There are many ways to SAFELY remove your garden stains. Below are methods listed by Hortus.
Grass, foliage and flowers - Work earth friendly soap or detergent into the stain, then rinse with cool water. Do not use chlorine it is toxic but you could buy earth friendly bleach.
Mud - Let the stain dry and then brush it well to remove the soil. If the stain remains then treat it as if it were grass and use the above method. Strains from iron rich clay's should be treated as rust.
Rust - spreat the stain portion on a pan of boiling water and squeeze lemon juice on it, and spread the fabric in the sun to dry. Rince and repeat if needed.
Berries - Remove all the excess berries with a spoon. Thoroughly saturate the stain with cold water and work earth friendly soap or detergent into the area. Allow to be for 5 minutes. With the stained area stretched over a bowl in the sink, pour hot water through the back of the stain to force it out of the fabric.