Question: There is a bluish green substance growing on my trees. Is it harming them and is there something I can use to get rid of it?
Answer: From your description it sounds like you have lichens growing on your trees. This interesting creature is not a plant, but rather a combination of a fungus and algae that grows as a single entity living symbiotically. The fungus provides the algae with water and minerals from the air and the tree it is growing on, while the algae provides the fungus with food in the form of carbohydrates and vitamins it creates through its ability to photosynthesize.
The trees hosting the lichen growth are not being harmed. They merely serve as a suitable anchor spot. Lichens can be found growing on rocks, tortoise shells, window panes, bark, moss and various other plants. Most lichens are specific to a particular substrate. Lichen normally found growing on rock will rarely be found growing on bark. As of the last count there were 79 different types of lichens growing on Stonehenge.
There are approximately 18,000 species of lichens divided into four categories according to their body type; crustose, foliose, fruiticose, and squamulose. Crustose lichens are flat and grow close to the surface. Foliose lichens are leaflike and are not connected to the surface at all points. Fruiticose lichens are bushy and have many filaments that grow upright to about 10 centimeters high. Lastly, the squamulose group is a mixture between the foliose and crustose groups.
They will be more prevalent on stressed plants because the amount of foliage is less and more sunlight is able to reach the bark and allow the lichen to photosynthesize. They can be an indicator of poor health but are never the cause of the decline. One exception is the hemlock looper moth that lays its eggs on lichens, and has been reported in Canada to negatively impact forest trees. Otherwise they are an important player in nature's ecosystem.
Three basic necessities need to be provided for lichens to become established; an undisturbed area, time and clean air. The presence of lichens in your area is an indicator of good air quality. In areas with heavy air pollution, it is unlikely you will find lichens growing on any surfaces. As you gradually move away from these areas the population of lichens will gradually increase.
So, if you are still interested in decreasing the lichen population in your area, improving the health of your trees and shrubs is your best method. Increased foliage in the canopy of your tree will decrease the amount of light available for the lichen photosynthesis.
Mona Bawgus is a certified master gardener and consumer horticulturist with Rutgers Cooperative Extension of Atlantic County. Write to her c/o Rutgers Cooperative Extension, 6260 Old Harding Highway, Mays Landing, NJ 08330. Email: