The default material to make bushes has for decades been lichens. They can be bought at your LHS, model train shops, craft stores and if you live near the woods, plucked from trees or growing on the forest floor. The model railroad lichens are commonly dyed in hideous colors not found in nature, so a quick shot from the airbrush or spray can is necessary to render them a more appropriate color. The main problem with lichens is that they don't look much like real bushes, namely because they don't have any leaves. This can be fixed by hitting them with a blast of spray-glue, such as 3M's Super 77, sprinkling on dried herbs such as oregano, basil, parsley leaves or Woodland Scenics "Coarse Turf" shredded foam and painting when dry. You can also use the same technique with tree roots, but after the spray-glue treatment, cover the roots with Woodland Scenics "Poly Fiber", trim to shape with scissors, spray again with spray-glue, and cover with herb leaves or shredded foam. Paint when dry.