A major problem for gardeners and arborists is the development of lichen on trees. You are no doubt familiar with lichen, even if you don’t know the actual term. Lichen is that strange, hard growth that grows along the bark of trees and nearby surfaces in a rainbow of blue, green, white, and gray.
It can be frustrating to have watched a tree sprout from a small seed into a beautiful tree, only to be confronted with the reality of lichen. Lichen often forms patches all over the bark, covering the tree with green, brown, or even blue growths that are alien to everything a gardener knows. This often leads novices to go into damage . . .
Lichen is a unique lifeform constructed by two others: fungus and algae. Sometimes a cyanobacterium can take the place of the algae if it’s not available. These organisms form a symbiotic relationship, meaning they benefit one another and use each other to survive. This symbiotic relationship creates a vegetative body called a thallus, which is the lichen you see on the trees.
The majority of lichens are harmless, and since they are made of fungi, algae, or cyanobacteria, a little soap and water are all you need. If you're worried about damaging your tree by using sprays or chemicals on the lichen, consider avoiding them all together.
Lime sulfur is a potent plant-killing agent. It is composed of calcium hydroxide and sulfur, a potent combination. It can only be used on dormant plants and should not be spread around other parts of the tree or shrubbery as it destroys any that it touches.
Lichen grows best in cool, moist areas that only receive sun part of the time. If you're unsure about the best way to get rid of lichen, consider branch thinning. It would be a good idea to remove regular water sources, like a sprinkler, if they keep hitting that side of the tree.
If you are struggling with lichen in your garden, you’re not alone. These fungi/algae/cyanobacteria combinations grow in cold climates all around the globe and seem to enjoy trees because they provide an ideal location away from the dangers of ground.