When mildew gains a foothold, homeowners often turn to diluted chlorine bleach (sodium hypochlorite), a.k.a. your standard household whitener. It appears to make mildew vanish, but on porous surfaces, such as wood or vinyl siding, chlorine evaporates too quickly to get to the fungi’s “roots,” guaranteeing regrowth. Chlorine can also damage foliage and stain clothing.
A better alternative: oxygen bleach (hydrogen peroxide). Less volatile than chlorine, it removes mildew on and below the surface—as well as algae, dirt, and oxidized paint—but won’t harm plants or clothing. Choose a liquid concentrate such as Scotts Outdoor Cleaner Plus OxiClean ($9.99 per gallon; Scotts), which is easy to use in a garden sprayer or pressure-washer.