Do not harvest asparagus spears in the first year. This allows all the plants to develop a strong root system. Harvest mid to late spring in the third year after planting. After the third year, the harvest will extend 6-10 weeks. Harvest when spears are ¼-¾” (6-19 mm) in diameter and about 5-8” (13-20 cm) long by snapping offshoots at the base. Harvest before the stems toughen and the scales at the tips begin to open.
For more information on growing Asparagus, check out this great guide from Harvest To Table: