http://www.kurtz-fernhout.com/PlantStudio/index.htm
It's a simple click-through plant creation and you can export as .dxf and .3df which is importable with blender. (I have successfully created a plant and exported it as a .dxf, imported it into blender, but I am still trying to learn export from blender for use in sandbox.)
I was hoping maybe someone else who is more advanced than myself might be able to give it a spin to see if it might be useful for sandbox since I'm still learning? You know, just in case I do something wrong and write it off as 'unusable' when it can be used. I mean, there's a bunch of pre-made plants for download too (http://www.kurtz-fernhout.com/PlantStudio/exchange.htm.
Edit to add: I guess PlantStudio also exports as .obj which can be used straight from the plant program to sandbox? Again, it's not going for me at this moment, but I'll keep plugging at it to see if I can't get something to work.
Second edit to add:
I got it to work finally! Hurrah! The .obj export works just fine though I had to save without colors and create a skin myself - as well as the obj.cfg.
But here's a quick test plant:
So yes, I guess this is a workable program for still plants. Animated ones, I have yet to try.