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Moldy Trees? Or Is It Something Else?
by Nikky Witkowski - posted 10/07/13

“What do I do about my tree? It’s molding!” This has recently become a very common question I hear from gardeners in Northern Indiana, but is most likely a “problem” across the Midwest. Many individuals are seeing this “moldy” growth on the bark of trees or on branches that appear to be dead or dying back. It can be green or white; sometimes it might have yellowish tones. Usually it lies pretty flat against the tree, but occasionally it sticks out up to 1/2 inch from the trunk. The good news: it is common and it is a fungus. The bad news: it’s also an alga, and not hurting the tree. It is called lichen.

How could lichen be a fungus and an alga? Think back to high school biology and symbiotic relationships. There are relationships where two things can hurt each other, like parasites (for example: people and ticks or mosquitoes). However, there are beneficial relationships that can be formed as well. You might argue it’s like a man and his dog, but a closer comparison is legume plants to the fungi called mycorrhizae. The mycorrhizal fungus, which makes nodes in the roots, provides the plants with nitrogen while the plant provides carbohydrates for the fungus. They benefit each other. This is exactly how lichens are with a fungus and alga.

Now, you might wonder what the tree’s role in the process is. It is simply somewhere the lichen grows. Lichens could grow on rocks or bare soil as well, but we can find them on trees and shrubs. They interact such as paint on a wall. They make the trunk colorful and decorate it without harming it. There was a study done to see if lichens were an indicator of plant health. The results showed that while more lichens were present on sick trees, it did not prove that trees with lots of lichen growths were sick or dying.

So the stance is that lichens are a complex of two things, and they don’t harm trees. The question then is: what do they do or why are we seeing them? While there may be some conflicting reports on this, most resources seem to believe that lichens are an indicator of our environmental status. Seeing more of them could mean that air pollution is lower or there are less heavy metals in the area that could be harming the environment. The other theory I wonder about is this: Perhaps lichen are being noticed more because we have a lot a trees that are sick or being killed by emerald ash borer and everyone sees the lichen more due to canopy loss on trees. Both of these theories could have valid truth, and both could be easily wrong depending on individual situations. If you are very vigilant over your trees and shrubs and notice more lichen, maybe it is an environmental improvement. If you noticed your trees or shrub thinning and then saw them, maybe they were always there hidden under the foliage.

My take on these interesting organisms is this: We all like to make our house a home. Lichens can be nature’s way of making plain trunks and branches look beautiful or interesting — just like I would paint a wall or hang up pictures. I have personally seen some lichen structures that make me think, “Wow, that is so cool! And no one had to do a thing to make it that way. Nature did it all itself.”

So when you are walking around your garden, take the time to get close to examine the lichens, not just smell the roses.

Photo courtesy of Nikky Witkowski.


Nikky Witkowski is the Agricultural/Natural Resources and Horticulture Purdue Extension Educator for Lake County, Ind. She is the Lake County Master Gardener Coordinator and has a bachelor’s and master’s degree in horticulture from Purdue University.