The devil to get rid of. Unfortunately washing has probably set the stain. They say you should brush away as much of the pollen as possible before washing. Try a paste of bicarbonate of soda ..leaving it on the garment for a good few hours and then wash again .Ecover Laundry bleach is very good..soak for a few hours. .Or a good rub with some laundry soap ..Fairy for example or Sunlight ..I think they still make it..
You may need to keep on soaking and washing before it finally disappears.