Lichens and moss are often found growing on rocks, tortoise shells, windowpanes or even plants. They are harmless, but could indicate that something is wrong with a plant or tree. Have you ever heard of the old saying “A rolling stone gathers no moss”? It’s a good analogy to describe why lichens or moss often don’t grow on young, healthy, actively growing trees. As long as an object is moving, moss or lichens can’t take hold. On the other hand, stressed trees or shrubs grow very slowly and often have moss or lichens growing on them.Not pathogensLichens and moss aren’t 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 thins and allows 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 growth. Lichens are often found growing on trees planted in small islands in the middle of parking lots. These trees are stressed by limited soil and root growth, compacted soils and heat stress due to paved surfaces. Moss tends to grow on the north side of old, slow-growing trees under heavy shade. Signs of poor healthIf you see lichens or moss growing on trees or shrubs in your landscape, this is a clue that something is causing your plants to grow slowly and decline in health. This could be a combination of factors like plant competition, drought stress, root stress, over watering, soil compaction, poor nutrition, improper soil pH or improper pruning. If you remove what’s stressing your trees or shrubs, the lichens or moss will go away.