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How do I Remove Green Algae Stains From White River Rock?

Kelvin O'Donahue

Green or black algae stains tend to be common on stone or concrete exterior surfaces, especially in humid climates and where shaded from direct sun. Though unsightly stains caused by algae are typically only surface coatings, they can be removed with common household cleaners and some elbow grease.

Once removed, growth can be kept under control with regular maintenance.

  1. Flush the stones with clean water. Loosen the algae deposits with a stiff-bristle brush and rinse away the debris with more water. Rinse the brush or broom thoroughly to remove loose particles.

  2. Mix 1 ounce of trisodium phosphate and 1 cup of bleach into 1 quart of water. Pour the mixture into a spray bottle.

  3. Spray the cleaning solution over the entire area that is stained with algae. Coat the stones liberally and keep them wet for several minutes. The solution will kill most algae. Wear safety glasses when spraying this solution as it is caustic.

  4. Scrub the stones with a stiff brush to clean away the remaining matter. Rinse all surfaces thoroughly with clean water.

  5. Tip

    For larger areas, use a stiff-bristle broom instead of a brush and a garden sprayer instead of a spray bottle. For very large areas, a power washer can be used instead of a scrub brush or broom.


    Do not use any sort of acid on natural stone surfaces.

    Cover plants that may be hit by the overspray. When finished, thoroughly rinse plants and leaves with clean water.