Gray, green, and yellow "moss" (lichen) on sick trees

I have gray, green, and yellow moss growing on dead branches of my piñon trees and am having a disagreement with my husband about it. He says it does no harm, but the trees it is in are all thin and look sick. Some of the living branches have the moss, so I think they are dying. I want to save my trees. A nursery suggested that I use a herbicide that they called Roundup. They said I should spray it on the branches to kill the moss. My husband still says that I shouldn't do it. Can you suggest something better?

The "moss" on the branches of your tree is an interesting symbiotic combination of fungus and algae. Symbiotic means that they live closely together and help each other. The fungus creates a moist environment and catches nutrients for the algae which uses the water and nutrients to create sugar and other food for itself and for the fungus. If you will notice, there are lichen growing on rocks as well as the trees. The ones on the trees may be closely attached to the branches, or may be foliose, that is leafy-looking. The ones on the rocks will usually be just closely attached and not foliose. In climates with more moisture, they will even coat the telephone lines in some areas.

Lichen (the plural as well as singular spelling) do not take food from the plant on which they are growing. You noted that the trees with the most lichen were looking sick. These lichen are a "result" rather than the "cause". The lichen need sunlight and do better on the trees which are not healthy, so they are more apparent on the "sick" trees. A sick tree will also "leak" nutrients and sometimes sugars and other things which the lichens can use to grow. This causes increased growth of lichen on the "sick" trees.

As for the use of an herbicide on the trees, it may kill the lichen, but will also kill the trees. The best you can do is be sure that the trees receive extra water in the driest times. If there are many trees in the area, you can thin the weak trees by removing them so that the healthy trees can receive extra sunlight and less competition for soil nutrients and water.

Lichen are interesting but not harmful. Unfortunately, they prefer situations which make them look guilty of harming trees, when they are not the guilty culprit. They are only innocent bystanders taking advantage of the habitat provided by conditions which have harmed the trees.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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