Amaranthus cruentus (red amaranth)

Amaranthus cruentus is an annual herbaceous plant originating from Central America and cultivated since ancient times for its grain. Introduced to many countries, it is now widespread and naturalized in many parts of the world. A. cruentus is listed as invasive in China, Israel and Italy, with evidence of impact. Although also reported as invasive in New Caledonia, New Zealand, Papua New Guinea and the Philippines, no further information is provided on its impact in these countries. This species is also described as a casual alien, an escape from cultivation and agricultural weed; it is listed in the Global Compendium of Weeds.

The genus Amaranthus includes about 60 species (Stevens, 2020) that are spread worldwide in the temperate, subtropical and tropical areas. The genus has three species that are important for grain production (pseudocereals): A. caudatus, A. hypochondriacus and A. cruentus (Marin et al., 2011).

Much of the difficulty in taxonomic discrimination of species within the group can be attributed to attempts at recognizing taxa based on pigmentation or growth forms, which are extremely variable within amaranths (Sauer, 1967). However, examination of floral parts can result in constant characters from which discontinuities can be used to define well-established taxa. In this sense, tepal (petals and sepals are combined in a single floral whorl) number and morphology are commonly used in taxonomic keys (Kole, 2011).

The following description is from Flora of Editorial Committee (2015):

Plants almost glabrous or slightly pubescent distally, especially when young. Stems erect, green or reddish purple, branched distally, 0.4-2 m. Leaves: petiole 1/2 as long as to ± equaling blade; blade rhombic-ovate or ovate to broadly lanceolate, 3-15(-20) × 1.5-10(-15) cm, occasionally larger in robust plants, base cuneate to broadly cuneate, margins entire, plane, apex acute or subobtuse to slightly emarginate, with mucro. Inflorescences terminal and axillary, erect, reflexed, or nodding, usually dark red, purple, or deep beet-red, less commonly almost green or greenish red, leafless at least distally, large and robust. Bracts narrowly spathulate, 2-3 mm, equaling or slightly longer than tepals, apex short-spinescent. Pistillate flowers: tepals 5, oblong to lanceolate, not clawed, equal or subequal, 1.5-3 mm, apex acute; style branches erect or slightly reflexed; stigmas 3. Staminate flowers at tips of inflorescences; tepals 5; stamens (4-)5. Utricles obovoid to elongate-obovoid, 2-2.5 mm, smooth or slightly rugose distally, dehiscence regularly circumscissile. Seeds usually white or ivory, with reddish or yellowish tint, sometimes dark brown to dark reddish brown, broadly lenticular to elliptic-lenticular, 1.2-1.6 mm diam., smooth or indistinctly punctate.

The following information is provided by Grubben (2004):

As early as 6000 years ago, A. cruentus was domesticated as a pseudocereal (grain amaranth) in Central America from the weed A. hybridus. Escaped plants from cultivation also occur in the wild. The vegetable form of A. cruentus was probably introduced in the tropics and subtropics of the Old World during colonial times. At present, A. cruentus is a widespread traditional vegetable in all countries of tropical Africa. It is the main leafy vegetable in Benin, Togo and Sierra Leone and very important in many lowland areas e.g. in southern Nigeria, DR Congo, Kenya and Tanzania. It is more popular in humid lowland than in highland or arid areas. It is also an important vegetable in many tropical areas outside Africa e.g. in India, Bangladesh, Sri Lanka and the Caribbean. The Bangladesh type has big fleshy stems, which are consumed with the leaves. A. cruentus is grown as a leaf vegetable throughout Southeast Asia, although to a lesser extent than A. tricolor. In Indonesia, it is grown in mountain areas, where the climate is too cold for the more common A. tricolor (Ebert et al., 2011). Grain amaranth, a cultivar group of A. cruentus with yellowish white or pale brown seed, is traditionally grown as a cereal crop in Latin America (e.g. Mexico, Guatemala, Ecuador, Colombia). Since colonial times, it has been successfully introduced as a pseudocereal in India and Nepal, in mountain areas as well as at lower elevations and it has become well established as a popular food plant. Thinnings of young seedlings of the grain crop are frequently used as a vegetable. Grain amaranth is produced commercially in hot and dry areas of the United States, Argentina and China. Apart from some try-outs in Zimbabwe, Kenya, Uganda and Ethiopia, grain amaranth is not cultivated in Africa. Ornamental types of A. cruentus characterized by big bright-red inflorescences can be frequently found in tropical and subtropical countries.

Presumably native to Central America, A. cruentus is now widespread in tropical and subtropical regions of the world; the red form (which is probably of cultivated derivation) is cultivated as an ornamental in temperate regions also, occasionally escaping there (Townsend, 1985).

Amaranthus cruentus is an ancient food and in the famous Tehuacan caves in central Mexico, archaeologists have dug up remains -both the pale grain and the bundles of plants brought in for threshing - at a dozen levels, dating back 5500 years. The species is still grown in the region and popped amaranth seedcakes are sold on the streets of the towns. A. cruentus has also survived as a grain crop in a few Indian villages of southern Mexico and Guatemala and as a crop used to extract a red dye for colouring corn-based foods in the Indian pueblos of the arid southwestern United States, where it probably became established in prehistoric times (National Research Council, 1984).

During the 19th century, this deep-red form was adopted for use as cooked greens by gardeners throughout the tropics. It became a more important crop in tropical Africa than anywhere else. A. cruentus was evidently introduced to Africa by Europeans. But then it passed quickly from tribe to tribe, probably as a weed in millet and sorghum (Sorghum) seed. It outran European exploration of the interior, so that Livingstone and others found it already in cultivation when they arrived. Today, it is being planted and gathered year-round in the humid regions of much of Africa (National Research Council, 1984).

Seed dispersal of Amaranthus species is by wind, farm machinery, water, birds, animals, and the spreading of manure and compost. As both seeds and fruits float easily they are dispersed by raindrops, streamlets produced on the soil by rain, surface irrigation, and water courses. Viable seeds can be dispersed after ingestion and elimination by mammals and birds (Achigan-Dako et al., 2014).

Amaranthus cruentus is part of the so-called A. hybridus aggregate, a group of species in which taxonomic problems are far from clarified, especially because of apparently common hybridization and nomenclatural disorder caused by names being commonly misapplied. Some recognized species of this aggregate are cultivated taxa. A. cruentus is one of these, as are the other grain amaranths, A. caudatus and A. hypochondriacus, A. cruentus also seems to be closely related to A. hybridus, a weed that is its putative progenitor. Transitional forms between these four species can be found (PROTA, 2016).

Although some cultivated forms of A. caudatus-A. quitensis are suspected to be the result of interbreeding with A. cruentus, the South American amaranths are not thought to readily hybridize with the North American members of this cluster (Kole, 2011).

In East Africa, A. cruentus [as A. hybridus subsp. cruentus] is described as a weed of cultivation, found on broken waste ground, in short grassland and in shaded places at forest edges, and at elevations of 20-1790 m (Townsend, 1985). In Hawai’i, A. cruentus is an escape from cultivation (Wagner et al., 1999). In New Zealand, it has been observed among crops, in gardens and waste places in settled areas (Webb et al., 1988). In Papua New Guinea, this species is probably associated exclusively with areas under cultivation at altitudes of 1200-2200 m (Womersley, 1978). In the hot, humid regions of Africa, Southeast Asia, southern China, southern India and the Caribbean, A. cruentus is grown as a soup vegetable or for boiled salad greens (pot herbs) (National Research Council, 1984).

According to Grubben (2004), wet rot or stem rot caused by the fungus Choanephora cucurbitarum is the main disease in amaranths. Damping-off caused by Pythium aphanidermatum and Rhizoctonia is often serious in seedbeds. Pythium also attacks older plants. No viral diseases have been reported.

A review of the main fungal diseases affecting A. cruentus worldwide and more specifically in Poland is given by Pusz (2007); the most common are leaf spots caused by Alternaria alternata, Phoma levellei [P. leveillei], Phoma medicaginis and a range of other fungi, and stem rot caused by Phomopsis amaranthicola. Leaf and stem blight caused by Phomopsis amaranthophila has been reported in Brazil while Alternaria leaf spot occurs on A. cruentus in Tanzania (Ebert et al., 2011). In Taiwan and other countries, white rust (Albugo bliti [Wilsoniana bliti]) and amaranth anthracnose (Colletotrichum erumpens var. amaranti) may turn into serious diseases (Ebert et al., 2011).

In the USA, the lygus bug (Lygus lineolaris) has severely damaged grain amaranth yields by piercing the developing seed and sucking out the juices (Olson and Wilson, 1990; Wilson and Olson, 1992). Leaf miners have also been found on both grain and vegetable amaranth. In Lucknow, India, serious damage to both grain and vegetable types has often been caused by spider mites. The stem weevil (Lixus truncatulus [H. truncatulus]) is a major pest of amaranth; its grubs damage foliage and roots and cause the plants to wilt. Leaf rotters (H. recurvalis [S. recurvalis]) also cause considerable damage during rainy seasons (National Research Council, 1984). Infestations of the weevil, Conotrachelus seniculus, were observed on the roots of A. cruentus in Arizona in 1984 (Terry and Lee, 1990). Significant damage to cultivated A. cruentus in southern Nigeria (Agunloye and Osisanya, 1985) and Brazil (Oliveira et al., 2012) by Herpetogramma bipunctalis has been documented.

Bacterial wilt caused by Ralstonia solanacearum has been reported on A. cruentus in Benin (Sikirou et al., 2019).

Amaranthus cruentus is listed as invasive in China, Israel and Italy with evidence of impact although no specific details are available (Celesti-Grapow et al., 2009; Xu et al., 2012; Dufour-Dror and Fragman-Spair, 2017; GBIF, 2020). According to PIER (2018) this species is also invasive in New Caledonia, New Zealand, Papua New Guinea and the Philippines but further details of its impact have not been provided. Although A. cruentus is noted as an agricultural weed in some countries, there is no specific information on its economic or environmental impact. In Italy, it is a weed in soyabean and maize fields and reported to have developed resistance to common herbicides (Zanin et al., 1981; Zanin et al., 1984; Sattin and Zanin, 2003).       

Amaranthus cruentus is useful as a grain or a leafy vegetable. The grain types have white seeds; the vegetable types (as well as those used to extract red dye) usually are dark seeded. It is probably the most adaptable of all amaranth species, e.g. it flowers under a wider range of daylengths than the others (National Research Council, 1984). is used as an African leafy vegetable (Grubben and van Sloten, 1981) and also widely grown as a dye plant, ornamental and pot herb in Central America, Europe, China, India, southeast Asia, and Africa. The leaves and the softest portions of the shoots are usually boiled in several changes of water and then separated from the cooking liquid (Martin and Telek, 1979) though they traditionally are steamed in Uganda (Stafford et al., 1976). Amaranth leaves are combined with condiments to prepare soup in Nigeria (Okiei and Adamson, 1979; Oke, 1983); used in salad, boiled and mixed with a groundnut (Arachis hypogaea) sauce in Mozambique (Oliveira and de Carvalho, 1975) or pureed into a sauce and served over vegetables in West Africa (Martin and Telek, 1979). In Ethiopia, A. cruentus is used as a tapeworm expellant while in Sudan ash from the stems is used as a wound dressing (Achigan-Dako et al., 2014). In Gabon heated leaves are used on tumours (Grubben, 2004).

Amaranthus cruentus is an important leaf vegetable crop cultivated throughout the tropics (Grubben and van Sloten, 1981). The leaves are rich in vitamin A, calcium and potassium (Early, 1997). The seeds of grain amaranth are valued for their high protein content (up to 15%) (Musa et al., 2014). A. cruentus is used in Mexico to produce typical sweets called alegría, in which the amaranth grains are toasted and mixed with honey or chocolate (Maestri, 2014). The economic value of A. cruentus as a popular market vegetable ranks high. From market surveys, it appears as one of the main African leafy vegetables, possibly the number one in quantity and area. No statistical data are available, since in most cases all leaf vegetables are recorded as one single group. In national or FAO statistics, they are not recorded at all. Correct registration is hampered by the short growing period (3-6 weeks), scattered occurrence of small plots of cultivation and the dispersed sales in small street markets. In the big cities in Benin, the average quantity of fresh leafy vegetables bought daily at the markets was 42 g/head/day, 31% of which was A. cruentus. There is some unregistered export of amaranth from African countries, as well as from Latin America (Caribbean, Suriname) to Western Europe (PROTA, 2016). 

The potential of grain and vegetable amaranth as a food resource has been reviewed extensively by Haas and Kauffman (1984), Saunders and Becker (1983), National Research Council (1984) and Sánchez Marroquín (1980). Dye use seems limited to cultures that do not grow amaranth as a grain crop (Sauer, 1950). The red dye from amaranth leaves is used to colour alcoholic beverages in Bolivia and north western Argentina, to colour maize (Zea mays) dough in Mexico and the southwestern United States (Sauer, 1950) and to dye foods and beverages in Ecuador (Jain and Hauptli, 1980).

Amaranth is high in protein, lysine, calcium, iron and fibre, all of which are useful as functional ingredient in cereal products. Amaranth oil is high in squalene, a powerful antioxidant used as a dietary supplement for diabetes and those suffering from hypertension and metabolic disorders (Pal and Khoshoo, 1974; Teutonico and Knorr, 1985). A. cruentus forage meal and extruded grains have been used as feed ingredients for broilers (Tillman and Waldroup, 1988; Fraga et al., 1993; Miazzo et al., 1994).  

Solymosi (2000) reported that chamazulene showed excellent phytotoxic activity against A. cruentus with a post-emergence application of 100 ml/12.5 m². Pyridate was found to provide effective control of A. cruentus in field studies in Andalusia, Spain (Giménez-Espinosa and De Prado, 1997). Trials in maize fields in Egypt showed that acetochlor (750 ml/feddan) gave good control of A. cruentus (Mahmoud, 2010).