Grown for its succulent young stems, the asparagus is any of a large group of erect or climbing plants widely distributed from Siberia to South Africa. The best-known and most widely cultivated species, however, is the common, or garden, asparagus, when allowed to go to seed, reaches 4 to 10 feet and has tender shoots, or spears, which are widely eaten as a vegetable. The shoots are low in calories but contain relatively large amounts of calcium and phosphorus. They also contain vitamins A and C and the B vitamin niacin. Generally, the shoots are eaten either green or white. The green shoots are usually sold fresh, while the white ones are used mostly for canning or freezing.
A typical asparagus plant ranges in height from 6 inches to 2 feet (15.2 to 61 cm) and grows from either a tuberous root system or an underground stem called a rhizome. Unlike most other plants, the asparagus does not produce true leaves. Instead, it bears leaflike branchlets, called cladodes, that produce many small scalelike structures, called cladophylls, which perform the functions of true leaves.
The flowers of the asparagus are greenish yellow in color. They are borne singly or in short spikes or flat-topped clusters, and in the fall each flower ripens into a small red berry, which contains about three seeds. There are male and female plants.
Asparagus has been known and cultivated for more than 2,000 years, ever since early Roman times. Although the majority of species are found in tropical and subtropical climates, the common asparagus is most successfully grown in temperate regions, where the winter rest period is more favorable than a continuous growing season all year round.
Almost any kind of soil can be used for raising asparagus, as long as the soil is well drained and as long as the water level is about 4 feet below the planting level. The plants grow best in fertile, nearly neutral soils, and are usually started indoors from seeds. Acid soils should be avoided, and the best soil is considered to be a mixture of sandy loams and loose clays. The soil should also be enriched with organic manure and chemical fertilizer.
When the seedlings are about a year old, they are transplanted to fields. Asparagus is also propagated by means of cuttings from roots of older plants. As the slender shoots come up, they are cut off at the base. Each season the stems grow thicker, but they are cut back until they grow thick enough for marketing. The asparagus plant needs three to four years of maturing to grow shoots of marketable size. A single plant may produce for as long as 16 years.
Blanched, or whitened, asparagus is produced by heaping soil over the budding stalks to keep them from sunlight. Generally, the rows for white asparagus are from 4 to 6 feet apart, while those for green asparagus are usually about 3 to 5 feet apart. Asparagus may be marketed canned, frozen, or fresh in bunches.
Diseases and Pests
While the plants are growing, they must be protected against various diseases and insect pests. Their most destructive enemy is asparagus rust (Puccinia asparagi), a fungus that is very difficult to control. The most effective method of combating this disease is to plant rust-resistant varieties, such as the Mary Washington.
The most serious insect pests are the asparagus beetle (Crioceris asparagi), the garden centipede (Scutigerella immaculata), and various kinds of cutworms. The asparagus beetle and the garden centipede can often be controlled through the use of insecticides, and the best method of combating cutworms is to flood the field in late December or early January.
Harvesting should not begin before the third year, and the shoots are usually cut in early June. Harvesting is always done by hand. The white shoots are cut to a length of 4 to 7 inches long; the green shoots are cut from 5 to 8 inches long. The shoots are then graded, trimmed, and tied in bundles.
Immediately after harvesting, manure and fertilizer should be added to the soil to produce strong crowns for the next season. In warm regions where there is no winter resting period, the grower creates a resting period for the plants by stopping the supply of irrigation water.