Don, it's 'static grass' mat, sometimes called 'hairy grass'. It's available in rolls of differing shades, or can be bought in bags of fibre, although they take a lot more work to us. They have to be sprinkled onto a pre-painted base, which is then coated in adhesive and, when set, a static charge passed over the surface, to make the fibres 'stand up'.
The mat can be cut to size and glued down using PVA. When set, any 'bare' ares, such as pathways etc, can be created by slightly wetting the required area, and scraping off all, or as much as required, of the fibres. When dry, the area can then be painted or textured as required, and the 'spare' loose fibres removed can be used to create 'clumps' of grass or weed.
Have a look at Part 1 of my 'Diorama Guide' I posted some years ago as, from memory, it explains a bit more about diorama materials and preparation, and material for bases etc. I noticed you intend to use a sheet of expanded polystyrene - this is not a particularly good material to use for the actual base, but very useful for creating contours or depth.