Mold is a fungus that forms on virtually anything, including a patio made of concrete. Moisture is the main reason mold forms. Concrete can be particularly vulnerable -- it acts like a sponge and traps moisture inside. The primary way to keep mold from forming on patio concrete is to keep the area dry. Follow a other few basic procedures to help prevent mold from growing on your patio.
Don't allow standing water to collect on the concrete patio. Remove any water immediately with a broom or mop.
Find the source of the water, and take steps to make sure it does not collect again. Verify that water is not draining down onto the patio from a gutter or the home's roof. Make sure gutter downspouts are pointing away from patio. Remove or cut back any plants or tree limbs that are touching or hanging over the patio.
Keep the concrete patio clean at all times. Mold needs food to live, so removing any dirt or debris helps restrict its growth. Clean the patio as needed with a solution of 1/2 cup detergent per gallon of water, using a scrub broom. Rinse off the patio with a hose. Allow it to dry.
Apply a commercial mold-prevention product to a concrete patio that continues to develop mold growth.
Keep patio as dry as possible at all times. Expose the patio to as much sun as you can. Check for mold on items such as furniture and flower pots, and keep them clean.