesize, when there is more light from a thinning, weak tree they grow better. So if you want to discourage lichen growth, take steps to maintain the health of your trees by applying a light application of an organic fertilizer, top-dress under the canopy with compost, composted manure or chopped up leaves, treat for winter moth larvae and water deeply once a week during times of drought.
Although the best response is to pay attention to the health of your plants and ignore the lichen, it can be suppressed by spraying the tree with a copper fungicide. Such applications will need to be repeated, however, and are not recommended near bodies of water because copper is toxic to fish and other aquatic life. Be aware that copper fungicides can also stain houses and other structures blue.
Lichen grows best in areas where the air isn't polluted, so probably the best approach is to view the coating on your trees as a signal that your air is clean. So perhaps the best approach is to take a deep breath and decide to like your lichen!