If you are wondering how to rid oak trees (Quercus spp.) of green fungus, you should be aware that most of the time, a green substance on an oak tree trunk is not actually a fungus at all but rather a lichen. While lichens do not pose a threat to trees, ridding oak trees of them does not require much effort.
What Are Lichens?
Not quite a moss or fungus, lichens are actually the result of a symbiotic relationship between a fungus and algae. Lichens are not plants but rather the result of a specialized fungus growing on the bark of a tree that provides shelter for a species of green algae, blue-green bacteria, or both. The algae or bacteria provide nutrients for the fungus through photosynthesis, and the fungus allows the algae to grow.
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Lichens are classified by the type of fungus, and the green lichens growing on your oak tree may be either a foliose or crustose species. Lichens are not parasitic and are not feeding off the tree; they are simply using the tree as a place to grow. This means they pose no threat to the health of the tree. There are thousands of these long-lived and slow-growing lichen species worldwide. Their growth sites are not limited to trees; they can grow on rocks and buildings and in extreme environments.
Lichens will grow on anything that is slow-growing itself provided that adequate light and nutrients are present. Fast-growing plants shed their bark more often than slow-growing ones, making it difficult for lichens to latch on. If you notice lichens on a fast-growing plant, it may indicate a problem with the plant. The issue, however, was likely not caused by the lichen.
Removing Lichens From Oak Trees
Removal of lichens from an oak tree is primarily done for cosmetic purposes. Unless the lichen has grown so heavily on the branches that it is interfering with photosynthesis, lichen removal would not provide much health benefit to the tree. Lichens can be unsightly, especially during the winter months, which is as good a reason as any for removal.
Begin by scrubbing the tree bark with a soapy solution. With a scrubbing brush, lightly rub at the lichen, being careful not to damage the bark. Bark damage can leave the tree vulnerable to other diseases or pests.
You can even add baking soda to the water with which you're scrubbing if you want to try a different type of chemical-free remedy first. You could also choose a copper-based fungicide that would kill the fungus potion of the lichen. If the lichen is growing on the branches, you can prune the tree with anvil pruners, cutting away the affected branches.
Lichen Prevention on Oak Trees
There isn't a surefire way to prevent lichens from growing on trees since there is only so much you can do to control the environment. However, the best method to prevent lichens from growing is to modify the surrounding area. Lichens grow best in areas that are partly sunny, wet and relatively cool, like shaded areas with a lot of overgrowth.
Thin out the tree branches to allow more sunlight in the area. Your tree will likely appreciate this regardless. In addition, you can move any sprinkler systems if possible to ensure the area stays warm and dry.