You may have noticed Watering Cans are available in the FarmVille Market and on the 'Free Gifts' page. These are needed to grow Mystery Seeds from your Orchard. Each Mystery Seed needs to be watered 10 times, so ten watering cans each are needed.
You can send Watering Cans to your friends or request them through the news feed. They can also be purchased in the FarmVille Market for 1 Farm Cash each.