Grass stains are tricky to remove, and since your trousers are white it will be most efficient to soak them in bleach. Rinse out the stain as best you can and then soak it in bleach. When the stain is gone, rinse the garment and wash it as you normally would. If the stain is still there after 15 minutes of soaking, it is probably not going to come out.
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In the past I have soaked clothing in hot water and then put shampoo on spots and either used an old toothbrush or fingernails to rub in and scrape off and go with the (grain) of material not against it
There are several methods to use to remove grass stains off of new shoes such as laundry detergent, water and vinegar as well as stain remover products. Read the directions completely prior to use.
Here's a great article on how to remove grass stains from jeans: http:/www.jeanshub.com/JeansCare-GrassStains.aspx use a enzyme presoak or put enzyme directly on the stain!!!!
The sooner you start to remove grass stains, the better luck you will have. Scrape off any residual mud and then pretreat the stain. Wash the clothes in hot water.