You can create meshes for a forest in a 3d program like blender, 3ds , maya,… After that place them with the foliage tool.
Here are some tutorials:
- Tree (very basic version):
- Grass/Flowers/Bushes:
It takes some time until you get a good result, but after that it will be very easy for you to create vegetation meshes.
More useful tutorials for meshes:
- LOD : ?v=MH-icn_L2mk
- LOD Blender:
- Collision:
- Lightmap: