Flagstone is aesthetically pleasing and durable for outdoor patios, but stones and mortar can become stained and mildewed over time. Prevent this problem by cleaning your flagstone patio at least twice a year -- at the end of winter and the end of summer. Flagstone is fairly easy to maintain, but some flagstone detergents are strong, so take precautions accordingly. If you do not have a drainage system built into your patio, opt for a "green" cleaning product.
Mix up the flagstone cleaning solution. For about four gallons of water in the bucket, use slightly less than a cup of flagstone cleaner. However, the ratio is subject to the concentration of your chosen cleaning solution, so read the package for specific measurements. Stir cleaner and water into the five-gallon bucket.
Apply cleaning mixture to the flagstone. Use the stiff push broom to spread the mixture thoroughly over the stones. Make sure the solution covers all the mortar and stone. Put a little extra on any stained areas.
Let the mixture sit for about two minutes, or as directed by the cleaning solution manufacturer. This gives it time to work on the stains.
Scrub the flagstone patio with the stiff push broom or hard-bristled brush. Work especially hard on stained areas. Apply more cleaner if needed, and continue scrubbing until stains dissipate.
Rinse the flagstone patio with water, using the garden hose. Wash away all the cleaning fluid, and wipe off excess water. This reveals any remaining stains or blemishes that need more attention.
Use the push broom and more cleaning mixture to remove any final remaining stains. Rinse again.
Let the flagstone patio dry completely. This may take a few days. Temporarily disregard any loose sand or small rocks resulting from the scrubbing process. Rinse all solution from push broom and put it aside to dry as well.
Sweep the flagstone patio with the clean push broom. Now your patio is spotless and ready for use.