Historical wargamers have used coconut fiber mats for field grass and crops on their battlefields for over 30 years...and it's still a useful way to model fields. I incorporated the easily usable material into area terrain for my Warmachine battlefields. Tall grass makes a great +2 DEF cover terrain- not strong enough to stop a bullet and act as cover, but thick enough to obscure the target(especially a Rhulic Dwarf of course!).
The individual based 'clumps' can be removed from the area base to make way for figures or they can be used separately on a terrain board without a larger base.
The picture above shows a standard spring/summer version of the field grass.
And above you see some trenchers advancing through that version(sorry the pic is a bit bright).
For a complete "how to" make the terrain elements and to see how I use them in a WM battle take a look at the vids below...