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.
Normally when your stamina falls below 50 you will not be unable to play another game until you have waited for it to increase again or have used energy drinks. If you set your phone's date to manual instead of network-controlled and then move the date forward 24 hours or more you will see that your stamina has been replenished when you play the game. Now change your date settings back to normal.