You could optimize your plants by baking them down to much simpler models and use alpha mapping to cut out the leaves. If you were to do this you could probably use less than 50 polygons for each plant. It would also make it possible to paint in unique details on the leaves to make them more interesting. I'd add moss with…
Thanks for the vertex painting link. For the moss, I was thinking of using this technique here: http://www.polycount.com/forum/showthread.php?t=130038&highlight=rock+moss But I will play around with both methods to see which one I prefer :) I will also try to use alpha maps more moving forward.