Grass has natural pigments from chlorophyll that make it particularly tough to get out of fabric. Act quickly, before the stain has time to set. Combine dish soap with hydrogen peroxide and soak the stain in the mixture. Let sit for 30 minutes, then scrub the spot and rinse with cold water. Another option is creating a white vinegar and baking soda rub, then allowing it to soak for 30 minutes before putting it in the washing machine.