Hello!
Lets look at the lollowing link:
woodlandscenics.woodlandscenics.com/.../GroundCover
I'd say a mixture of Underbrush, Clump Foliage, Bushes and Foliage Clusters with small addition of Foliage and Poly Fiber should cut it. It might also be worthwhile to form the main outline of a hedge with some material like styrofoam, then paint it dark green and do the surface with the materials mentioned above.
First of all, get some nice photo material of the thing you want to model - that helps the most.
Good luck and have a nice day
Paweł