Things You'll Need
Copper hydroxide spray
Lichen is a combination of fungus and algae that grow on rocks, trees, and buildings. These organisms are not harmful to trees, but can be unsightly. They draw their nutrients from the air, as opposed to having a parasitic relationship with the tree. You can still kill the lichen on the trees if you don't like its appearance. There is both a chemical method and a more natural method of killing lichen on trees.
Purchase a copper hydroxide spray from your local gardening center. Buy the product in late winter or early spring, when the tree is dormant and will not absorb much of the chemical.
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Spray the tree with the copper hydroxide, according to the package directions. Focus the spray on the lichen on the tree trunk.
Repeat the spray the next year in the early spring.
Baking Soda Method
Fill a spray bottle three-quarters full with water. Fill it the rest of the way with baking soda. Shake the bottle until the baking soda dissolves.
Spray the lichen with the dissolved baking soda in late winter or early spring.
Repeat the application the next year.