If you have a garment that is grass stained, first try removing the stain with toothpaste and a toothbrush. Scrub the grass stain with the toothpaste and then launder as usual.
If the grass stain is still visible, do not dry the garment in the dryer. Instead, rub the grass stain with molasses and let it stand overnight.
Lastly, if you have a true stubborn grass stain that is resistant to everything, you can try rubbing methyl alcohol on it. Just be sure that the color in the garment has been set to prevent fading and running.