Q. A number of trees in our backyard and the oak tree in our front yard have pale green splotches all over their trunks. Can you tell me what this is? Is it dangerous to the tree, and what can be done about it?
A. From your description and photos, the “problem” is lichens growing on the bark of your trees. Lichens are a fungus living in a symbiotic (mutually beneficial) relationship with blue-green bacteria. The fungus surrounds the bacteria. They often grow on tree trunks, unfinished wooden fences, and even rocks.
Lichens pose no threat to your trees' health, or to you and your family. Their presence is actually a sign that the air in your neighborhood is relatively clean - they do not grow in areas where the air polluted. There is no reason to remove them, and you could damage the trees' bark in the process.