Lichens are bluish-green to gray paper-thin plants that grow on the outside bark of the tree. Their growth becomes very dense and upright on some plants, while on others they are flat on the bark of the tree.
Lichens can also be found on rocks and other inanimate objects indicating that they are receiving no nutrients from the host tree, as is the case with mistletoe. They are a combination of algae and fungi growing together in what is known as a symbiotic relationship. The trees are involved only in that they are the object on which the lichen is growing. That means they are almost always harmless to the tree.
Although found in most areas of Texas, lichens are more of a problem along the Gulf Coast and in more humid areas. They are most often thought of as an indicator of poor tree vigor. Lichens require a certain amount of sunlight, so if the tree is healthy and has dense foliage, the lichens do not have a chance to grow and develop. When the trees are weak, lichens can proliferate and cause a great deal of concern. Thus it is very important that trees be maintained to ensure maximum foliage.
In the GardenLine family, that means following the Deep Root Feeding tip sheet.
Currently there is no chemical control for lichens. In the past, we've always recommended copper-based fungicides such as Kocide. We've also found that any banner-based fungicides (also known as propiconizol - PPZ) also work to diminish the amount of Lichens.
Research has also shown that these fungicides can wash off very easily in a rain or even an irrigation cycle, so be sure to use them during a dry period of two or more days.