Should be an interesting looking diorama. I know that Woodland Sceenic makes a whole array of different foliage types to be used for whatever is neccessary. I've seen a couple of shots of a Shep Paine dio with a Sherman ploughing through some hedgerows, and it looked like a combination of lichen and tall grass.
If it's tall grass, again a variety of things will work. There are plenty of commercial products, but you can take an old broom, or thick brush, and use the wiskers.