Prota 2: Vegetables/Lιgumes
DC., Prodr. 13(2): 262 (1849).
Wild amaranth, wild spinach, pigweed (En). Amarante sauvage (Fr).Amaranto, bredo (Po). Mchicha (Sw).
Origin and geographic distribution
Amaranthus thunbergii is native to Central, East and southern Africa, where it has been recorded as a weed in many countries, from Congo east to Eritrea and Somalia, and south to South Africa. It has been introduced as weed into Australia and Europe, probably with sheep.
The main use of Amaranthus thunbergii is as a collected potherb. In Botswana it is a popular leaf vegetable eaten fresh or dried; it is eaten with milk or fat in combinations with sorghum or maize. In Namibia it is eaten with a porridge of pearl millet. The leaves are rather bitter when compared to other amaranth species. It is a popular vegetable also among the Asian population living in South Africa. It is used as a fodder for livestock. In Zimbabwe the ground and dried flower heads are used to make tobacco snuff milder.
Production and international trade
Amaranthus thunbergii is not commercially cultivated. It is occasionally sold at markets in Botswana and Namibia as a cheap vegetable.
The composition of Amaranthus thunbergii leaves is comparable to that of other amaranths. Leaves of amaranths contain on average per 100 g edible portion: water 84.0 g, energy 176 kJ (42 kcal), protein 4.6 g, fat 0.2 g, carbohydrate 8.3 g, fibre 1.8 g, Ca 410 mg, P 103 mg, Fe 8.9 mg, β-carotene 5716 μg, thiamin 0.05 mg, riboflavin 0.42 mg, niacin 1.2 mg, ascorbic acid 64 mg (Leung, W.-T.W., Busson, F. & Jardin, C., 1968). The leaves and stems contain nitrate, most in the stems, and also oxalate at a level comparable to other green leaf vegetables. Most people cook amaranth in ample water and the cooking water with soluble nitrate and oxalate is thrown away. To reduce bitterness the leaves may also be cooked twice. The presence of a rather high content of hydrocyanic acid and oxalic acid makes it less suitable for fresh consumption by humans and as fodder for animals. However, in a test with sheep no toxic effects of large rations of Amaranthus thunbergii were found.
Adulterations and substitutes
In dishes with green leafy vegetables or potherbs, Amaranthus thunbergii may be replaced by any other vegetable amaranth.
Small annual herb up to 55 cm tall, erect or ascending, simple or branched from the base and frequently also above; stem and branches stout, angular, glabrous or thinly hairy below, upwards increasingly furnished with long, crisped, multicellular, rather flocculent hairs. Leaves arranged spirally, simple, without stipules; petiole up to 4 cm long, sometimes longer than lamina; lamina narrowly elliptical to rhomboid or spatulate, (0.5)1.54.5(6) cm Χ (0.5)13(4) cm, cuneate to attenuate at base, blunt or retuse at apex, entire, glabrous or thinly pilose on the lower surface of the primary venation, sometimes with a dark purple blotch. Inflorescence an axillary cluster up to 1.5 cm in diameter, with male and female flowers intermixed but male flowers most frequent at the top of upper clusters; bracts up to 6 mm long, with long awn. Flowers unisexual, subsessile, with 3 tepals up to 6 mm long, having a long, fine awn; male flowers with 3 stamens; female flowers with superior, 1-celled ovary crowned by 3 stigmas. Fruit an ovoid-ellipsoid to pyriform capsule up to 3.5 mm long, with a short beak below the stigmas, circumscissile, obscurely wrinkled, 1-seeded. Seed 11.5 mm long, shining black or dark brown, feebly reticulate.
Other botanical information
Amaranthus comprises about 70 species, of which about 40 are native to the Americas. It counts at least 17 species with edible leaves. Amaranthus thunbergii resembles Amaranthus graecizans L., but may be distinguished by its stems furnished with long crisped hairs, leaves broadest above the middle and having a rounded top, and by the long awn of the tepals.
Growth and development
Emergence of the seedling takes place 35 days after sowing. The vegetative development is fast. Like maize and sugar cane, the genus Amaranthus is characterized by the C4-cycle photosynthetic pathway, which means a high photosynthesis at high temperature and radiation. Flowering may start 48 weeks after sowing and occurs from February to May in southern Africa. The growth of new shoots continues after the start of flowering. Pollination is effected by wind but the abundant pollen production causes a high rate of self-pollination. Seeds mature after 12 months.
Amaranthus thunbergii occurs on waste places, as a weed on cultivated ground, grassland and rocky soil, mostly in arid highland areas. In Botswana it grows on various soil types under a wide range of environmental conditions, but is most frequently found in seasonally wet localities, e.g. along watercourses. Amaranthus thunbergii is very resistant to adverse climate and soil conditions.
Where Amaranthus thunbergii is appreciated as a vegetable, selective weeding may be applied by removal of all other weed plants. Cultivation of Amaranthus thunbergii is only occasionally recorded, e.g. in Botswana, where its cultivation during the dry season is being tested. Presumably cultivation is relatively easy; the same technology as for Amaranthus blitum L. may be applied. Once that the plant is established, it is self-seeding.
Propagation and planting
The seed is scattered and gives rise to spontaneous new plants. It is also spread with cow dung, and may remain viable in the soil for many years. It germinates at the surface or in the upper soil layer of less than 3 cm.
Diseases and pests
Although data are lacking, Amaranthus thunbergii seems to be very tough and resistant to pests and diseases.
Young plants and young tender shoots are picked as a vegetable. People start to collect the young shoots three weeks after the rains have started. Repeated harvesting stimulates the growth of new shoots.
Handling after harvest
If collected for the market, shoots and leaves are often sprinkled with water to keep a fresh appearance. In Namibia, the leaves are boiled and prepared in flattened cakes that are sun-dried for use during the dry season.
Amaranthus thunbergii is widespread and usually occurs in disturbed habitats, and thus does not seem to be threatened by genetic erosion. It is not included in germplasm collections.
Amaranthus thunbergii is a tasty and nutritious traditional wild vegetable. The prospects for domestication and cultivation as vegetable are poor because it would have to compete with more popular vegetable amaranths. In breeding of cultivated amaranths Amaranthus thunbergii might be used as genitor of resistance genes.
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Correct citation of this article:
Grubben, G.J.H., 2004. Amaranthus thunbergii Moq. In: Grubben, G.J.H. & Denton, O.A. (Editors). PROTA 2: Vegetables/Lιgumes. [CD-Rom]. PROTA, Wageningen, Netherlands.