The folks at UC Davis experimented with their asparagus crops and came up with this happy conclusion: you can successfully get a spring and a summer/fall crop from your asparagus garden...if you are willing to plant twice as much. Here's how:
In early winter, when asparagus crowns are available from your favorite nursery, plant double the amount of asparagus needed for your household; start with 24 crowns, at a minimum. Divide your plantings into two areas. Remember, it may take two or three years before the emerging spears are big enough for dinner, about the thickness of a pencil.
In spring, harvest half of the plants as you normally would; then, allow the foliage to grow for the rest of the season.
In the second half of your asparagus bed, allow the uncut spears to develop into ferns throughout the late spring and early summer.
Then, cut the ferns back to two inch stubs in July or August. This causes the crowns to send up new spears, which can be harvested in late summer and early fall. A light mulch will help keep the soil surface from becoming too hard, allowing the shoots to break through easily. Be sure to keep separate the harvest time for these two areas of asparagus. Otherwise, next year's plants may be too small to harvest.
Although a lot of work initially, these green taste treats are guaranteed to feed your family for a long time. A healthy asparagus plant can produce spears for more than 20 years.
http://farmerfredrant.blogspot.com/2009/08/fresh-garden-asparagus-in-september.html