When you have the Zen Garden water the plants until they don’t need any more water. Then change the Date on your device to another day. When you do this the plants will need watering again. Keep repeating this – buy new plants, grow them to full size, sell and repeat to earn money. This trick also be used to get Marigolds in Crazy Dave’s shop when they are out of stock. You can use this trick to fill up your Zen Garden and make as much money as you desire without having to close out of the game.