James C. Schmidt
Extension Specialist, Home Horticulture/4-H
University of Illinois Extension
Asparagus (Asparagus officinalis) is one of the most popular perennial spring vegetables and a well-cared for planting can last 20 years or more. It is an excellent source of vitamin A and also contains significant amounts of calcium, phosphorus, riboflavin, and vitamin C. A well-cared-for planting can last twenty years or more. Select the new hybrid asparagus varieties such as 'Jersey Giant', 'Jersey Prince', or 'Jersey Centennial'. These varieties out-yield the old 'Mary Washington' and 'Martha Washington' varieties, although they are still available. Because an asparagus planting will be in the same location for many years, it should be located at the edge of the garden where it will not be disturbed.
Thoroughly preparing the soil before planting will ensure a fertile site and good growth. In the fall before planting, spade the soil and incorporate liberal amounts of manure, compost, or peat moss. Also, add 8 to 10 pounds of a 5-10-5 fertilizer per 1,000 square feet. Avoid a site that is low, poorly drained, or shaded for most of the day.
Although it is possible to start asparagus from seed, it takes several years to produce asparagus ready for transplanting. Therefore, one-year plants or roots purchased from a nursery, garden center, or mail-order company is recommended. Two- or three-year plants are sometimes available but may have been over-crowded at the nursery and are more susceptible to damage when transplanted.
Asparagus can be planted in the spring as soon as the soil can be worked up but no later than May. Dig a trench 5 to 6 inches deep and space the plants 18 to 24 inches apart in the row. If more than one row is planted, space the rows 4 to 5 feet apart. This wide spacing is necessary because of the vigor of the fern growth during the first season and promotes rapid drying of the fern in the fall to prevent disease problems. A 50-foot row will supply 40 to 50 pounds of asparagus. You will need 8 to 10 plants per person. Cover the plants with 1 to 2 inches of soil and tamp down. As soon as the shoots begin to appear, add more soil around the plants but avoid covering them completely. Repeat this procedure as the plants grow until the trench is filled.
Asparagus requires little care once it is established. The biggest problem faced by gardeners is weed control. A weed control program should be started early. Weeds can be kept under control by carefully hoeing, cultivating, or using a rototiller. Cultivation deeper than 2 or 3 inches can damage the roots. Common salt has been used to control weeds in asparagus beds, but it is not recommended. The continued use of salt may eventually produce undesirably salty soil and lower yields. It can also be leached by rainwater into other areas of the garden killing other vegetables that are not as salt tolerant as asparagus. One good way to control weeds is to place some type of mulch 3 to 4 inches deep around the plants. Mulch reduces the need for cultivation, helps keep the soil cool, and delays the emergence of spears in the spring. Each spring before the spears appear, mix 1 to 1- 1/2 pounds of a 10-10-10 fertilizer (or comparable organic fertilizer) per 100 square feet lightly into the soil.
During the first year after planting, you should be able to harvest several times, depending on temperatures. There is no need to wait until two years after planting before you harvest. In fact, harvesting the first year after planting will stimulate more buds to be produced on the crown which means greater yields in later years. Spears can be harvested for a period of 2 to 3 weeks. In succeeding years, the length of harvest increases to about 4 to 6 weeks, or for as long as the spears are large.
Select spears that are 7 to 9 inches long with tight tips. As the tips begin to loosen, known as 'ferning out', the base of the spears begin to get tough. The diameter of the spear has no bearing on its toughness. Stop harvesting when about 3/4 of the spears are about the diameter of a pencil. These should be left to replenish the food supplies to the roots.
Asparagus is harvested by snapping the spears off at the ground or by cutting them off with a sharp knife just below the ground. If they are cut, care should be taken not to damage other nearby spears just below the surface. Asparagus should be used as soon as it is harvested, but it will remain fairly fresh for up to a week if kept at 35° to 38° F. with the cut ends in water.
Because the tops of asparagus plants produce and transfer food to the roots, they should be allowed to grow all summer. The tops can be removed when they die after a killing frost in fall, or they can be left in place during the winter for use as a snow barrier and removed in early spring. It may be important, however, to remove them before winter to prevent the carryover of asparagus rust and the over-wintering of asparagus beetles.
Pests and Diseases
Aphids, which are usually not a serious pest of asparagus, often appear in large numbers on the asparagus ferns. They can be washed off with a strong stream of water or with a detergent solution.
The most harmful insect pest of asparagus is the asparagus beetle. This beetle is a brilliant metallic bluish-black/reddish with six squarish yellow spots on the back. The larvae and adults can strip the fern, reducing spear production the following spring. Adults overwinter in protected areas and may feed on the spears in the spring. A small infestation can be picked off by hand, but a serious infestation requires an insecticide. Check with your local Extension office or garden center for which product to use. Do not use insecticides on the spears. Removing top growth, weeds, and debris eliminates places for the beetles to over-winter.
The most serious disease of asparagus is rust. It first appears as small, reddish-brown spots on the stems and then kills the leaves and shoots. Plant varieties that are resistant to rust and remove the tops of the plants in the fall to prevent the disease from over-wintering and infecting young shoots in the spring.