Green algae on a wooden fence is not only unsightly but it can also deteriorate the wood over time. It's not uncommon for algae to grow on wooden surfaces, especially in areas that are shaded from sunlight. The good news is you that don't have to replace fence boards or paint over the algae spots. You can clean green algae off a wood fence fairly easily.
Cover surrounding plants, grass and landscaping with plastic sheeting to protect them from the bleach solution. Wear rubber gloves and old clothing.
Fill a bucket with a solution of one part chlorine bleach and two parts warm water.
Dip a scrub brush into the bleach solution. Scrub the algae spots on the wood.
Let the bleach mixture sit on the wood for 15 to 30 minutes or until you notice the spots becoming lighter. Do not allow the bleach mixture to dry on the fence.
Rinse the bleach solution from the fence with water from the garden hose.
Apply the bleach mixture again if the algae is still there. Let it sit for 15 to 30 minutes. Rinse it off with water from a power washer. Use a power washer with 500 to 800 psi and put the tip on the widest spray setting. Hold the power washer tip about 12 inches from the wood surface and move the wand back and forth to rinse.