I know it's weird, but I actually love doing the cleaning and watering of gardens way more than planting seeds, so I frequently end up cleaning gardens others have harvested. I'd love it if untended gardens developed weeds--it can just be the wiggly greenery for 'dirty plot', or it could be a new graphic. The way I see it, if you don't [water or plant, depending on whatever you want to trigger it] for so many days, it spontaneously reverts to weeds/dirty plots. Or, if you're feeling really fancy, the weed trigger could result in either weeds or a random crop/herb.