Extending the season
In addition to the traditional spring crop, it’s possible to have a summer crop if you have a lot of plants. Choose one or two of the most vigorous and don’t harvest any of their spears in the spring. In early July, cut the whole plant back to the ground — it will begin producing dozens of thick spears. As with the spring crop, simply harvest until the new spears are too thin for the table. You can keep a healthy plant on this summer schedule for a decade or more, but if it begins to weaken, switch it back to a spring-harvest format.
Female plants are short-lived and less productive than males; they also tend to produce unwanted seedlings and become weedy. Most hybrid seed lines produce far more male plants, but open-pollinated types are closer to 50/50. Even crowns aren't guaranteed to be male. The only way to tell them apart is to wait until berries form in summer, which identifies the plant as female, then dig them out.
Mail-order sources for both seeds and plants usually give you the best choice. Older, open-pollinated varieties, such as 'Mary Washington' and 'Martha Washington', are still available but are not productive or disease-resistant enough to compete with the more modern types. 'Viking' (also sold as 'Viking KB-3'), developed at Vineland Station in Ontario, is probably the best asparagus overall for cold-climate growing; it's hardy to Zone 4. An open-pollinated cultivar, it produces quite a number of female plants, so expect to do a lot of culling. You might want to try the newer, extra-hardy 'Guelph Millennium'. It has yet to be widely tested but so far has proven hardy to Zone 3 and quite disease-resistant.
The California hybrids ('Jersey Giant', 'Jersey Knight', 'Jersey King') and a few other hybrid strains, such as 'Supermale', are highly productive and nearly all male but are only hardy to Zone 5.
Finally, novelty varieties such as 'Purple Passion' and 'Purple Jumbo', both disease-resistant and hardy to Zone 5 (4, with mulch), have attractive deep purple rather than green spears, which unfortunately turn green when cooked.
Green vs. white
Europeans have customarily grown white asparagus, maintaining that it’s tastier and more tender than the green. The plants are the same, but the spears of white asparagus are blanched (cut off from light while growing). This is done by either mounding up soil over the plants in spring or covering the plants with an opaque cloche, called an asparagus jar. Mounds and jars are removed after harvesting; asparagus plants will die if permanently deprived of light.
Today’s asparagus is much less fibrous than older cultivars, and many North Americans actually prefer the green spears. You can decide for yourself by blanching one plant while leaving its neighbour unblanched, then doing a taste-test.