Lichens are a hybrid of moss and algae that grow on wooden decks, walls and paving stones where it is damp. Lichens are mostly self-sufficient and cause little damage to the surfaces they grow on and they're relatively easy to remove and control. Lichens growing on trees and shrubs can be left alone unless they prevent light from reaching the leaves.
Pressuring Lichens Out
Pressure washing will remove lichens from hard surfaces like wooden decks. Blasting the edge of each lichen patch with a spray set at a shallow angle to the surface they are growing on is an effective way to expose the base of the lichen and lift it off the surface. Once you've gotten the lichens up, you can throw them in the trash or compost. Pressure washing older decks may damage them, so use a gentler setting on the pressure washer or use other methods.
On more delicate surfaces, you can scrape the lichens off. Lichens grow slowly, so the time you put into removing them will pay off. Scrape them off with a sharp knife or a flat-bladed scraper. Use a stiff rubber spatula on more delicate surfaces to remove lichens you've softened with water. Careful scrubbing with a stiff brush will remove any remaining pieces of lichen.
Chemical Lichen Control
Many products designed to control algae and moss are also effective at killing lichens. Sprays that contain the active ingredient potassium soap of fatty acids and are mixed automatically when attached to a water hose are effective at killing lichens. These sprays are designed to kill lichens and loosen them from the surface they are growing on. After thoroughly coating the lichens with the soapy solution, scrub the surface with a stiff bristled broom or brush and then rinse with water.
Preventing the Return
Lichens only grow in damp areas, but they spread slowly. You can prevent lichens from regrowing by repositioning sprinklers and drip irrigation lines or fixing leaky gutters and downspouts that are spreading water on surfaces sprouting lichens. Occasionally scrubbing the surfaces before lichens begin to form will remove them before they get a chance to grow large enough to be an issue. Decreasing shade and improving air flow by removing overhanging branches or brush can help keep the area dry. Digging shallow drainage ditches and lining them with gravel channels water away from paved areas in the garden or areas with poor drainage.