Congratulations! The flower clock seems like a huge accomplishment! I never got it before on any of my towns, but most of them didn't last too long. :/ How long have you had your town? Or does it depend on how "perfect" your town is?
Ive had it Since the 14th (or about that) June. So about a month And i think how perfect youre town is does make a difference. The clue is Jacob's Ladder's flowers will appear (always by the cliff). Then you have to talk to Isabelle at the town hall to see the satisfaction of your residents. And eventually she'll say that they all love your town and you'll have unlocked the town hall upgrade and flower clock in the projects list.