Thanks guys for the comments. The grass is cut off clumbs of a cheap nylon broom. It was an old broom just sitting in my shed so no harm done and the wife none the wiser. Yes it was tedious and time consuming. What I did was push holes in the foam base with a pencil, filled the holes with white glue and then started cutting off the clumbs of nylon from the boom and slotting them into the holes I had made. Simple yes, but I had to do a small area at a time until the glue dried as it was falling out whenever I touched it. When I finished it I thought now how do I paint the grey colored nylon strands not to mention the bright blue foam base? Really that wasn't all that hard and the airbrush did the trick nicely.
I have always wanted to do a tank in long grass and whilst I am sure you can buy better looking grass on line, the trouble is you have to spend quite a bit of money buying an electro static applicator to make the grass stand up and I wasn't prepared to go to that expense. Plus, I wasn't entirely convinced the applicators work all that well on really long grass/reeds. Hope this helps.
Phil