Green or dark growth in patio joints thrives where moisture and debris remain. Cleaning the surface helps, but lasting improvement depends on drainage, sunlight, and keeping organic matter from filling the joints again.
Remove loose debris
Sweep leaves and soil from the patio, then wear gloves and eye protection. Keep children and pets away from the wet work area. Do not mix cleaners, especially chlorine products with acids or ammonia.
Use a measured cleaner
Start with warm water and a small amount of detergent, scrubbing with a stiff nylon brush along the joints. For persistent growth, use a patio cleaner labeled for concrete and follow its dilution, dwell time, and rinse instructions. Avoid pressure that blasts joint sand into the garden or against the house.
Rinse away residue
Work toward a safe drain or lawn area approved for the product. Rinse until no slippery film remains and do not let concentrated cleaner collect around plants. Let the patio dry fully before deciding whether another pass is needed.
Reduce repeat growth
Trim vegetation that shades the joints, correct downspout splash, and sweep organic debris regularly. Refill washed-out joints with compatible material after the surface is dry. Seal only when the concrete and joints are clean, dry, and suitable for the chosen sealer.
Cleaning is most effective when it changes the wet conditions that allowed mildew to return.