Ditto on the Woodland scenics. Their static grass is probobly the best for short grass, and comes in a variety of colors, or can be painted with a dry brush technique or airbrushed once set.
If you want to do it yourself....
Trees: the roots of woody plants make good trees. The bark and branches are "in scale" and you can get some very cool shapes. If you use a very fine piece with lots of branches you can hit it with spray glue and roll it in any of the leaf products, oregano, chopped tea leaves or any ground up plant material for leaves. (Found another good use for my wifes food processor)
Tall grass: Hemp or cisal rope, frayed, makes good grass. The cisal twine again, good for grass but a differnt thickness. Makes good straw for barnyard scenes. Tedious to make a field, but worth the effort. You can paint it if you don't like the dried gass color.
Bushes: Again the roots, but also cruise the dried flower section of the craft stores. Some of the plants used there, make good shrubs and flowered plants.
Some of the better artificial aquarium plant products have some good potential. The more expensive the better the detail and the more realistic.
Kitty litter makes good gravel and ground material. Split peas and anti skid carpet pads make good cobblestones. Broken drywall makes great rubble. Vanilla wafers, not good for dioramas, but are sure tastey and great with milk.
Once you realize the potential for "at home products" to be used in different applications for dioramas you will never look at anything the same again. Now if I can just figure out what to do with all this dog hair....
"Imagination is the dye that colors our lives"
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"