As the asparagus harvest season comes to an end, it is time to evaluate steps that need to be taken in order to maintain or improve a planting's productivity. From now until the end of the growing season this fall, asparagus plants (ferns) must produce and store compounds that are a result of the photosynthetic process. In essence, they are "plant factories" that supply the energy to the crown and storage roots for next year's crop. Anything that a gardener can do to keep healthy, vigorous ferns throughout the post-harvest growing season will increase asparagus yield and quality the next season.
Asparagus (Asparagus officinalis) is a member of the Asparagacea plant family and thought to be native to the eastern Mediterranean region of the world. 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. Interestingly, written records of that era reveal the methods for growing asparagus did not differ greatly from those we use today.
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.
Asparagus was brought to America by the first European settlers and has ben an important perennial vegetable since colonial times. Classified as a cool-season vegetable, Michigan leads the U. S. in commercial asparagus production, followed by California, New Jersey and Washington. In Missouri, asparagus primarily is a home garden vegetable.
If an established asparagus planting begins to produce small, thin spears, one or more cultural problems might be the cause. This condition may be a sign of low fertility or a protracted harvest of spears the previous growing season. Although asparagus seldom is damaged by winter conditions, an abrupt decline in production could be the result of severe winter temperatures which may have damaged some of the larger roots.
Another influence on spear size is the gender of the plant. Asparagus is a dioecious species, having separate male and female plants. Female plants produce red berries while male plants do not. As a general rule, male plants tend to be more productive and produce thicker spears than female plants. Unfortunately, there is no way to distinguish between male and female seedlings until they become large enough to flower and produce berries.
In recent years, a number of male hybrid asparagus varieties have been introduced. For example, Rutgers University released the "Jersey line" of male hybrids (e.g., Jersey Giant, Jersey Supreme, Jersey King and Jersey Knight). Thus, planting crowns of a popular variety such as Jersey Giant will assure an "all-male" stand of asparagus.
A new asparagus bed may take several years before abundant, thick spears are produced. When production declines, it is time to increase the vigor of the planting. Well planted and established asparagus beds can be productive for as long as 20 to 30 years or more, if soil fertility is maintained and weeds are controlled.
To maintain soil fertility. liberal amounts of fertilizer should be applied as soon as each harvest season has ended in June. Well-rotted manure or compost can be applied at the rate of about one bushel per 30 square feet of bed area. Alternatively, a compete, balanced fertilizer such as 10-10-10 or 12-12-12 may be added at the rate of about 1½ cups per 10 feet of row. Scatter the fertilizer lightly and uniformly but do not allow it to touch the stems of the asparagus plants. Plant appearance can be used as a guide for fertility needs. If excessively vigorous growth is occurring, a lighter application of fertilizer might be adequate.
Since vigorous growth of ferns is important from the time harvest ends until fall, good weed control is essential also. Although cultivation and hand pulling always are options, in larger plantings there are several herbicides labeled for the control of weeds in asparagus that might be employed. Pre-emerge herbicides with long residual activity (e.g., trifluralin or simazine) can be applied in the early spring before spears begin to emerge to control annual weeds. Pre-emerge herbicides will not control established weeds. To accomplish that, a postemergence herbicide is needed.
Postemergence herbicides may be applied either before asparagus spears emerge, during harvest or after the harvest season ends, depending on the herbicide used. For example, glyphosate, a non-selective herbicide, can be used to control established weeds after harvesting the final cutting of spears and no asparagus foliage remains above ground. It is important not to allow non-selective herbicides to contact asparagus spears or ferns, since plant injury may occur. Whatever the method employed, weeds are easier to control with postemergence herbicides when they are relatively small (less than six inches tall).
Table salt once was commonly used to kill weeds in asparagus plantings, since asparagus is fairly salt-tolerant and most weeds are not. This practice is now discouraged, since the prolonged use of salt leads to the deterioration of the physical properties of soil and a reduction in plant growth.
Another pest that warrants monitoring is the asparagus beetle, a common insect that feeds on asparagus. Both adult and larvae must be kept under control for maximum plant growth and future spear production. Pesticides labelled for control of asparagus beetle (e.g., carbaryl) should be used whenever the pest becomes problematic. Normally, chemical control is warranted when 5 to 10% or more of the ferns show beetle infestation. When using any pesticide, always read and follow label directions carefully.
Additional information on growing asparagus in the home garden can be found in MU Extension Guide 6405 which is available from your county's University Extension Center, or can be found online at https://extension.missouri.edu/publications/g6405.
REVISED: June 4, 2021