Camophlage:

The name for that brush stuff is 'lichen', and if you look around, sometimes you can get it cheap in floral shops. Beware though, when dry, its VERY crumbly.

I used to use lots of it for my miniatures battlefields (BattleTech) and found a place just outside town where I filled a 30 litre cooler full of the stuff. Then home to cook it up with a 30% glycerin solution and some dye to color it. That funny smell in the store-bought bag is the glycerine. About every 2 years you gotta re-cook lichen as the glycerine leeches out of the 'bushes' and they gets brittle.

There is a new alternative that uses extruded foam-plastic, and it supposedly won't get brittle with age as the lichen does. I've seen it in the hobby shops in the model railroad section. Can't remember the maker, but will try to find it if you like.

I will warn ya though... your cats may wanna nibble on it. My cats used to snack on my gaming table all the time... were unholy terrors on the buildings too.