Spray the area with a garden hose with a sprayer attachment to remove dirt and particles. Do not use a high-pressure washer as this can damage the surface fibers of the wood.
Scrub the area with a bleach-free commercial mildew remover or with oxygen bleach. Both of these products will kill mold and mildew and deeply clean the wood. Scrub using a soft, plastic-bristle scrub brush. Do not use a metal scrub brush as this will also damage the surface fibers of the wood.
Rinse the area with fresh water.
Follow up the mildew-cleaning agent by scrubbing the area with a solution of water and a bleach-free, ammonia-free household detergent such as liquid dish soap.
Rinse the area with fresh water and allow the wood to dry.
Refinish your cedar siding every two to four years, depending on the moisture levels where you live. Use a finish with synthetic resin, as natural resins have ingredients that are favorable for mold and mildew growth. Refinish your cedar siding directly after cleaning stains if the stain growth was extensive.