Lichen is a fungus that grows with algae and spreads on rock, trees, concrete and other solid surfaces to create a crust-like substance. Lichen is most common in humid climates and near bodies of water, such as lakes, rivers, creeks or ponds. Cleaning lichen off of concrete can be a challenge, but it is possible to remove with some items from the hardware store paired with aggressive cleaning.
Apply a fungicidal wash, available for purchase at many hardware or garden supply stores, to the affected area. Wear gloves and protective eye wear and apply the wash during dry weather when there will be at least a few days of sun.
Wait three to four days after application for the wash to dry up and kill the organisms in the lichen.
Scrub the dried lichen off the concrete with a long-handled stiff cleaning brush.
Spray the area with water to rinse, using a garden hose or power sprayer or by pouring buckets of water over the concrete. Consider adding an anti-vegetation detergent to the water for increased prevention of future growth.
Scrub away lichen with the long-handled brush as soon as it begins to grow on concrete, and then rinse well.