Lichens are a fungus body, usually one of the Ascomycetes with apothecia, enclosing a green or blue-green alga. The fungus receives some food from the alga and the alga some food and protection from the fungus. This relationship is termed symbiotic. Lichens frequently grow on living trees and shrubs, but their injury is indirect, an interference with light or gas exchange to stems or foliage, rather than from penetration of living cells of the host plant. There are three types associated with plants: crustose, a crust closely appressed to bark of main trunk or larger limbs; foliose, leaflike, prostrate but not so firmly attached to the substratum; and fructicose, bushlike, erect or hanging (Spanish moss).
Lichens are more abundant on garden shrubs – boxwood, camellias, azaleas, oaks, fruit trees, almonds, etc., on the Central Coast. Spanish moss is most common in the “fog belt” along the coast. It flourishes in neglected gardens and orchards, and in shady, damp locations, and may sometimes kill twigs and branches of weak trees growing on poor sites.
In most gardens control is unnecessary. If lichens become too disfiguring or too abundant for plant health, they may be killed by spraying affected parts; spray when the lichens are dry. They may be removed from main trunks by rubbing the bark with a steel brush after they are softened by rain.