Asparagus officinalis (PROSEA)
- Protologue: Sp. pl.: 313 (1753).
- Family: Liliaceae
- Chromosome number: 2n= 20
- Asparagus (En)
- Asperge (Fr)
- Indonesia/Malaysia: asparagus
- Philippines: asparago (Tagalog)
- Cambodia: tum'-peang barang
- Thailand: nomai-farang
- Vietnam: măng tây.
Origin and geographic distribution
The origin of asparagus is believed to be the eastern Mediterranean; however, it grows wild in Europe, the Caucasus and western Siberia. It is also naturalized in the Americas and New Zealand, and occurs now worldwide as a crop plant. In South-East Asia it is found mainly in Indonesia, Malaysia, and Thailand.
The major use of asparagus is to eat the lightly cooked tender young unexpanded shoots (spears). The spears are also processed either by quick-freezing, or by canning (or bottling) in brine. Spears may be harvested prior to emergence as white asparagus, or after emergence when 18-25 cm tall as green asparagus. The green spears should be all green, while the white spears should be all white. It is normal to peel the white spears prior to cooking, while the green spears are normally eaten unpeeled. There are references to the seed being used as a coffee substitute.
Production and international trade
It is estimated that there are about 150 000 ha of asparagus grown worldwide, producing some 500 000 t/year. Production predominates in North America (52 000 ha) and Europe (45 000 ha), with South America (30 000 ha) and Asia (20 000 ha) increasing in importance. Australasia (6000 ha) and Africa (3000 ha) are relatively unimportant. The world price fluctuates tremendously, but tends to be about 1000 US$/t. In the South-East Asian region asparagus is gaining in importance, with production predominating in Thailand and Indonesia, followed by Malaysia. Current statistics (1990) are: Thailand (1804 ha) producing 7966 t (85% green, 15% white; 85% is consumed locally, and in 1989 there were 900 t of fresh export and 186 t of canned export); Indonesia (1000 ha) producing 2100 t (60% green, 40% white; in 1990 80% was consumed locally, and there were 105 t of fresh export and 315 t of canned export). No data are available for the other countries, but Malaysia has much less production than either Indonesia or Thailand. There are plantings in the Philippines, chiefly in Mindanao.
The composition of the edible spears depends on whether one is considering white or green asparagus. Green asparagus contains per 100 g fresh weight: water 92 g, protein 2.8 g, fat 0.2 g, carbohydrates 2.2 g, vitamin A 980 IU, vitamin B1 0.23 mg, vitamin B2 0.15 mg, niacin 2.2 mg, C 48 mg, Ca 24 mg, Fe 1.5 mg, P 52 mg. The energy value is 113 kJ/100 g. White asparagus is slightly lower in protein (2.1 g), and considerably lower in some of the vitamins and minerals (vitamin A 50 IU, vitamin B2 0.08 mg, niacin 1.1 mg, C 28 mg, Ca 16 mg). The average weight of 1000 seeds is 40 g.
- Herbaceous, dioecious, climbing or erect perennial, up to 2 m tall, with a robust woody rhizome comprising a number of bud clusters and many long (1.5-2 m) unbranched, fleshy storage roots.
- Young stem fleshy when still underground; aboveground stem strongly branched, with fine, needle-like foliage.
- True leaves reduced to minute bract-like triangular brownish scales; in the axils of the scales, 3-6 subterete, green, needle-like, thin branchlets (cladodes), 1-2 cm long are present and seem to represent the leaves.
- Flowers solitary or in pairs in the leaf-axils, unisexual, small, tubular-campanulate, pendulous; tepals 6-8 mm long in male, 4-6 mm in female flowers.
- Fruit a globose berry, red, 1-6-seeded.
- Seed black.
Growth and development
Germination is normally slow, with the optimum temperature being 25-30 °C. Initially a single shoot, and a single root develop, but once the first shoot has fully expanded, a second shoot develops from the junction of the initial shoot and the root. This is the origin of the primary bud cluster, but in time secondary bud clusters develop in the axils of some of the primary buds. It is normal for each bud to develop two storage roots at about the time the bud develops into a shoot. There is very strong apical dominance on each bud cluster, and the next bud on the cluster does not normally develop until the previous bud is fully developed into a shoot (or the spear is harvested). In the tropics the foliage remains green and the plant never goes dormant. In temperate climates the aerial parts senesce during autumn, and growth is continued the following spring by the shooting of buds from the rhizome. Because the spears from these buds comprise the marketable yield, it is necessary initially to establish a large pool of stored food reserves in the swollen roots before harvesting, so that it is general practice not to harvest until 2 years after planting, and from the third to the fifth year slowly to increase the harvest period from 3-4 weeks to 10-12 weeks. It seems that senescence of asparagus in the tropics starts earlier than in temperate areas due to the absence of dormancy. In western Europe, plants of over 100 years of age have been reported, although it is normally not economical to harvest longer than ca. 10 years. In the tropics the crop is mostly used up after 5-6 years.
Other botanical information
Asparagus is normally dioecious, but occasionally andromonoecious plants occur. The plants are insect-pollinated. Many wild Asparagus L. species exist, mainly in Europe, Africa and Asia. In South-East Asia, A. cochinchinensis (Lour.) Merrill (distributed from Indo-China to Japan and the Philippines) produces edible tubers and A. racemosus Willd. (distributed from Africa through southern Asia and Malesia to Australia) edible roots. In Europe, many cultivars of A. officinalis exist.
Asparagus does not appear to have a daylength response. Photosynthetic activity appears to increase up to 300 W/m2 PAR (Physiologically-Active Radiation) as in most C3 plants. The optimum temperature for dry matter accumulation is 25-30 °C, but the optimum temperature for the accumulation of food reserves in the roots may be slightly lower. High relative humidity is a distinct disadvantage due to the problems of foliage diseases. The crop can be successfully produced at low altitudes even in the tropics, though spear quality may not be as high as that produced at higher altitudes. Absence of frost during the growing season is important. Deep well-drained sandy loams or volcanic soils are preferable, with an adequate supply of nutrients, particularly nitrogen and potassium. Asparagus appears to be able to grow in a very wide range of pH, though 5.8-6.5 appears optimum.
Propagation and planting
Propagation is primarily by seed. There is increasing interest in vegetative propagation (using tissue culture methods) to clone up high quality plants, but this method is still in the experimental stage. Seed may be sown either directly in the final growing site (uncommon), or in the field in a seedling nursery, or in modules under protective cultivation (increasingly important with the higher costs of seed). Because the crop is long-term, the choice of cultivar is critical. Plant spacing is normally 1.50 m × 0.30 m.
Good control of weeds is essential, not only to reduce competition, but also to enable the young spears to be seen at harvest. Staking is sometimes done in the tropics when using the "mother fern" system (see under Harvesting). The only pruning might be to remove the tops of shoots in the tropics to reduce damage by wind. Irrigation requirements will depend on rainfall, but because the crop is deep-rooted it is not normally considered important except in arid areas.
Diseases and pests
In the humid tropics the major diseases are those attacking the foliage, namely Stemphylium botryosum , Cercospora asparagi and Phoma asparagi. Control is by regular spraying with fungicides (e.g. Mancozeb). Some cultivars developed in New Jersey (United States) appear to have some resistance to these diseases. Fusarium species are a major problem in all climates, and appear to be stress-related due to excessive harvesting. Fusarium as well as Phoma can also be kept under control with good drainage and a balanced fertilizer application.
In temperate climates the young spears are harvested in spring for up to 12 weeks, and the foliage is then allowed to grow to replenish the stored reserves in the roots for next year's crop. In tropical climates, harvesting is usually at any time of the year, using a "mother fern" growing system, in which (once a plant is well established) any newly developed spears are harvested at the appropriate stage, while at the same time maintaining 3-5 mature photosynthesizing shoots. For green asparagus, spears are cut at (or just below) ground level with a knife when they are about 18-25 cm tall. For white asparagus the spears are cut 10 cm above the rhizome just before the spears emerge through the soil surface.
Worldwide the average yield is 3 t/ha per year, but because of the long period of establishment, yields from established crops are 20% higher. In South-East Asia, although the yield potential is probably higher, the actual yields average only 2.1 t/ha per year in Indonesia, and 4.4 t/ha per year in Thailand, although these should be increased for established crops by about 40% to account for plantings which have not yet come into production.
Handling after harvest
Asparagus spears have a high respiration rate and therefore deteriorate very rapidly after harvest. They should be removed from the field as soon as possible after harvest and then stored at high humidity and 2 °C (for up to 4 weeks).
Important germplasm collections of A. officinalis cultivars are held at the Crops and Food Central Research Institute, Lincoln, Canterbury, New Zealand, and by USDA, United States.
The major breeding objectives are related to the development of improved cultivars for disease resistance, and for improved yield and quality. Appearance and low fibre content are particularly important in white asparagus. In Europe, breeding efforts are directed towards the production of male hybrid cultivars with big spears of uniform quality. Male plants live longer and yield better than female plants.
There is increasing interest from affluent countries in obtaining fresh asparagus year-round. This asparagus is obtained from northern and southern hemisphere sources during the appropriate spring periods, but could be supplied from the tropics during the remaining 6 months of the year. With the increasing interest in fresh rather than processed vegetables, the potential for this crop in the tropics appears excellent, particularly when related to the low labour costs in many tropical countries. The major challenge is to develop cultivars which are better adapted to the tropics, and the appropriate technology for production in the humid tropics through the correct choice of site (high, medium, or low altitude) and harvesting strategy. Other priorities are the control of foliage diseases and the establishment of sound post-harvest and transportation infrastructures.
- Hung, Lih, 1980. Special aspects of growing asparagus in Taiwan. Journal Chinese Society for Horticultural Science 26(1): 1-10.
- Nichols, M.A., 1990. Asparagus - the world scene. Acta Horticulturae 271: 25-31.
- Nichols, M.A., 1991. Asparagus production in the tropics. Acta Horticulturae (in press).
- Robb, A.R., 1984. Physiology of asparagus (Asparagus officinalis) as related to the production of the crop. New Zealand Journal of Experimental Agriculture 12: 251-260.
- M.A. Nichols