Grass stains are particularly hard to get out of jeans. Denim is a thick fabric that loses color with bleach and other harsh cleaners. However, there are some methods that do work to remove those stubborn stains.
Check to see if the jeans are colorfast
With a white cloth, rub straight white vinegar into a hem or the inside of the waistband. If the cloth is still white, then the jeans pass the colorfast test. If some of the color comes off on the cloth, then your jeans are not colorfast.
If the jeans are colorfast, pour white vinegar directly onto the grass stain. In many cases, this removes the entire grass stain. If some staining still remains, soak the jeans in vinegar for up to 2 hours. If the jeans are not colorfast, dilute the vinegar with an equal amount of water and then soak your jeans for 2 hours. After soaking the jeans, wash as normal.
Use baking soda toothpaste
If you don't want to use vinegar, rub a baking soda toothpaste into the stain with an old toothbrush. The grittiness in the toothpaste helps loosen the grass stain. Once the stain is completely covered, wash the jeans as you normally do.
Use stain removing products
If you don't want to use home remedies, there are some laundry products on the market designed to remove grass stains. Follow the manufacturer's directions. Some products work better than others.
Let the jeans air dry
Once you have pre-treated the stain and laundered the jeans, let them air dry completely. It's almost impossible to tell if a stain has been removed when jeans are wet. If you accidentally dry them with the stain still present, you risk setting the stain forever. Once the jeans are completely dry, look at them in bright sunlight to see if any of the stain remains.
Repeat if necessary
If the grass stain is still there, then either repeat the same treatment procedure or choose a different one. Launder and air dry again. Check the spot to make certain the stain is completely gone.