Lichens

By:
Gary H. Brinen, Extension Agent - Horticulture (retired)

Lichens are flaky moss-like organisms that grow on the bark and branches of our landscape trees or shrubs. They do not attack and kill. They are a very natural part of landscapes and wooded areas on every continent of the world.

Lichen may look like moss, but it is actually an algae and a fungus living together mutualistically, an association which is advantageous to both organisms. The bulk of a lichen is comprised of fungal hyphae called rhizoids. Rhizoids serve to attach the lichen to things like rocks, bark, branches, etc. Rhizoids also obtain minerals from rain water, plant leachates and organic debris, like bird excrement and wind blown particles.

The alga gets water and nutrients from the fungus. The alga is a plant capable of manufacturing food (photosynthesis) for the fungus. The association with the alga is essential to the survival of the fungus. The fungi of lichen are only found as a component of lichens; they do not exist independently.

Lichens are long lived and well adapted to extremes of heat, cold, and drought. For example, masses of lichens, related to the reindeer "mosses" (not a true moss), cover the northern arctic and serve as pasturage of musk ox, caribou and domesticated reindeer.

We tend to notice lichens more in late winter when plants are bare, and we are beginning to set expectations for our landscape's spring explosion. This is all well and good you might say, but it looks ugly and it is killing my tree/shrub. Most people believe lichens are diseases which have killed or are killing their plants. However, there is no documented proof that lichens parasitize or kill plants.

Plants covered with lichens may look trashy, but there are reasons why they are covered. They grow rapidly when exposed to sun and a bare plant or a plant with a thin canopy create ideal location for lichen to proliferate. If a plant is declining and covered with lichen, there is some other cause. Lichens are just taking advantage of a sunny situation. Consider other possible causes of the plant's poor growth like stress, drought, diseases or insects. Lichens are not considered pests. So there are no chemicals we can spray to legally control lichens. You can try picking them off bark or pruning out covered branches, which may already be dead.

Lichens do serve an important role in the ecological community. They initiate soil formation from bare rock by a mechanical breaking up of the rock surface. When the lichen is wet it adheres tightly to the rock. When it dries, the lichen contracts, breaking away tiny fragments of the rock surface. In addition, lichens have a number of economic uses. They serve as food for animals, they are used in tanning and dyeing processes and they are a common source of litmus in chemical laboratories.

So, after knowing lichens a little better, perhaps we should not panic and blame them for killing our landscape plants. Rule out lichens as the cause and start checking for other possibilities, like decline from insect, disease or water problems.

(Fact Sheet #36)