I have had several questions regarding the same issue on plants in the past few weeks. Most of the plants effected have been oaks, maples and azaleas. I have been asked, what is this and what is it doing to my plant?
What everyone is talking about is lichen. A combination of algae and fungus, lichen is able to photosynthesize and to protect itself from extreme environmental conditions. They can be found on plant stems or trunks, wood fence posts, soil and rocks. Because of the need to photosynthesize, these living organisms are not usually found in full shade. They need an absence of leaves (shade) to survive.
Lichens are most often found on dogwood and azalea plants that are growing poorly and have leggy stems with little or no leaf cover. It is the absence of leaves from the stem that allow lichen to receive enough sunlight to flourish. While it is a reasonable assumption that the growth of lichen is causing the plant to die based on the visual decline of the plant, lichen is not responsible for the poor growth nor the decline of the plant. It is taking advantage of the poor growth to capture energy and increase its size. More often the plants are in stress due to water logged soil or soil that has a high pH level. Most azalea prefer a soil pH somewhere between 5.0 and 5.5. Dogwood can tolerate neutral pH but also do well in an acidic soil as low as 5.0.
The best way to prevent the growth of lichen is to ensure good health of the plant. If a new plant is not doing well then try transplanting to a new location. If an older existing plant suddenly becomes weak then it is time to look at what conditions have changed since the plant was put in the ground. Over time, other plants may grow and add shade or plants may be taken down and add more sun to a growing area.
Pruning weak limbs can induce new growth of the plant as well as remove some lichen from the plant. Check to see if the soil has changed. If organic matter has been added over time, the soil could be holding more moisture than before or the opposite could be the case. If the soil has become compacted, then it will not hold as much moisture and have very little oxygen.
Other plants that are commonly found with lichen are oaks, sweet gum, maple, and magnolia. That is not to say that lichen will not grow on other trees or shrubs. It could simply be that these are the plants we pay attention to.
The Extension office is open Monday - Friday, located in Kennett, Missouri at 233 North Main Street. For horticulture questions contact the horticulture specialist at 573-686-8064. MU is an equal opportunity/ADA institution.