Some
trees and shrubs can develop a layer of fungi, algae, lichens
or moss on their bark. These are non-parasitic organisms and
do not injure the plants on which they grow, but they can
be unsightly when they cover the branches and trunks.
Lichens and moss, particularly, fall into this category. Such growth is most inclined to grow on plants which are lacking in vigour, particularly those which are already beginning to die back. In these circumstances the growth is often unjustly blamed for the poor condition of an affected plant.
Lichens and moss also grow more profusely on trees and shrubs which have been neglected - especially where the branches have become overcrowded. Lichens and moss can also develop on vigorous plants in areas where the humidity is high.
Control
Lichens can be controlled to a certain extent by improving air circulation by pruning out overcrowded branches, and cutting back overhanging vegetation. Following this, try to stimulate new growth by feeding, mulching, watering and applying a foliar feed. Once new growth appears, badly affected shoots can be pruned out.
There are no approved chemicals for treating lichens and moss on trees and shrubs.