Start at the bottom to avoid streaking and use your garden hose to rinse off each section before it dries.
How to clean grass stains off vinyl siding.
Hot water strong detergent such as tsp substitute siding cleaner or spic and span scrubber cleaners for stain removal choose one grass grease oil.
For a stronger solution mix.
Basic vinyl siding cleaning.
Apply the vinyl siding cleaning solution with a soft bristle cleaning brush scrubbing the full length of each lap.
Enzyme digester isopropyl alcohol a cloth white vinegar.
Well while vinegar can truly help with the stains you can also use lemon since the acid it has works as good as that of the vinegar.
And for a complete cleaning try power washing your siding every once in a while.
Fantastik murphy oil soap windex or lysol tar.