They may take two years before you can harvest them. But once established they'll come back every season for 10 or 20 years. From TheHomesteadSurvival.com, How to Succeed at Planting Asparagus | thisoldhouse.com
How To do it : Choose a sunny part of the garden with good drainage. Dig a trench and check the pH, which should be 6.5 to 7.5. Plant the crowns about 8 in. deep and 15 in. apart. Cover initially with 2 in. of dirt, and gradually fill the trench as the spears emerge.