I can't prove it, but I think I got the gold watering can for donating ten (or eleven) different plants. I know I had at least the original five, and the mysterious stranger sold six more, I donated one of each and the next day I had the watering can, so I assume it has something to do with how many plant varieties you have or possibly donate. I wish there was an FAQ for this game! There are so many basic gameplay questions I'd like answered.
I didn't use the spoiler thingy because this is pure conjecture.