When you have dried in grass stains or just dried grass stains in clothes that you want to get out you can use vinegar and water and soak the stain in the vinegar and water solution for 15 to 30 minutes.
To get dried grass stains out of clothes soak the stain for 15 to 30 minutes in a solution of two parts water to one part white vinegar.
Then using a white clean rag dab the stain until it comes out.
Then wash the grass stained clothing in the washing machine with an enzyme-based laundry detergent and the grass stain should come fully out of the clothing.
If not you will need to use some more vinegar and possibly some baking soda and then gently scrub the stain with a toothbrush and then wash the clothing again in the washing machine.