Has anyone heard of PlantStudio?
I have used it for a few projects.
It is free, it has a small library of plants which you can edit or use a randomizer to change. You can make your own from scratch, it is easy to use and has built in tutorials.
I also use XFrog and this is no Xfrog but sometimes the simple tool is the best tool.
You can export 3ds, lwo, obj, dxf, pov inc and vrml.
I have only used the lwo export option, it worked well.
PC only. Try it you will probably like it.
Download it here
http://www.kurtz-fernhout.com/-Steve Horstmeyer