Asparagus is our most popular perennial vegetable. A meal of early asparagus is a good "spring tonic" and sets the stage for an entire growing season of more healthful eating because of the availability of fresh vegetables from the garden. March though mid-April is an ideal time to establish a planting of asparagus in Missouri which, with proper management, should last 10 to 15 years of more.
The association of vegetables with good health is more than coincidental, and asparagus represents and excellent example of the latter. The scientific name of a plant often holds clues to the plant's original use by humans. For example, the scientific name for asparagus is Asparagus officinalis. The specific epithet of its name (officinalis) comes from the Latin officina, a term describing the storeroom of a monastery where medicines and other necessities were kept. It is fairly safe to assume, then, that the delectable spring vegetable asparagus first was consumed for its perceived medicinal properties. Evidently, such is the case, since history records that ancients first used it as a medicine to treat ailments ranging heart trouble to toothache.
The first use of asparagus dates back over 5000 years ago to the ancient Egyptians. Later, Greeks and Romans not only ate asparagus fresh, they dried it for winter use. History records that the Romans actually went to the trouble of freezing asparagus high in the Alps for the Feast of Epicurus. The Roman emperors were said to be so fond of the vegetable that they maintained a special fleet of ships to retrieve the frozen delicacy, with great haste.
By the 15th century asparagus was cultivated in French monasteries. Evidently, French monarch Louis XIV was so fond of asparagus that he had special greenhouses built so that it could be grown year-around. Later, asparagus would find favor in England and the remainder of Europe. The early colonists are credited with introducing asparagus into the New World.
Mature asparagus plants. (Credit: University of New Hampshire)
Asparagus is a perennial, herbaceous plant that can grow to a height of nearly five feet. Its needle-like leaves botanically are known as "cladodes," which actually are modified stems. It has a deep, adventitious root system and overwinters as a crown, which is a collection of rhizomes (modified stems) and lateral roots. Asparagus (mainly) is a dioecious species with both male and female plants. Female plants bear small red "berries" (seed pods) which are toxic to humans.
The emerging shoot or "spear" is the edible part of asparagus. Harvest begins in spring as soon as spears begin to emerge and normally lasts until the early part of June. Green asparagus simply is asparagus that develops chlorophyll as the spears emerge. White asparagus, which has a milder taste than green, is asparagus that has been shielded from the sun, thus preventing the development of chlorophyll. This traditionally has been accomplished by the labor-intensive task of hilling asparagus beds and (later) removing the soil to expose the white spears.
White (blanched) vs. green asparagus
Because asparagus is a perennial vegetable, attention should be given to choosing the best planting site. Like most vegetables, asparagus will not tolerate wet, soggy soil. Choose well-drained soil, or use raised beds to promote drainage. Since weeds are a major problem, try to select a site with as few weeds as possible. Growing a cover crop during the summer (e.g., buckwheat) and the fall and winter (e.g., rye or wheat) the year before you wish to plant asparagus will suppress weed growth and increase organic matter in the soil. Once the bed is planted, there is little that can be done to improve the soil.
A soil sample should be taken the fall or spring before planting to determine its nutrient status. The optimal pH for asparagus is 6.5 to 7.0; lime may need to be incorporated into the soil before planting. Before planting, broadcast and incorporate fertilizer about 20 pounds of 10-20-10 or a similar fertilizer per 1,000 square feet of garden area.
For best results, asparagus should be planted in the spring as early as the soil in the garden or field can be worked. In southern regions of Missouri, asparagus can usually be planted in late March or early April; in central and northern regions, early to mid-April.
Dormant asparagus crown. (Credit: University of Maine)
Normally, asparagus is propagated vegetatively using dormant crowns. When available, male crowns should be used, since male plants are up to three time more productive than female plants. Examples of all-male asparagus varieties include 'Jersey Giant,' 'Jersey Supreme' and 'Jersey Knight.' Select healthy, 1-year-old crowns from an inspected nursery. Separate crowns by size, and plant similar-sized crowns together to encourage uniform growth. If crowns cannot be planted immediately, store them in a refrigerator.
To plant, make a 4- to 6-inch-deep furrow. Cover the fertilizer or mulch with an inch of soil, and space the crowns 12 to 18 inches apart in the furrow. If a variety produces large-diameter spears, you should reduce spacing within the row to decrease spear size. Each row should be no less than 5 feet apart so the ferns can close canopy and shade weeds during the summer. If rows are spaced too close together, spear size may be reduced. Cover the crowns with about 2 inches of soil and, as the plants emerge and grow, gradually fill in the furrow throughout the summer.
Illustration of planting furrows. (credit: University of Minnesota)
Alternatively, asparagus can be started from seeds. Seedling plants often are available in the spring along with other bedding plants. Or, asparagus can be directly seeded into the soil. If the latter is done, use eight to nine seeds per foot of row. Seeds should be planted about an inch deep and will require two to three weeks to germinate. Be aware that if asparagus is started from seeds, one half of the resulting plants will be female and less productive than all-male types.
Weed control is the most challenging component of successful asparagus production. Asparagus is a poor competitor with weeds. On small plantings, very light cultivation with a hoe may be used to remove weeds, but avoid using a power rotary tillers or any other tillage implements that can damage the crown, reduce yields and promote diseases. Organic mulches such as grass clippings, wood chips, straw/hay or compost can be applied 4 to 6 inches thick to suppress weeds.
Several herbicides are labeled for weed control in asparagus. Glyphosate can be used as a contact spray to control winter annual and biennial weeds early in the spring before the spears emerge and after the last harvest. Cover crops such as rye or wheat may be spring-seeded in row middles to suppress weeds. Common rock salt once was used to control shallow-rooted weeds in asparagus because asparagus is deep rooted and can tolerate some salt. This practice is no longer recommended because the salt can damage soil structure by creating a crust that impedes water infiltration.
Asparagus can be harvested for a limited time (two weeks) the second year after planting crowns. Overharvesting one year can weaken the plants and decrease yields the following year. Three years after planting the crowns, asparagus can be harvested for five to eight weeks. Each year during the first several years of production, yields will increase if the planting is managed properly.
Asparagus spears are best harvested by snapping them off by hand near ground level. Most gardeners prefer to snap the asparagus spears when they reach 7 to 9 inches in length in cool weather (less than 70 degrees F), or 5 to 7 inches in warmer weather (more than 70 degrees F), and the spear tip is tight. Cutting with a knife is generally not recommended because it may spread diseases. Expect to harvest every one to three days as temperatures increase. Harvesting should stop when the majority of spears are the diameter of a pencil (less than 3/8 inch). Harvesting for a longer period will weaken plants and lead to poor production the following spring.
Spears ready for harvest. (Credit: Cal Poly University)
After harvest season has ended, the asparagus planting should be fertilized to stimulate summer and fall fern growth. A complete, balanced fertilizer (e.g., 13-13-13) can be applied at a rate of about 1.5 cups per 10 feet of row. Herbicides can be applied after harvest to control weed growth. Frost will desiccate the ferns, and they can then be cut in late fall or early winter. Do not mow ferns in early fall while they are still green because this will reduce the following spring's harvest. In northern Missouri, growers may mulch the crowns to protect them from low-temperature injury. The mulch can be raked to the row middles the following spring (early April), and spears will emerge for another harvest season.