the over spray of a power washer. I would suggest getting your garden hose, a 5 gallon bucket , some rubber gloves and with a good soft bristle car wash type brush use a mixture of warm/hot water, laundry detergent and bleach and just soak and wash the mildew away...you can have much more control like this and won't damage any plants.

Rinse well and move right along...if no plants, just get a brush that you can screw a handle into an "Go To Town!" You'll be done before you know it! Hope this helps some, best to you. JL