Create background scenery with images

If they can be just 2D planes of imagery on flats then we usually hide the spot where that image plane meets the ground plane with some debris just in front of the plants. So some true 3D rocks scattered about or some 3D grasses (hair or mograph). If you really want to go crazy you can use the image as a deformation map in the material and that way the bushes will have a bit of outward geometry (not true of course but that far back it should work).

Its all about breaking up the alpha of the plants where they touch the ground.

Make sure to layer your shrubbery ("with a nice little path..Neee!") so some is in front and some behind and that way they parallax when the camera moves. You can also use a target tag to make sure they always face camera so you don't ever see they are truly flat. This only works for subtle camera moves - too much and you will realize they are rotating in relationship to everything else.