Amaranthus retroflexus
(redroot pigweed)

Identity

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Preferred Scientific Name

• Amaranthus retroflexus L. (1753)

Preferred Common Name

• redroot pigweed

International Common Names

• English: carelessweed; common amaranth; redroot
• Spanish: amaranto; aracu; atacu; bledo; bledo rojo; marxant; quelite; quentonil
• French: amarante récourbée; amarante réfléchie
• Portuguese: caruru gigante; moncos-de-Peru

Local Common Names

• Argentina: atac; ataco; caa-ruru; yuyo colorado
• Bolivia: ataco coman; chiori
• Brazil: bredo; carura aspero; caruru
• Canada: amarante a racine rouge
• Denmark: opret anarant; tilbagebjet anarant
• Finland: vihrea revonhanta
• Germany: Amarant (Rauhhaariger); Fuchsschwanz; Krummer Fuchsschwanz; Rauhhaariger Amarant; Zuruckgekrummter; Zurueckgebogener Amarant
• Iran: taj khoroos
• Italy: amaranto; amaranto comune; biedone
• Japan: aogeito
• Madagascar: amatarika
• Netherlands: papegaaienkruid
• Norway: duskamarant
• Peru: yuyo
• Sweden: svinamarant
• Turkey: horoz kuyruga; kirmizi koklu tilki kuyrugu
• Venezuela: pira
• Yugoslavia (Serbia and Montenegro): hrapavi stir

EPPO code

• AMARE (Amaranthus retroflexus)

Taxonomic Tree

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• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Caryophyllales
•                         Family: Amaranthaceae
•                             Genus: Amaranthus
•                                 Species: Amaranthus retroflexus

Notes on Taxonomy and Nomenclature

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A. retroflexus has a diploid chromosome number of 34 (Murray, 1940; Grant, 1959). It readily hybridizes with closely related species (A. hybridus, A. powellii and A. caudatus), but the F1 generation is highly sterile (Murray 1940). Hybrids often have oddly shaped inflorescences.

Description

Top of page A. retroflexus is a monoecious, erect, finely hairy, freely-branching, herbaceous annual growing to 2 m tall; taproot pink or red, depth varies with soil profile; leaves alternate, egg-shaped or rhombic-ovate, cuneate at base, up to 10 cm long, margins somewhat wavy, veins prominent on underside, apex may be sharp, petiole shorter or longer than leaf; flowers numerous, small, borne in dense blunt spikes 1 to 5 cm long, densely crowded onto terminal panicle 5 to 20 cm long but may be smaller on upper axils; three spiny-tipped, rigid, awl-shaped bracteoles surround the flower, exceeding the perianth, length 4 to 8 mm, persistent; tepals five, much longer than fruit, usually definitely recurved at tips, obovate or highly spatulate, one pistil and five stamens; style branches erect or a bit recurved; fruit a utricle, membranous, flattened, 1.5 to 2 mm long, dehiscing by a transverse line at the middle, wrinkled upper part falling away; seed oval to egg-shaped, somewhat flattened, notched at the narrow end, 1 to 1.2 mm long, shiny black or dark red-brown.

Distribution

Top of page A. retroflexus is thought to be a native riverbank pioneer of the central and eastern USA and adjacent regions of south-eastern Canada and north-eastern Mexico (Sauer, 1967). It has become naturalized throughout the temperate regions of the northern and southern hemispheres. A. palmeri is also native to North America, but its distribution is confined primarily to the southern USA and Mexico (Sauer, 1955).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Habitat

Top of page A. retroflexus is found on a wide variety of soil types and textures. It grows particularly well in fertile soils and has a high N requirement. It tolerates soil pH from 4.2 to 9.1 (Feltner, 1970), but is less common on acid soils, such as those of the south-eastern USA, where the related species A. palmeri is more abundant. It is common in cultivated fields, gardens, waste places, roadsides, river banks, and other open, disturbed habitats where annual weeds predominate. It is seldom found in closed or shaded communities (Weaver and McWilliams, 1980).

Habitat List

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Category	Sub-Category	Habitat	Presence	Status
Terrestrial

Hosts/Species Affected

Top of page A. retroflexus has a worldwide distribution and is a common weed of most field and horticultural row crops in the temperate areas of the world.

Biology and Ecology

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A. retroflexus is an annual that reproduces solely by seed. It is a prolific seed producer, with single vigorous plants capable of producing between 230,000 and 500,000 seeds (Stevens, 1957). Seed production declines beneath crop canopies where light is limited (McLachlan et al., 1995). Germination requirements and dormancy patterns are highly variable depending on distribution and local climatic and ecological conditions and, as such, generalizations should be treated with caution. Recent research suggests that germination is stimulated by light and high temperatures (Gallagher and Cardina, 1997; Oryokot et al., 1997). The seeds are small and most germinate near to the soil surface, with optimum emergence from about 1 cm depth (Wiese and Davis, 1967; Siriwardana and Zimdahl, 1983). Weaver and McWilliams (1980) reported that seeds can remain viable in the soil for many years, but Egley and Chandler (1978) reported a 90% decline in viability after seed burial for 18 months. Seeds are dispersed by wind, animals and as contaminants of crop seeds or farm machinery.

A. retroflexus has the C4 pathway of photosynthesis, typical 'Kranz' leaf anatomy, a low carbon dioxide compensation point and high water use efficiency (Weaver and McWilliams, 1980; Tremmel and Patterson, 1993).

More detailed information on the biology and ecology of A. retroflexus is provided by Weaver and McWilliams (1980) and Holm et al. (1997).

Natural enemies

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Notes on Natural Enemies

Top of page A. retroflexus is a host plant for a variety of insect pests and diseases which attack crops, although damage is rarely severe enough to serve as biological control. However, several authors have suggested the pigweed flea beetle (Disonycha glabrata) (Tisler, 1990) and various pathogens (Burki et al., 1997) as potential control agents.

Impact

Top of page A. retroflexus is an aggressive and competitive weed in a variety of row crops. It causes substantial yield loss in soyabean, maize, cotton, sugarbeet, sorghum, and many vegetable crops (Weaver and MacWilliams, 1980). Holm et al. (1997) list the many countries and crops in which A. retroflexus is a significant weed problem. It has been reported to have allelopathic effects on both weeds and crops (Athanassova, 1996). It has the capacity to accumulate and concentrate nitrates in stems and branches in amounts which are poisonous to livestock (Mitich, 1997; Torres et al., 1997) and leaves have been reported to have oxalate levels as high as 30% (Nuss and Loewus , 1978). Amaranthus species can cause allergic reactions in humans, primarily due to wind-borne pollen (Mitchell and Rook, 1979; Wurzen et al., 1995).

A. retroflexus is an alternative host for a number of crop pests and diseases, including the parasitic weed Orobanche ramosa in tomato in the USA, the green peach aphid, Myzus persicae, in orchards, and cucumber mosaic cucumovirus in peppers (Weaver and McWilliams, 1980).

Uses

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A. retroflexus is palatable to sheep and has a nutrient composition and digestibility equivalent to that of alfalfa (Marten and Andersen, 1975; Moyer and Hironaka, 1993). Closely related Amaranthus species are used as pot herbs, cultivated grains, and ornamental or dye-plants, particularly in Central and South America (Wesche-Ebeling et al., 1995; Mitich, 1997). A. retroflexus  is eaten as a vegetable in many places of the world and is used for many food and medicinal uses by Native American groups. A. retroflexus may have traits useful to breeding programmes for the cultivated grain amaranths.

Similarities to Other Species/Conditions

Top of page Immature plants of A. retroflexus are similar in appearance to those of A. hybridus and A. powellii. Flowering plants of the latter two species have thinner, longer branches of the inflorescence, and straight, un-reflexed perianth segments. In A. hybridus, the perianth segments are acute and the bracteoles, although a little shorter than those of A. retroflexus, are distinctly more spiny to the touch. A. retroflexus also resembles A. palmeri and the waterhemps, A. rudis and A. tuberculatus, but the stems and leaves of the three latter species are smooth and hairless, and male and female flowers occur on separate plants. The latter three are all north American species. A. palmeri has not spread significantly outside the USA and Mexico (Sauer, 1955), but has become increasingly important weed in soyabeans, groundnuts and cotton in south-eastern USA (Webster and Cole, 1997).

Many other Amaranthus species are superficially similar to A. retroflexus. Of those included in this compendium, A. spinosus has spines, A. viridis has much smaller flowers, A. blitum has indented leaf tips, and A. graecizans and A. blitoides have mainly axillary inflorescences. Further species can occur as weeds on a local basis and reference to local floras or expertise may be necessary

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Control

Cultural Control

Seedlings can be controlled by cultivation, but older plants often recover from mechanical damage and produce axillary inflorescences. Soil solarization under plastic can provide control of A. retroflexus if high temperatures are achieved for prolonged periods of time (Mas and Verdu, 1996).

Chemical Control

A. retroflexus is readily controlled by most herbicides which inhibit photosynthesis, such as atrazine, simazine, metribuzin, linuron and bromoxynil. It is also highly susceptible to the synthetic auxin herbicides, such as 2,4-D or dicamba, and sulfonylurea and imidizolinone herbicides, such as imazethapyr, thifensulfuron-methyl, rimsulfuron and nicosulfuron. Most other herbicides for control of broad-leaved weeds also provide good control including acifluorfen, fomesafen and pendimethalin (Weaver and McWilliams 1980; Bauer et al. 1995; Carey and Kells 1995; Mamarot and Rodriguez, 1997). Its pattern of intermittent germination throughout the growing season, however, makes the application of residual soil-applied herbicides, or sequential post-emergence treatments, necessary in heavily infested fields. A. retroflexus can be controlled by the soil fumigant methyl iodide (Zhang et al., 1997).

Herbicide Resistance

Populations of A. retroflexus resistant to atrazine have been reported in the USA, Canada, France, Germany, Hungary, Switzerland, Spain, Poland, Chile and the Czech Republic (Heap, 1997). Many of these are cross-resistant to metribuzin and linuron (Daban and Garbutt, 1996). Populations resistant to ALS inhibitors (sulfonylureas and imadizolinones) have been reported in the USA and Israel (Heap, 1997). In the past 20 years biotypes resistant to 15 herbicide active ingredients have been reported in 15 countries (LeBaron and Gressel, 1982; Benbrook, 1991).

Biological Control

Biological control of A. retroflexus with fungal pathogens has been reported (Burki et al., 1997). The pigweed flea beetle, Disonycha glabrata, is native to the southern USA and South America. It has been promoted as a control agent of A. retroflexus (Tisler 1990), but it gives incomplete control in the field because beetle populations develop too slowly and too early in the season to prevent competition with the crop and seed production.

References

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Aellen P, Akeroyd JR, 1993. Amaranthus L. In: Tutin TG, Burges NA, Chater AO, Edmondson JR, Heywood VH, Moore DM, Valentine DH, Walters SM, Webb DA, eds. Flora Europaea. Volume 1. Psilotaceae to Platanaceae. 2nd edition. Cambridge, UK: Cambridge University Press, 130-132.

Anderson RL, Nielsen DC, 1996. Emergence pattern of five weeds in the central Great Plains. Weed Technology, 10(4):744-749; 37 ref.

Athanassova DP, 1996. Allelopathic effect of Amaranthus retroflexus L. on weeds and crops. Seizième conférence du COLUMA. Journées internationales sur la lutte contre les mauvaises herbes, Reims, France, 6-8 décembre 1995. Tome 1., 437-442; 10 ref.

Baliousis E, 2014. Recent data from the flora of the island of Limnos (NE Aegean, Greece): new alien invasive species affecting the agricultural economy of the island. Edinburgh Journal of Botany, 71(2):275-285. http://www.journals.cup.org/action/displayJournal?jid=EJB

Bauer TA, Renner KA, Penner D, 1995. Response of selected weed species to postemergence imazethapyr and bentazon. Weed Technology, 9(2):236-242

Benbrook C, 1991. Racing against the clock, pesticide resistant biotypes gain ground. Agrichemical Age, 25:30-33.

Bnrki HM, Schroeder D, Lawrie J, Cagßn L, Vrablova M, El-Aydam M, Szentkirßlyi F, Ghorbani R, Jnttersonke B, Ammon HU, 1997. Biological control of pigweeds (Amaranthus retroflexus L., A. powellii S. Watson and A. bouchonii Thell.) with phytophagous insects, fungal pathogens and crop management. Integrated Pest Management Reviews, 2(2):51-59; 3 pp. of ref.

Brenan JPM, 1961. Amaranthus in Britain. Watsonia, 4:261-280.

Burgos NR, Talbert RE, 1996. Weed control and sweet corn (Zea mays var. rugosa) response in a no-till system with cover crops. Weed Science, 44(2):355-361; 26 ref.

Carey JB, Kells JJ, 1995. Timing of total postemergence herbicide applications to maximize weed control and corn (Zea mays) yield. Weed Technology, 9(2):356-361; 20 ref.

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Daban ME, Garbutt K, 1996. Herbicide cross-resistance in atrazine-resistant velvetleaf (Abutilon theophrasti) and redroot pigweed (Amaranthus retroflexus). In: Brown H, Cussans GW, Devine MD; Duke SO, Fernandez-Quintanilla C, Helweg A, Labrada RE, Landes M, Kudsk P, Streibig JCP, eds. Proceedings of the Second International Weed Control Congress, Copenhagen, Denmark. Slagelse, Denmark: Department of Weed Control and Pesticide Ecology, 505-510.

Egley GH, Chandler JM, 1978. Germination and viability of weed seeds after 2.5 years in a 50-year buried seed study. Weed Science, 26(3):230-239

Feltner KC, 1970. The ten worst weeds of field crops. 5. Pigweed. Crops and Soils, 23:13-14.

Gallagher RS, Cardina J, 1997. Soil water thresholds for photoinduction of redroot pigweed germination. Weed Science, 45(3):414-418; 14 ref.

Grant WF, 1959. Cytogenetic studies in Amaranthus. III. Chromosome numbers and phylogenetic aspects. Canadian Journal of Genetics and Cytology, 1:313-328.

Heap IM, 1997. International Survey of Herbicide-Resistant Weeds. Annual Report, Weed Science Society of America.

Holm LG, Doll J, Holm E, Pancho JV, Herberger JP, 1997. World Weeds: Natural Histories and Distribution. New York, USA: John Wiley & Sons Inc.

Holm LG, Pancho JV, Herberger JP, Plucknett DL, 1991. A Geographic Atlas of World Weeds. Malabar, Florida, USA: Krieger Publishing Company.

Ladonin VF, Kramarev SM, Klyavzo SP, Golovko AI, Kovalenko VD, Bondar' VP, Lerinets FA, 1994. Characteristics of the behaviour of herbicides of the maize complex with different methods of application on normal chernozems of the Ukraine steppe. Communication 1. Effectiveness of the utilization of different herbicides in maize crops. Agrokhimiya, No. 11:80-86; 5 ref.

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LeBaron H, Gressel J, 1982. Herbicide Resistance in Plants. New York, USA: John Wiley and Sons.

Maigre D, 1991. Availability and efficacy of atrazine in acid soils: the Ticino case. Revue Suisse d'Agriculture, 23(3):167-171

Mamarot J, Rodriguez A, 1997. Sensibilité des Mauvaises Herbes aux Herbicides. 4th edition. Paris, France: Association de Coordination Technique Agricole.

Marten GC, Andersen RN, 1975. Forage nutritive value and palatability of 12 common annual weeds. Crop Science, 15(6):821-827

Mas MT, Verd· AM, 1996. Soil solarization and control of amaranth (Amaranthus retroflexus): heat resistance of the seeds. Seizie^grave~me confe^acute~rence du COLUMA. Journe^acute~es internationales sur la lutte contre les mauvaises herbes, Reims, France, 6-8 de^acute~cembre 1995. Tome 1., 411-417; 5 ref.

McLachlan SM, Murphy SD, Tollenaar M, Weise SF, Swanton CJ, 1995. Light limitation of reproduction and variation in the allometric relationship between reproductive and vegetative biomass in Amaranthus retroflexus (redroot pigweed). Journal of Applied Ecology, 32(1):157-165

Mitchell J, Rook A, 1979. Botanical Dermatology: plants and plant products injurious to the skin. Vancouver, Canada: Greengrass.

Mitich LW, 1997. Redroot pigweed (Amaranthus retroflexus). Weed Technology, 11(1):199-202; 35 ref.

Moyer JR, Hironaka R, 1993. Digestible energy and protein content of some annual weeds, alfalfa, bromegrass, and tame oats. Canadian Journal of Plant Science, 73(4):1305-1308; 9 ref.

Murray MJ, 1940. The genetics of sex determination in the family Amaranthaceae. Genetics, 25:409-431.

Nuss R, Loewus FA, 1978. Further studies on oxalic acid biosynthesis in oxalate-accumulating plants. Plant Physiology, 61:590-592.

Oryokot JOE, Murphy SD, Thomas AG, Swanton CJ, 1997. Temperature- and moisture-dependent models of seed germination and shoot elongation in green and redroot pigweed (Amaranthus powellii, A. retroflexus). Weed Science, 45(4):488-496; 2 pp. of ref.

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Sauer JD, 1955. Revision of the dioecious amaranths. Madrono, 13:5-46.

Sauer JD, 1967. The grain amaranths and their relatives: A revised taxonomic and geographic survey. Annals of the Missouri Botanic Garden, 54:103-137.

Siriwardana T, Zimdahl R, 1983. Competition between barnyard grass (Echinochloa crus-galli) and red-root pigweed (Amaranthus retroflexus). Abstracts of the 23rd Weed Science Society of America Conference, 23:61.

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Terry LI, Lee CW, 1990. Infestation of cultivated Amaranthus by the weevil Conotrachelus seniculus in southeastern Arizona. Southwestern Entomologist, 15(1):27-31

Tisler AM, 1990. Feeding in the pigweed flea beetle, Disonycha glabrata Fab. (Coleoptera: Chrysomelidae), on Amaranthus retroflexus. Virginia Journal of Science, 41(3):243-245

Torres MB, Kommers GD, Dantas AFM, Lombardo de Barros CS, 1997. Redroot pigweed (Amaranthus retroflexus) poisoning of cattle in southern Brazil. Veterinary and Human Toxicology, 39(2):94-96; 33 ref.

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Wesche-Ebeling P, Maiti R, García-Díaz G, González DI, Sosa-Alvarado F, 1995. Contributions to the botany and nutritional value of some wild Amaranthus species (Amaranthaceae) of Nuevo Leon, Mexico. Economic Botany, 49(4):423-430; 32 ref.

Wiese A, Davis R, 1967. Weed emergence from two soils at various moistures, temperatures and depths. Weeds, 15:118-121.

Wnrtzen PA, Nelson HS, LOwenstein H, Ipsen H, 1995. Characterization of Chenopodiales (Amaranthus retroflexus, Chenopodium album, Kochia scoparia, Salsola pestifer) pollen allergens. Allergy (Copenhagen), 50(6):489-497; [20 pl.]; 16 ref.

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Distribution Maps

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