According to the website MrsCleanUSA, grass stains are a mixture of protein and other organic matter including chlorophyll and other relatively stable pigmented compounds, such as xanthophylls and carotenoids. These materials closely bind to the fibers in cotton when rubbed together, making the removal of grass stains a hard task for many. By pre-treating the stain and using some elbow grease, you too can get rid of pesky grass stains on khaki pants.
Brush off any excess dirt or grass left on the pants. Do not rub it into the pants. Simply use your hand to brush the excess into a wastebasket.
Pre-treat the stain by pouring the proper amount of liquid detergent for your load of laundry directly onto the stain itself. With most bottles of laundry detergent, the amount needed to pour into the wash is marked on the lid to the bottle; this amount should be about 1/2 of the lid to the laundry detergent bottle.
Scrub the detergent into the stain itself using the scrub brush. Put some force behind the scrub, and try to scrub it until the stain begins to dissolve on its own.
Let the pants sit out 10 to 15 minutes before you start the wash. Wash the pants with like colors in cool water.
Once the wash cycle is complete, remove the pants and check for grass stains. Do not dry the pants until the grass stain is completely removed. You can repeat steps 2 to 4 until the stain is completely gone from the pants.