Just tootling around the internet today, and I found this:

http://www.winternet.com/~chuckg/dictionary.html

I think it is the most extensive dictionary I've seen as it includes the grammatical usage in botanical Latin (for those of us interested >>> me).

And HERE'S the BIG KICKER: Chuck Griffith, the author, is a rock gardener! But wait, there's more:

Chuck is a friend and long time member of our Minnesota Chapter of NARGS ! He is just full of surprises. I am embarrassed not to have known about this, but he's going to get a write up in our next newsletter, that's for sure.

Rick