Not sure if this is what you are after but here's a really quick tut on trees.
Cut an organic, rounded shape from really thick card for your base. Cut two and glue them together with PVA for a more solid base. Bevel the edge with a *sharp* craft knife. Slap some polyfilla on it to texture the ground a bit (this could be skipped if you are not a perfectionist), when dry paint it with a watery wash of Cadian Green. You should end up with something that looks like this:
Then pick a piece of foliage from your ridiculously huge box of plants, which may include aquarium plants, seed pods, large bits of stuff from pot pourri, or those wooden plants that middle aged people like me think look good as an acceptable alternative to fresh flowers in a vase.* Your box of plants should look like this:
Using green stuff, affix the plant to the base. Repeat until you are happy with the effect:
Then flock or static grass the base. If you have used a bit of imagination and different types of plants then it should look a bit like this:
Try and get plants of different heights to make it look good.
Well, that's how I do it. Its an expensive way to do it with bought plants, but it's quick and I think it looks good. If you wanted to do a crater you could just use the polyfilla'd base with a little sculpting. Haven't tried it myself, but it should look alright.
Jpunk (who should be revising but is painting a land-raider and listening to the creepshow. Whoops.)
*Disclaimer, I am merely middle aged, I don't like wooden plants as a form of interior decoration.