Hello and a happy new year,
I have not seen it mentioned here, so maybe some don't know it yet. There is a source of plants (with the ability to make your own) in a free program called Plant Studio
http://www.kurtz-fernhout.com/PlantStudio/It is a little dated and the plants don't have a high polygon count, but they look great in populations. You can also save them in various stages of the plant's development (botanologically correct)
The program is free (registration name and code are given in the order page) and the included plants, plus those in the exchange page are in the public domain.
It exports to OBJ, which when inserted into Terragen promptly crashes and closes Terragen. So, I imported the object into Wings3d and reexported it to OBJ without changing anything. This file did not crash Terragen, but to be on the safe side, I saved as a TGO. The objects are huge, so you need to scale them way down. The one I tried, started to look acceptable at 0.05 scale.
Also, for trees, there is the GPL Arbaro
http://arbaro.sourceforge.net/ It exports to OBJ too and the objects don't crash in Terragen, but you will have to add textures in a 3d program first, like Wings3d. From within Terragen, AFAIK you can only give them a single color, although once textured, you can add images for colors and displacement.
Helen