Clean Algae from Patio Stones

Algae turns shaded patio stones into slippery hazards, blooming green-black wherever moisture lingers and sunlight rarely reaches. The north side of the house, under deck overhangs, beneath dripping gutters—these are algae's favorite real estate. Left unchecked, it doesn't just stain; it creates genuine fall risk when wet and works its way into the porous surface of natural stone and concrete pavers. Cleaning algae properly means more than hosing it off. You need to kill the organism at root level, lift the staining it leaves behind, and address the conditions that invited it in the first place. Done right, you'll have clean stone and several seasons before it returns. Done poorly, you're back out here in six weeks with the same green film.

1. Clear the Stage First. Remove all furniture, planters, and decorative items from the patio. Sweep thoroughly with a stiff-bristled broom to remove loose dirt, leaves, and debris. Pay attention to joints between pavers where organic matter accumulates—this debris holds moisture and feeds algae growth.
2. Protect Plants Now. Use a garden hose to thoroughly saturate any nearby plants, grass, or garden beds. Then wet down the entire patio surface. This pre-wetting dilutes any cleaning solution that might splash onto vegetation and prevents the stone from immediately absorbing your cleaning mixture before it can work.
3. Let Chemistry Work. In a pump sprayer or large bucket, mix oxygen bleach powder with warm water according to package directions—typically one cup powder per gallon of water. Apply generously to the algae-affected areas, working in sections. Let the solution sit for 15-20 minutes while it breaks down the algae. You'll see the green darken as it dies.
4. Dig Into Joints. Using a long-handled deck brush with stiff bristles, scrub the patio in overlapping strokes. Work the bristles into the joints between pavers where algae hides. For stubborn spots, apply additional solution and let it sit another 10 minutes before scrubbing again. The combination of chemical action and mechanical agitation lifts algae the hose alone won't touch.
5. Flush Everything Clean. Starting at the highest elevation of your patio, rinse with strong water pressure, working toward drainage points. Use a pressure washer on low setting if available, or a hose with a high-pressure nozzle. Ensure all cleaning solution is completely removed—residue can leave white streaks on dark stone.
6. Target Stubborn Marks. For any remaining dark stains, apply undiluted white vinegar directly to the spots. Let it sit for 10 minutes, scrub with the deck brush, and rinse. Vinegar's acidity breaks down organic staining without the waiting time of oxygen bleach. This is your detail pass for perfectionists.
7. Stop Algae Before It Starts. Once the patio is dry, identify where water pools after rain. Add sand to low spots between pavers or adjust the soil grade if needed. Trim tree branches or shrubs that create constant shade over the patio. Algae needs moisture and darkness; removing either breaks the cycle.
8. Seal the Cracks. For pavers with wide joints, sweep polymeric sand into the gaps and activate it with a light mist of water. This sand hardens to prevent weed and algae growth in joints while allowing water to drain through. It's optional but extends the time between cleanings significantly.