If you soak the clothes with the grass stains soon after then the grass stains will usually easily come out during the wash cycle.
What I do to remove grass stains from clothes is to wet the grass stain with isopropyl alcohol and let it sit for awhile.
Then I put the clothes into the washing machine and let it soak for an hour and add some baking soda and vinegar to the washing machine and let that work on the grass stain.
Then I start the washing machine up and let it run through the cycle and the grass stain usually comes out during one wash.
Resolve on grass stains also helps get rid of the grass stains and it's very important to treat the grass stain as soon as you can because the longer the grass stain stays on the clothing the harder the grass stain is to get out of the clothes.