http://simonschreibt.de/gat/airborn-trees/
http://wiki.polycount.com/wiki/Foliage
for hedges, you can use an opaque mesh for the center, and cover it with alpha masked cards. then you can edit the normals, with either the Normal Edit Modifier set to radial, or transfer the normals from another mesh, using the Data Transfer Modifier. the mesh you transfer normals from should be a sculpted blob that is roughly the shape of the bush.