From the community gardens thread it seems that I'm not the only one who was a little confused about how they worked at first (and thus just harvested, watered and hoed everything in sight).
So! I propose that when a new player first enters a Community Garden (or perhaps the Magic Rock informs them of the nearest one, whichever) they get a quest to harvest something, feed it to a piggy, collect the seeds from said piggys plop, then plant and water the seeds. Maybe with a sentence or two explaining how the Community Gardens work.
It wouldn't stop people who irk others deliberately, but I think it would help people who are new understand a lot more easily.