Green stains on siding are usually the result of mold, mildew, algae or moss. These stains occur naturally over time. When house siding begins to turn green, it diminishes the exterior appearance of the home. Fortunately, siding can be cleaned to remove not only green stains, but other dirt and stains that may be taking over the homes' exterior. It is best to clean the entire sided surface rather than just the green-stained area. This will leave a clean, even appearance over the entire home.
Cover any electrical outlets on the exterior of the home with duct tape. Be sure that all windows and doors are shut inside of the home. Cover landscaping and plants around the house with plastic sheets to protect them from the cleaning solution.
Spray down the siding with plain water from the garden hose. Set the nozzle on medium force and begin at the top, spraying the water down. Avoid spraying the water from the ground up, as large amounts of water may seep behind the siding, causing damage.
Mix a solution of trisodium phosphate and warm water into a large bucket. Generally, use ½ lb. trisodiium phosphate per two gallons of water. Refer to the label on the trisodium phosphate for specific mixing instructions. Test a small area of the green stains to be sure that the TSP solution will remove the stains. If it does not, mix a solution of one part chlorine bleach to four parts water to clean the siding. This means that the stains are likely mold and will need to be cleaned with a stronger solution.
Scrub the siding with a long-handled, stiff scrub brush dipped into either the TSP or bleach solution. Scrub from side to side, applying extra force over the green stains. RV brushes work well for cleaning siding. Begin working at the top of the house and work your way down. Work in sections of about 20 feet in width and the full height of the structure.
Rinse each 20 foot section with plain water from the garden hose as soon as you are done scrubbing. Again, rinse with the hose pointed towards the ground to reduce water seepage behind the siding.
Use oxygen bleach as an alternative to TSP or chlorine bleach. Mix a solution of oxygen bleach and water into a large bucket. The exact ratio of bleach to water may vary depending on the concentration of sodium percarbonate in the oxygen bleach. Generally, use between 4 oz. and 6 oz. for every gallon of water. Clean the siding in the same fashion as noted above using the oxygen bleach mixture. Oxygen bleach is a safer alternative to chlorine bleach and is also effective at killing and removing mold and mildew.