Or, well, I have completed it, but the game hasn't noticed. I've broken my watering can; in the quest log, the "break a watering can" condition is checked off as completed, which is the only requirement; but it isn't giving me the reward or clearing it from the log.
I actually broke the can several times--break, repair a small amount, break again--to try to trigger the quest completion. In several cases, it took a few minutes for the game to notice that I had worn out all the uses in the can and to change it to broken in my inventory.