Grass stains, with their greenish hue, can be quite stubborn, primarily because they are a combination of protein and chlorophyll, which acts as a natural dye. Dye stains are some of the toughest stains to remove, so this may require several repeats to remove the stain fully.
With all protein stains, never put the garment in the dryer before the stain is removed, as heat sets stains, especially proteins.
Apply a small amount of liquid laundry detergent like Tide directly to the stain. Gently rub the fabric together or use a soft-bristled toothbrush or use a soft microfiber towel to work the soap into the fibers.
Allow the garment to soak in a basin of cold water and oxygen-based bleach, like OxiClean, for about 15 minutes. Gently scrub once time has elapsed to see if the stain is removed. Apply more detergent and soak again if the stain persists.
Wash the item in the hottest water, safe for the fabric. Remember to check the care label first!
Pro Tip: If the fabric is synthetic, like athletic wear, stain fighters designed specifically for synthetics, like Sweat X Sport Extreme Stain Remover, can make a huge difference in getting the stain out fully and fast. Use it instead of laundry detergent at the pre-treat stage.