An Acknowledgement of Lichens

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What is that green stuff growing on my tree, and will it kill it? a homeowner wonders.

They are lichens, and they are not harmful to your tree but an indication that you live in an area with good air quality. It can be common to see lichens growing on trees in Illinois, but they can also grow on rocks, homes and barns.Lichens are a symbiotic combination of algae and a fungus.

Depending on the algae and the fungus, the combination can cause the lichen to have different colors (green, yellow, white, blue or gray). They form beautiful crusty patches.The fungus collects nutrients, usually from the air, and transfers them to the algae which then convert the nutrients into usable food.

The fungus receives a share of the food. Because the algae and fungus are symbiotic, they must work together for survival.Lichens are beneficial to the environment in which they live because they absorb water and nutrients and hold them for longer periods of time. Also, they break down complex molecules into simple nutrient molecules used for plant growth, which might not be as abundant without the lichens.

"More than 30 years ago, it was very difficult to find any lichens growing in Illinois or the Midwest because of air pollution," says Jim Schuster. Jim Shuster is a retired University of Illinois Plant Pathologist and regular guest on the MidAmerican Gardener television show. He says, "The Clean Air Act forced changes, and our air has become a lot cleaner. The cleaner our air becomes; the more kinds of lichens find their way back into our environments.

"It is best not to worry about lichen growth unless the population is heavy. It may seem that lichens are most often found on ailing trees that lack life and are not healthy. However, it is not the lichens that are the culprit of tree decline. Younger trees usually grow fast enough to prevent lichens from sticking to the bark. Lichens grow slowly and take time to form a colony. Shuster gives an example, "…a young tree, with a lot of lichens, might mean the tree was planted too deep or has entangling roots. In most cases, by the time vast numbers of lichens are seen growing on small trees, it is too late to save the trees."He says, "Lichens on a large or old tree and a few on small or young trees should be ignored."