There is an excellent modeler tool, that allows you to devise a high definition tree or bush (High Poly in modeling language), which you can fine here:
It's written in java, hence multi-platform, and can be downloaded on this page: http://sourceforge.net/projects/arbaro/
Once you've defined your vegeteal, you can export the result in Blender (using the .obj format). Form there, it is possible for someone knowing Blender to create a Low Poly version, which can be used ingame.
There are tutorial vids in English, and quite detailed ones, such as:
One tutorial is planned here to give you the bases for using and proposing models to be used for Khanat.