It depends on the application.
I use everything! and I do mean everything. For 1/35 scale I have found using real foliage (grasses,herbs,leaves all dry of course) and grinding them with mortar and pestle; so I can regulate the consistance; works great. The final product looks good and the colors already close to what you'll need.
Paint takes well and gluing is a snap!
... oh ya for stuff like pine needles I use nylon rope. I take a razor and cut an 1/8 inch or so and it frays out to tiny needles.
and one last thing for palm leafs and the like I have used soda cans and cut out the shapes. It is the perfect thickness for most 1/35 tropical plant life.
Anyway I hope I helped, Have fun