Everything you need to learn about asparagus harvest, production and growth!
Introduction to Asparagus:
Asparagus is known in colloquial terms as sparagrass, sparrowgrass, and among larger growers of the crop, as “grass”. Asparagus is a perennial, dioecious, monocot plant that produces new above ground stems each year from a fleshy underground “crown”. The crown is not a typical rhizome but consists of unelongated basal internodes of old stems.
Increase in size of the crown result from the development of buds both apical and lateral. On the crown are located many buds, each potentially capable of producing an upright stem. New roots develop from the bases of actively growing buds. The roots are very thick and fleshy and are the primary food storage organs of the plant.
The small bracts at each node of the spear are the true leaves but are not photosynthetically active. The cladophylls which are actually modified stem functions as photosynthetically active organ.
Climate and Soil Required to Grow Asparagus:
Asparagus is a cool season crop that is very winter hardy. Mature plants especially when in the fern stage are also relatively tolerant to heat, drought and salinity. However, newly emerging spears are damaged by frost. Asparagus can be grown on many types of soil, but good drainage is imperative.
The best soils are those which are deep, loose and light. Sandy loams are ideal. Asparagus roots may reach a depth of 3 meter and the soil should allow full development of the storage roots. Soils ranging from slightly acid to slightly alkaline (pH. 6.0-7.5) are best. Asparagus will tolerate highly acidic and highly alkaline soils, but yields on such soils are low.
Seedling and Crown Production of Asparagus:
Asparagus seeds number approximately 40,000 per kilo. Approximately 500 g seed is needed to produce enough crowns (11,000-15,000) for one acre of irrigated production. For a transplant nursery, plant 3-4 cm deep and 2-4 cm apart in rows 60 cm apart. Seed should be planted in late spring in well- prepared fertile soil which is around 25°C.
Crowns are grown through the summer and are dug in the following spring for transplanting to commercial fields. During the spring, cut and remove the old tops and dig the crowns leaving 20-30 cm of storage root. Crowns should be dug just before the buds have started to grow.
Crown Planting of Asparagus:
Under dry land farming conditions, 5,500 crowns per acre are required. Use only well-graded 1-year old, large, best crowns. Planting is done in furrows taken at 1.0 – 1.5 m apart, 15-20 cm deep, with crowns 50-60 cm apart. The soil must be deeply tilled and trenches are made. Spread out roots with buds up, cover with a shallow layer of soil (not more than 4 cm) and irrigate.
The shallow trench or furrow is gradually filled in as the spears develop during the season, until at the end of the season the field is again level. On irrigated land, 11,000-15,000 crowns per acre or more are planted. Direct seeding of asparagus is not recommended but can be recommended where irrigation and proper planting and tillage equipment is available.
The fern should remain standing as long as it is green. Once the fern has turned brown, it is no longer providing storage reserves to the crown and should be removed by chopping, disking or burning. If rust is a problem, fern should be removed and not returned to the soil.
Varieties of Asparagus:
The advent of new hybrids and all-male asparagus varieties is a major achievement. Seed production of these (“all-male”) hybrids is based on vegetative propagation (usually by tissue culture) of the parents. Seed of all- male varieties produces only male plants. These all-male hybrid plants are high yielding and tolerant to diseases such as Fusarium wilt and asparagus rust.
They have greater longevity than female plants or mixed plantings. They do not produce seed, eliminating the problem of volunteer asparagus which becomes a “weed” problem. Seed of conventional varieties such as Mary Washington produces both male and female plants.
Asparagus varieties are of two types, based on the color of the spears. The more important group produces dark green sprouts when grown in sunlight and includes Mary Washington, Martha Washington, Reading Giant and Palmetto.
Standard Varieties – Mary Washington (tips remain tight and slow to fern), Jersey Queen (a giant female selection from Mary Washington).
All-Male Varieties – Jersey Giant (produces very large sprouts), Jersey Knight.
Synthetic Varieties – Plants may be a mixture of males and females- Synthetic 4-56 (mostly male plants but 30-40% females).
Standard Hybrids – Standard hybrids (plants are 50:50 males and females) – Jersey Centennial, UC 157.
Novelty – Purple Asparagus – Viola (a tetraploid also known as Purple Passion). Purple asparagus turns green when cooked as the purple pigment is destroyed by heat.
Recommended Varieties in India – UC 72, UC – 66, SL 831, Perfection, Brock Imperial 84, Mary Washington, Martha Washington, Conovers Colossal.
Hybrids – Dariana, Franklin, Geinlim, Andreas, Larac, Venlin.
Harvesting, Handling and Storage of Asparagus:
Asparagus is not harvested in the first year (the year of crown establishment). Harvesting is usually limited to 4-6 weeks depending on crop vigor during second year. Full-season harvest is conducted in the third year but full production usually occurs about the 4th year. Asparagus fields last about 12 to 15 years although some fields may remain productive for over 20 years.
Emerging spears should only be harvested for 6 to 8 weeks during the spring or the storage reserves of the crown will be depleted and spear production will be reduced the following year. Most commercial plantings are generally renewed at about every 10 years or when a dramatic decline in yield occurs.
An acre of asparagus may produce from 125,000 to 150,000 spears. Generally, spears are harvested when 8 to 12 inches tall while the tips are still very tight, with a special long handled knife. White asparagus can be produced from any variety by the exclusion of light by mounding soil over the row to a much greater depth than needed for green asparagus.
Spears are cut while still under the surface when they push the soil upward or when a wet spot is noted on the surface of the soil just before the spear breaks through. A long knife is then used to cut the spear. Spears must be handled in the absence of light to keep them white.
White asparagus can also be produced under black plastic tunnels. These are low covers supported by wire hoops. One side of the tunnel is buried by soil, the other held down by steel rods, wood or some other weights. Tunnels are opened periodically for harvest and then closed again.
Nearly all green asparagus is harvested with a little white on the butt end. Harvest intervals are generally 24-36 hours apart depending on weather. Spear growth begins, but is slow when soil temperatures reach 10°C, reaching a maximum rate at temperatures of 24-29.5°C with high soil moisture.
Spear length cut on any given day should be set in consideration with the interval to the next scheduled cutting, soil moisture, and air temperatures expected during that interval. If a long harvest interval is expected, and temperature and soil moisture are high, shorter spears should be cut, or else they will be unmarketable by the next cutting.
Conversely, if temperatures are cool and harvest interval is short, the shorter spears should be left for the next harvest. Cut asparagus should be cooled immediately, otherwise quality and food value degenerate rapidly. Control, loss of moisture by placing butts in cold water or in contact with other moisture-holding material.
Failure to reduce the field temperature promotes growth, causing loose tips. High quality spears have high sugar (4 to 5% soluble solids) and low fiber. When comparing spears of the same height, large diameter spears generally have less fiber than thin ones. To slow fiber formation and the loss of sugars, asparagus should be hydro cooled immediately after harvest to 0 to 1°C. Asparagus can be stored at 0 to 1°C and 95% humidity for up to 3 weeks.
Toughness of Asparagus:
The development of toughness in asparagus is due to the formation of fiber cells in the stalk. These cells are thick-walled and contain lignin, a supporting material. They are located in the internal vascular tissues as well as just under the surface of the stalk (the pericycle).
Facts about Fiber in Asparagus:
1. The occurrence of fiber cells and toughness is always greater at the base of the spear than the tip.
2. Smaller spears are considered to be tougher than spears of larger diameter because they contain a higher percentage of fiber on a weight basis. Avoid harvesting small-diameter spears (less than 1 cm).
3. White asparagus generally has higher fiber content than green asparagus; since light decreases fiber formation.
4. Due to the narrow genetic base of asparagus varieties, there appears to be little difference between cultivars in fiber content or development.
5. Asparagus developing during cool temperatures (10-12°C) is higher in fiber than spears harvested after warmer weather. This is due to reduced spear growth rate (size), but not due to reduced fiber cell development which continues during cool temperatures. Thus a fast growing spear out-paces fiber development. This is the greatest contributing factor in asparagus fiber levels. During cooler weather, harvest shorter spears. In warm weather, harvest before spears reach 25 cm.
6. Snapped asparagus (versus cutting) leaves a higher proportion of the fiber-heavy lower stem in the field, and over the period of harvest this method tends to keep overall fiber levels lower. Try to snap spears at ground level versus cutting below ground.
7. After harvest, several factors contribute to fiber formation. Most post-harvest fiber development occurs within 24 hours. This can be slowed dramatically by cooling promptly to 1°C. Cool the crop as quickly as possible after harvest. There is an increase in fiber with storage, especially at higher than optimum temperatures, but the increase is low under proper storage.
Storage temperatures should be from 0°C (short-term) to 1°C (long-term). Fiber content (or at least its integrity) can even be reduced under CA (controlled atmosphere) storage with low oxygen (2-3 %) and high carbon dioxide (5-10 %) levels. Water-loss after harvest also increases fiber development, making humidity in storage an important consideration. Film wraps or placing butt ends on water pads also helps to reduce fiber development.
Storage of Asparagus:
The recommended holding and shipping temperature is 0°C, with a relative humidity of 95%. Keep asparagus upright in containers with moisture pads whenever possible.
Fresh asparagus is highly perishable and deteriorates rapidly at temperatures above 5°C. Thus, the spears should be cooled immediately after cutting, preferably by hydro cooling.
In addition to general deterioration, spear growth, loss of tenderness, loss of flavor, loss of vitamin C, and development of decay take place at moderately high temperatures. Asparagus can be kept successfully for about 3 weeks at 0.5°C and 4-5 weeks in Controlled Atmosphere storage. It can be held for about 10 days at 0°C, but it is subject to chilling injury when held longer at this temperature.
High relative humidity is essential to prevent desiccation, particularly at the butt ends. Commonly, the desired relative humidity is obtained by placing the butts of asparagus on wet pads. A high relative humidity can also be obtained by prepackaging spears in perforated film. Non-perforated film is not acceptable because the extent of increase in carbon dioxide and decrease in oxygen may be injurious and because enough ethylene may accumulate and toughen the spears.
Bacterial soft rot, which can occur at either the tip or butt of the asparagus, is the principal cause of decay. Asparagus may be damaged by exposure to ethylene, and should not be stored with apples, or other ethylene generating material. Adverse exposure may result in undesirable elongation, curving, and toughening.
Controlled Atmosphere storage is beneficial to asparagus even for a short period because it retards decay and toughening, which occur rapidly after harvest. Gas concentrations should be 2-3% oxygen and 5-10% carbon dioxide at 0.5 to 1.0°C. If temperature control is uncertain and might exceed 7.5°C, the carbon dioxide concentration should not exceed 7 %; but if the temperature is maintained at 0°C, a 12 % concentration is suggested. Brief exposure to 20 % carbon dioxide will reduce soft rot at the butt end.