Here's my quick and dirty way of making mushrooms for my bases or terrain in general (what Mordheim/Warhammer themed scenario or miniature is complete without shrooms?)
1) Get some small nails. I've used two sizes of brass nails (the kind you use for hanging up small pictures and crap).
2) I stuck 'em on a piece of discarded polystyrene and went to town on them with white paint.
Just get down and dirty.
Nevermind thinning out your paint, thick paint in this case just adds texture.
3) Next up, slap some red paint on the hats. I don't bother with the foot (or what the hell you foreign devils call it) since it won't show much once you've placed it out on your base.
4) Just to be extra serious I slapped some blood red ink on them for a wee bit more depth. In this case it was Windsor & Newton calligraphy Ink- Indian Red. Very good ink when doing blood & gore on your zombies fyi. Once dry, do a slight drybrush with the same red as in step 3.
5) Add some white dots, mount on the base and voilá: instant shrooms.
I think the whole process took about 20 minutes including dry-time.