Years ago my first models saw grass coming from packs of cheap flock powder. Dried sawdust is wonderful stuff except that it came in fairly lurid shades which promptly faded to pale brown as soon as they came into contact with sunlight.
Nowadays we can be a little more sophisticated. Static grass is all the fashion - little nylon fibres charged with electricity standing up properly. I built myself a device to do this some time ago and it works very well. What it doesn't do is shoot fibres into corners. For that the trusty Heiki puffer bottle is required.
In theory, the polythene bottle imparts a charge to the fibres as they are blown through the holes in the cap making them stand up as required. I'm not convinced as the power of puff tends to overcome the static, although if you do some careful vacuuming afterwards you can sort this out apparently. What you do get is grass in every crevice (on the layout, stop sniggering at the back) which looks OK to me.
Wanting some dense undergrowth I then shot cheap hair lacquer over this and drifted on some flock. which gives a real unkempt look to my waste ground. I've even sprinkled a few little flowers in to pretty things up a bit.