It's not too difficult to get trees growing in blender. Open blender, install the addon, append a twig and start growing a tree by adding a Grove4 mesh. Even with the default setup the trees look pretty good.
The problem is exporting these trees to Terragen. The twig system is great if you use it with the Particle System. But as far as I see this method is not transferable to Terragen. You will have to grow the tree with object instances of the twigs. And that makes blender extremely slow and it is consuming a lot of RAM (>10GB for a pretty simple 8 year old tree).
If someone knows a way how to use the Particle System approach in Terragen, I would be very interested. Without this I cannot recommend Grove4 for Terragen users.