Hudson & Allen made some excellent tall grass a while back. I'm not sure if they still make it anymore, but I do know where you can get essentially the same thing. This used horsehair that was still on part of the hide. it was perfect b/c it narrowed to a fine tip at the end like grass. they had a summer grass that was dyed green and a winter one that looked to be a naturally blonde hair color.

here's what the summer grass looks like:


(can't get it to work, what else is new? here's the url- http://gallery.kitmaker.net/showphoto.php/photo/132233

if you can't find any hudson & allen, try heading to a fishing store, espeically one where you can buy supplies to make fly fishing lures. horse hair is commonly used to construct lures, and its sold much the same as hudson & allen did, still in the hide ( I should mention it isn't that difficult to remove it from the hide). they may even have some thats already dyed green or some other color, but if you can't find any you could try airbrushing whatever color you need.