It could work.
But like you were wondering, is there enough people to support a Green Thumbs forum? Here is a test, you should make a thread either in General Discussion, Members Forum, How-To or Advice (and I am leaning more towards General Discussion and Members Forum). The thread should be fairly clean, asking:
"Hey, are there any gardeners, plant workers, greenhousers out there? What do you like growing, or taking care of? I am wondering because I am an avid gardener myself, and I would like to talk with other Green Thumbs out there."
Something along those lines. We can judge responses, and overall activity of that thread to see if a forum would survive. Good luck
