Once you have the Zen Garden, water the plants until they don't need anything else. After that, change the Date on your PC to another day. As soon as you do, the plants will need water again. Repeat this -- buy new plants, grow them to full size, sell and repeat to earn money.
This also works to get Marigolds in Crazy Dave's shop when they are out of stock. You can fill up your Zen Garden and make all the money you need. You don't even have to close out of the game.
Additions by dragonfellow17