Asparagus is one of the first garden plants to emerge in the spring. California, Washington and Michigan are the chief producers in the United States. A cool-weather plant, asparagus is rarely grown commercially in America's Deep South. Asparagus harvest starts when the soil warms to about 50 degrees Fahrenheit and lasts for about four to six weeks. Harvesting spears that are past their prime robs the plant of energy that would otherwise go into the next year's crop.
Asparagus grows in a wide range of climates. According to an article by Barbara Pleasant in "Mother Earth News," asparagus is best suited to U.S. Department of Agriculture plant hardiness zones 3 through 8. Farmers and home gardeners have, however, grown asparagus in areas as diverse as California's Imperial Valley, where the summer temperatures can reach 115 degrees Fahrenheit, to northern Minnesota, where winter temperatures can drop to minus 40 degrees. In hot climates, fast-growing varieties will grow spears before the heat ends their season. Pleasant recommends "Apollo" or "UC-157." In cold climates, slow sprouting and frost-tolerant varieties help the plant survive late frosts. For these cold regions, Pleasant recommends "Guelph Millennium," which emerges a week or more later than most asparagus varieties.
Asparagus is typically grown from crowns, the below-ground buds from which the asparagus stalks emerge. In the ground, with proper mulch, asparagus crowns can survive below-zero temperatures. Once dug, however, they freeze easily. Asparagus crowns should be stored in cool temperatures, roughly 40 degrees Fahrenheit, and should be protected from freezing. Asparagus crowns should be planted in the early spring, when the crowns are still dormant. Plant them as soon as the ground can be worked, when the soil is roughly 40 degrees.
Asparagus sprouts begin to emerge when the soil temperature reaches 50 degrees Fahrenheit. As the temperature rises, the stalks begin to grow more quickly. When the temperature rises above 70 or 75 degrees Fahrenheit, the asparagus tips begin to fern out early, requiring that you harvest more frequently. You will find that plants with spears that could be harvested at 7 and 9 inches in early spring will begin to fern out at 5 to 7 inches as the weather warms. The ideal growing temperature for asparagus is 60 to 65 degrees Fahrenheit. At temperatures above roughly 90 degrees, the heads become loose and bolt before filling out.
Asparagus grows best in areas where freezing temperatures force the crown into dormancy. In warmer areas, however, autumn drought can serve the same function. The key to a successful winter is allowing the late spears of the plant to go to seed. These late shoots should not be cut until they are dead and dried out. At that point they can be cut a few inches about the surface of the soil. In cold climates, especially very cold climates or cold climates with very little snow, a thick layer of mulch will help prevent winter damage to the roots.