Fast fix: Not required.
Home remedy: Apply a pretreatment product, such as Spray ’n Wash or OxiClean. Let the mixture soak in, then scrub lightly with an old toothbrush. Launder using a detergent with enzymes, such as Wisk or Tide. (Enzymes help break up protein stains, like grass, making them easier to remove.) If the stain persists, wash again with an oxygen bleach for colors or a chlorine bleach for whites.
Watch How To: Remove a Grass Stain