I'm not saying it's impossible to sell it, it's just that it will
likely be a long time to sell, and more importantly a great deal of work, and likely for a small return. Of the few garden centres or new startups looking for a domain, will they happen upon yours
and like it
and contact you?
When you factor in all your work, emails, telephone calls, research into websites, garden centres etc etc, then you time could be spent elsewhere on a name which will take less time, or even the same time but sell for at least mid to high $xxx and reap you some profit based on your time spent.
Your keywords are not really premium ("GardenCentre.com" etc), so is there not a lot of equal quality options to hand reg, for $10?
This is the sort of company which sells plants and trees:
http://www.homedepot.com/b/Outdoors-Garden-Center/N-5yc1vZbx6k
And they sell much more than "plants and trees" (compost, tools, fencing, decking, etc etc) so you should target garden centres in general and open up your potential market.
Good luck