Lichens are a combination of fungi and algae that occur naturally on rock and stone surfaces. Lichens do not cause damage to the rocks on which they grow, but you may not like the looks of them on your stone patio and wish to remove them. There are several methods you can follow to remove the lichens and restore the clean, fresh look of your patio.
Black Polyurethane Method
Cover the lichen with black polyurethane. Make sure you cover all the areas of lichen growth so that the black plastic blocks out all sunlight. After one to several months the lichen will die for lack of sunlight. There are faster and just as simple ways to remove lichens. This method has a slight advantage in that it kills all spores on the lichens and may be of additional help in preventing future growths from reoccurring.
Remove the dead lichens with a soft bristle scrub brush or a high-pressure nozzle attached to a garden hose.
Rinse the stone patio with cold water. You can add a small amount of oxygen bleach to aid in the prevention of re-growth for the lichens.
Make a poultice mixture of water, glycerin and porcelain clay particles. First mix the water and glycerin in a 1:1 ratio. Add dry porcelain clay until the mixture comes to the consistency of peanut butter.
Cover the lichen with the mixture and scrub hard with the soft bristle brush. The gritty nature of the mixture will help remove the lichen by grinding the vegetation and breaking up the plant.
Rinse the treated area with a garden hose and then apply a solution of water and oxygen bleach to prevent future growth.
Pressure Washing Method
Use a high-pressure cleaner that has an air compressor to increase the power and cutting action of the water spray. These machines can usually be rented from home improvement stores or commercial businesses for home cleaning projects.
Do a test strip in an inconspicuous area to make sure the sprayer does not gouge the rock. Some stones are softer and more susceptible to damage than others. There could be a problem if you have sandstone or other similar rock covering your patio. The power stream is adjustable, so begin with low pressure and adjust as needed to a higher setting to help avoid damage.
Spray the lichen-covered rocks with the power sprayer until all unwanted vegetation is removed.
Scrub the stones for a finishing touch.
Apply a mild solution of oxygen bleach and rinse with a garden hose.