It's better to coat it in whatever you're going to represent. For example, grass, earth, etc. If it's going to be grass,either paint the surface with a 'grass' or 'earth' coloured emulsion paint, then sprinkle on the ground-cover fibre/dust when the paint is almost dry, or, coat the surface in adhesive, either 'white' glue or hairspray or similar, then do the grass powder.
If you just paint it, it will not look very convincing, and the paint might crack after a time.