Asparagus officinalis L.

Family

Asparagaceae

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Chromosome number

2n = 20, 40

Synonyms

Vernacular names

Asparagus (En). Asperge (Fr). Espargo hortense (Po).

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Origin and geographic distribution

The origin of asparagus is believed to be the eastern Mediterranean region. However, it grows wild in Europe, the Caucasus and western Siberia. It is naturalized in the Americas and New Zealand, and occurs now worldwide as a crop plant, in Africa mainly in the subtropics. In tropical Africa, it is restricted to high elevation areas in East and southern Africa, but occasionally occurs elsewhere as experimental planting or for own use, e.g. in Rwanda and Ethiopia.

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Uses

The major product of asparagus are the tender young expanded shoots (spears) which are eaten lightly cooked. The spears are also processed either by canning (or bottling) in brine or by deep-freezing. They are harvested prior to emergence as white asparagus, or after emergence when 18–25 cm tall as green asparagus. Green spears should be all green and white spears all white, but harvest of the in-between stage is also practised. It is normal to peel the white spears prior to cooking, whereas the green ones are normally eaten unpeeled and only the lower fibrous part of the spears in the in-between stage is peeled.
The foliage of asparagus is occasionally used in flower arrangements.

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Production and international trade

There are about 255,000 ha of asparagus grown worldwide, equally divided between white and green, with an increasing trend towards the production of green, due to lower harvesting costs. Production predominates in Asia (102,000 ha, with China 90,000 ha), Europe (61,000 ha, with Spain 17,000 ha), North America (51,000 ha, with the United States 34,000 ha) and South America (29,000 ha, with Peru 20,000 ha). Africa (4,000 ha) and Australasia (7,000 ha) are relatively unimportant, although both regions have an important role in providing northern hemisphere markets with out of season fresh asparagus. In Africa asparagus is mainly grown in southern Africa (South Africa, Lesotho) and North Africa (Tunisia), whereas in tropical Africa it is found in the highlands of eastern Africa (Kenya, Uganda, Zimbabwe). The world price fluctuates tremendously, depending on whether the produce is for fresh use or for processing, and also on the time of the year, but tends to be about 1.0 US$/kg. Asparagus production for export is attractive for countries with cheap labour, e.g. Uganda and Kenya, which are able to harvest asparagus for fresh export during periods of low production in Europe.

Properties

The composition of white asparagus, per 100 g of raw edible portion (75% of harvested product) is: water 91.4 g, energy 103 kJ (25 kcal), protein 2.9 g, fat 0.6 g, carbohydrate 2.0 g, fibre 1.7 g, Ca 27 mg, Mg 13 mg, P 72 mg, Fe 0.7 mg, Zn 0.7 mg, carotene 315 μg, thiamin 0.16 mg, riboflavin 0.06 mg, niacin 1.0 mg, folate 175 μg, ascorbic acid 12 mg (Holland, B., Unwin, I.D. & Buss, D.H., 1991). Green asparagus scores higher in micro-nutrients (Fe 1.5 mg, ascorbic acid 48 mg) than white asparagus. The characteristic flavour of asparagus is due to sugars and bitter components. The key olfactory component is dimethyl sulphide, a degradation product of the amino acid S-methyl methionine.

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Description

Dioecious perennial herb with climbing or erect stems up to 2 m tall, and robust woody rhizome comprising a number of bud clusters and many long (1.5–2 m) unbranched, fleshy storage roots; stem fleshy when still underground, aboveground strongly branched, ultimate branchlets (cladodes) fine, needle-like, green and leaf-like, 1–3 cm long. True leaves reduced to minute, bract-like, triangular, brownish, prickly scales, with 3–6 cladodes in the axils. Flowers solitary or in pairs in the leaf-axils, unisexual, small, tubular-campanulate, pendulous, 6-merous, greenish yellow; tepals shortly united at base, 6–8 mm long in male, 4–6 mm in female flowers; male flowers with free stamens inserted near the base of tepals and a rudimentary ovary; female flowers with superior, 3-celled ovary, short style, 3-lobed stigma and rudimentary stamens. Fruit a globose berry up to 1 cm in diameter, red, 1–6-seeded. Seeds rounded, with a flattened side, black.

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Other botanical information

Asparagus comprises about 200 species. Many more than 100 of these occur in Africa, and most have been considered as belonging to a separate genus Protasparagus on the basis of bisexual flowers usually in fascicles or racemes, and globose seeds. However, species with intermediate characteristics occur, and Protasparagus can best be included in Asparagus. Some other species have been placed in another genus, Myrsiphyllum, mainly differing in cohering stamens, but this genus is also included in Asparagus, although usually distinguished as a subgenus. Some wild Asparagus species are used in a similar way as Asparagus officinalis.

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Growth and development

Germination is 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. Normally each bud produces two storage roots at about the time that the bud develops into a shoot. Apical dominance within each bud cluster is strong, and the next bud in the cluster does not normally develop until the previous one is fully developed into a shoot (or the spear is harvested). Flowering starts already in the first year and is continuous. The plants are insect-pollinated.
In the humid tropics the plants remain green and never go dormant. In temperate climates the aerial parts senesce during autumn, and growth continues the following spring as the rhizome buds form shoots. In the arid tropics dormancy can be induced by withdrawing irrigation. Under such circumstances production can be programmed for any time of the year. Because the spears from rhizome buds comprise the marketable yield, it is necessary to establish a large pool of stored food reserves, mainly long chain fructans, in the swollen roots before the start of harvesting. It is therefore general practice not to harvest until two years after planting, and to slowly increase the harvest period from 3–4 weeks to 10–12 weeks per year from the third to the fifth year. 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 economic to harvest longer than 10–15 years. In the tropics the crop starts to decline fast and becomes uneconomic after 6–8 years, and in lowland areas already after 3–4 years. Once the old asparagus plants have been removed, the same field cannot be used for new asparagus plantings for tens of years, probably because the remaining tough old roots contain phytotoxic compounds and are infected with Fusarium.

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Ecology

Asparagus has no daylength response. Photosynthetic activity appears to increase up to 300 W/m² PAR (Photosynthetically 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. A high relative humidity is a distinct disadvantage due to foliage diseases. The crop can be successfully produced at low altitudes even in the tropics, though yield and spear quality are not as high as at higher altitudes. Absence of frost during the growing season is important. Deep well-drained sandy loams or volcanic soils are preferable. Especially for white asparagus a light sandy soil is preferred, whereas for green asparagus the soil texture may be heavier. Asparagus appears to be able to grow in a wide range of pH, though 5.8–6.5 is optimum.

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Propagation and planting

Propagation is primarily by seed. The weight of 1000 seeds is 25–40 g. The import of one-year old rhizomes (crowns) of superior western European hybrid cultivars for large-scale asparagus production in the tropics has been economically disastrous in many cases because the initial investment is too high and the yield capacity and lifespan much lower than in a temperate climate. Besides, this practice increases the risk of the introduction of new pests and diseases. It is recommended that seedlings be raised in situ, which is the normal practice of smallholders. Because asparagus is a long-term crop, the choice of a cultivar is critical; recommended are ‘Lucullus’ for white and ‘Jersey Giant’ for green asparagus.
Seed is sometimes sown directly in the final growing site, but more commonly in a field nursery. Production of seedlings in containers under protective cultivation is becoming increasingly important with the higher costs of genetically improved seed. In the nursery, seeds are sown at distances of 15 cm in the row and 35 cm between rows. A nursery of about 100 m² needs 0.8–1.0 kg seed, to produce 15,000–25,000 suitable seedlings (crowns) for transplanting. In temperate climates 1-year-old crowns in the dormant stage are uprooted early in spring for planting in the field; in the tropics transplanting is done much earlier, after about 8 months. Plant spacing in the field is normally 1.8 m between the rows and 0.3 m in the row (18,000 plants/ha) for white asparagus production, and 1.5 × 0.3 m (22,000 plants/ha) for green asparagus production. Planting is in 15–20 cm deep furrows, the crown buds pointing up and the roots spread out. There is increasing interest in vegetative propagation, using tissue culture methods to clone elite plants, but this method is expensive and still in the experimental stage.

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Management

An adequate supply of nutrients, particularly nitrogen, phosphate, magnesium and potassium, is recommended. For a high yielding crop of about 5 t/ha/year, the annual fertilizer application could be, depending on the soil condition, 100 kg N, 35 kg P, 80 kg K and 60 kg Mg. Before the beginning of the harvest the rows need to be earthed-up into raised beds of up to 50 cm in height with soil from between the rows. 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 in the tropics might be to remove the tops of the highest shoots to reduce wind damage. Irrigation requirements 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 stems and 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 oxysporum is a major problem in all climates, and appears to be related to excessive harvesting. Fusarium as well as Phoma can also be kept under control with good drainage and a balanced fertilizer application.

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Harvesting

In temperate climates the young spears are harvested in spring for a period of up to 12 weeks, starting in the second year after planting. After the harvest period, the foliage is 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 the ‘mother fern’ or ‘mother stalk’ growing system, in which (once a plant is well established) 3–5 mature photosynthesizing shoots are maintained, while newly developed spears are harvested at the appropriate stage. For green asparagus, spears are cut with a knife at (or just below) ground level when they are about 18–25 cm tall. For white asparagus the spears are cut with a long-bladed knife just above the rhizome when they emerge through the soil surface.

Yield

Worldwide the average yield is 3 t/ha per year because of the long period of establishment, but yields from established crops are about 3.6 t/ha in average. In Western countries fields in full production (4–10 years) yield up to 6 t/ha. In arid subtropical climates, where it is possible to harvest 3 crops every two years, yields can be as much as 15 t/ha/year, but the crop duration is shorter than in cool climates. White asparagus yields 10–25% more than a comparable green asparagus crop, because the spears are thicker, and are harvested closer to the rhizome.

Handling after harvest

The spears are washed, cut at equal length (between 15 and 22 cm), sorted and packed for marketing. Asparagus spears have a high respiration rate and therefore deteriorate 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 for white asparagus; green asparagus has a much shorter shelf life and can be stored for only about a week).

Genetic resources and breeding

Germplasm collections of Asparagus officinalis cultivars are held at the Crops and Food Research Institute, Lincoln, Canterbury, New Zealand, and at the USDA Northeast Regional Plant Introduction Station, Cornell University, Geneva NY, United States.

Breeding

In Europe and America, many cultivars of Asparagus officinalis exist, but cultivars especially selected for the tropics are lacking and local plantings from farmers’ seed, resulting from old seed imports, are very heterogeneous. Major breeding objectives are related to the development of improved cultivars for disease resistance, yield and quality. Appearance and low fibre content are particularly important in white asparagus. In Europe, breeding efforts are directed towards the production of ‘super male’ hybrid cultivars with big spears of uniform quality. Male plants live longer and yield better than female ones. In general these hybrids are vigorous and early producing but susceptible to diseases and less suitable for the tropics than the traditional old open-pollinated cultivars such as ‘California 500’, ‘Mary Washington’, ‘Lucullus’, ‘Violette d’Argenteuil’ or ‘Jersey Giant’.

Prospects

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 (highland) 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 good, also because of the low labour costs. The major challenge is to develop cultivars adapted to the tropics, and the appropriate technology for production in the humid tropics through the correct choice of site (high or medium altitude) and harvesting strategy. Other priorities are the control of foliage and soil diseases and the establishment of a sound post-harvest and transportation infrastructure.

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Major references

• Benson, B.L., 1999. World asparagus production areas and periods of production. Acta Horticulturae 479: 43–50.
• Nichols, M.A., 1991. Asparagus production in the tropics. Acta Horticulturae 292: 149–153.
• Nichols, M.A., 1993. Asparagus officinalis L. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 91–93.
• 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.
• Uragini, A. (Editor), 2002. Proceedings of the 10th international asparagus symposium. Acta Horticulturae 589, ISHS, Leuven, Belgium. 392 pp.

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Other references

• Holland, B., Unwin, I.D. & Buss, D.H., 1991. Vegetables, herbs and spices. The fifth supplement to McCance & Widdowson’s The Composition of Foods. 4th Edition. Royal Society of Chemistry, Cambridge, United Kingdom. 163 pp.
• Hung, Lih, 1980. Special aspects of growing asparagus in Taiwan. Journal of the Chinese Society for Horticultural Science 26(1): 1–10.
• Messiaen, C.-M., 1989. Le potager tropical. 2nd Edition. Presses Universitaires de France, Paris, France. 580 pp.
• Rubatzky, V.E. & Yamaguchi, M., 1997. World vegetables: principles, production and nutritive values. 2nd Edition. Chapman & Hall, New York, United States. 843 pp.
• Standhardt, D. & Kecke, S., 1997. Evaluation of asparagus flavour quality for breeding purposes (Asparagus officinalis L.). Acta Horticulturae 479: 135–140.
• Tindall, H.D., 1983. Vegetables in the tropics. Macmillan Press, London, United Kingdom. 533 pp.

Afriref references

Sources of illustration

• Nichols, M.A., 1993. Asparagus officinalis L. In: Siemonsma, J.S. & Kasem Piluek (Editors). Plant Resources of South-East Asia No 8. Vegetables. Pudoc Scientific Publishers, Wageningen, Netherlands. pp. 91–93.

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Author(s)

•	M.A. Nichols Institute of Natural Resources, College of Sciences, Massey University, Private Bag 11-222, Palmerston North, New Zealand

Based on PROSEA 8: ‘Vegetables’.

Editors

•	G.J.H. Grubben Boeckweijdt Consult, Prins Hendriklaan 24, 1401 AT Bussum, Netherlands
•	O.A. Denton National Horticultural Research Institute, P.M.B. 5432, Idi-Ishin, Ibadan, Nigeria

Associate editors

•	C.-M. Messiaen Bat. B 3, Résidence La Guirlande, 75, rue de Fontcarrade, 34070 Montpellier, France
•	R.R. Schippers De Boeier 7, 3742 GD Baarn, Netherlands

General editors

•	R.H.M.J. Lemmens PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
•	L.P.A. Oyen PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Photo editor

•	E. Boer PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Correct citation of this article

Nichols, M.A., 2004. Asparagus officinalis L. [Internet] Record from PROTA4U. Grubben, G.J.H. & Denton, O.A. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. <http://www.prota4u.org/search.asp>. Accessed .

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