Homeowners throughout much of the southern United States find their oak trees becoming coated with mosslike, green, fungal growths. These growths commonly belong to one of three categories: Spanish moss, ball moss or lichens. True moss, such as Spanish moss, are epiphytes or organisms that live on a host plant. These species use the oak as a host and in some cases cause problems for a growing tree. Though epiphytes and lichens contribute to tree decline, these organisms are not considered parasites, according to the Texas A&M University website.

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Declining trees with little leaf growth often host lichens and Spanish moss.

Spanish Moss

Spanish moss is largely a problem in coastal and southern states because epiphytes favor humid, warm climates. While Spanish moss grows on and and around oak trees, it does not feed from the tree. It obtains nutrients from the air and the rain, according to the Clemson University website. Spanish moss usually selects trees already in decline to use as a host plant because it does not like to compete with healthy tree leaves for sunlight and water. As it continues to grow, Spanish moss can overtake the host tree, contributing to further decline. The weight of Spanish moss causes limb breakage and it chokes out oak tree leaves. Growing healthy, vigorous oak trees provides homeowners with the best method of combating Spanish moss. Water trees regularly and provide fertilization during the growing season.

Ball Moss

Oak trees occasionally feature spherical growths of a mosslike substance on their lower limbs and leaves. Ball moss is the most common cause these growths. Live oaks, especially those grown in shady areas, commonly host the ball moss epiphytes, according to the Urban Forest Management Program located in Austin, Texas. Like Spanish moss, ball moss merely uses the oak tree as a host plant and takes no nutritional value from it. Light to moderate ball moss growth on the live oak causes no real problems for the tree. When ball moss begins to invade the tree's canopy, control becomes necessary. Before establishing a control method, check adjacent trees for signs of ball moss growth, as the epiphytes often spread from tree to tree. Some homeowners choose to hand strip the tree of ball moss clumps, but this process requires a significant amount of labor. In addition to hand picking, ball moss responds to spray control methods. A mixture of baking soda and water in a pesticide sprayer helps break down ball moss, according to the Urban Forest Management Program.

Lichens

Oak trees featuring mossy growths on their bark most often are hosting lichens, rather than a true moss. Unlike Spanish moss or ball moss, lichens are composed of fungi and algae that grow together, according to the Clemson University website. Algae feed on sunlight and carbon dioxide, while fungi forms around the algae to prevent it from drying out. Lichen commonly are found on the trunks and limbs of declining oaks, where they form grayish-green scablike growths. Like Spanish moss, lichens typically choose trees in poor health. Lichens require significant amounts of sunlight and do not grow in the shade of a healthy tree's canopy. In order to control lichen growth, a homeowner must discover the true cause of the tree's decline. Once proper treatment is established and leaves begin to grow again, lichens fade away.