Most grass stains can be removed by simply washing the item according to the care instructions, especially if the stain is fresh. But if the stain has been allowed to set or proves to be difficult to remove, you may need to try other treatments.
Treat the stain as soon as possible, using a pretreatment product from your local grocery store. Test the product for colorfastness by applying to an unexposed area. Let stand for five minutes, then rinse. If the color is affected, don’t use the product. Also check the label on your laundry detergent for pre-treatment instructions. To remove any last traces of the stain, an all-fabric bleach may be necessary. Again, remember to test for colorfastness. Bleach the entire garment following the manufacturer’s instructions, then launder as usual.