Amaranthus retroflexus L., Amaranthaceae

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Present on Pacific Islands?  yes

Primarily a threat at high elevations?  no

Common name(s): [more details]

Chinese: fan zhi xian

English: careless weed, common amaranth, redroot, redroot amaranth, redroot pigweed, rough pigweed, wild-beet amaranth

French: amarante recourbee, amarante réfléchie

Japanese: aogeito

Spanish: amaranto, amaranto común, atacu, bledo, marxant, moco de pavo, penacho, quelite, yuyo

Habit:  herb

Description:  "A monoecious, erect, finely hairy, freely-branching, herbaceous annual growing up 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; 3 spiny-tipped, rigid, awl-shaped bracts surround the flower, exceeding calyx, length 4 to 8 mm, persistent; tepals 5, much longer than fruit, usually definitely recurved at tips, obovate or highly spatulate, 1 pistil and 5 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.  The species can be initially identified by the rounded (not pointed) shape of its recurved tepals and the shorter, more crowded branches of its inflorescence, and if nee be by a more detailed examination of the floral structures"  (Holm et al., 1997; pp. 51, 53).

Habitat/ecology:  "The weed thrives in cultivated fields, home gardens, waste places and on ditch banks.  It prefers open, sunny areas and appears quickly when soil is disturbed" (Holm et al., 1997; p. 56).

Propagation:  Seed

Native range:  North America, naturalized nearly worldwide (GRIN).

Presence:

Pacific
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
State of Hawaii Hawaiian Islands	Hawai‘i (Big) Island	introduced invasive	Wagner, Warren L./Shannon, Robynn K./Herbst, Derral R. (1997) (p. 51) Voucher cited: Herbst 5907 (US) Apparently naturalized, present status unknown.
Philippines Philippine Islands	Philippine Islands	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)

Pacific Rim
Country/Terr./St. & Island group	Location	Cited status & Cited as invasive & Cited as cultivated & Cited as aboriginal introduction?	Reference & Comments
Australia Australia (continental)	New South Wales	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Australia Australia (continental)	Queensland	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Chile (continental) Chile	Chile (Republic of)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
China China	China (People's Republic of)	introduced invasive	Li-ying, Li/Ren, Wang/Waterhouse, D. F. (1997) (p. 172)
China China	China (People's Republic of)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Colombia Colombia	Colombia (Republic of)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Ecuador (Mainland) Ecuador	Ecuador (Republic of) (continental)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Indonesia Indonesia	Indonesia (Republic of)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Japan Japan	Japan (country)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Japan Japan	Japan (country)	introduced	Mito, Toshikazu/Uesugi, Tetsuro (2004) (p. 182)
South Korea South Korea	South Korea (Republic of)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)
Mexico Mexico	Mexico (United Mexican States)	native	U.S. Dept. Agr., Agr. Res. Serv. (2011)
New Zealand New Zealand	New Zealand (country)	introduced invasive	Webb, C. J./Sykes, W. R./Garnock-Jones, P. J. (1988) (p. 107) "Waste places, cultivated ground".
Perú Perú	Perú (Republic of)	introduced invasive	Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James (1997) (p. 55)

Control:  Some varieties have developed resistance to herbicides (Holm et al., 1997; pp. 63-64).

Additional information:
Information and photos at Robert W. Freckmann Herbarium, Univeristy of Wisconsin at Stevens Point.
Information and photos at the Virginia Tech Weed Identification Guide
Description and other information from the Flora of North America
Photos and other information from Chileflora.

Additional online information about Amaranthus retroflexus is available from the Hawaiian Ecosystems at Risk project (HEAR).

Information about Amaranthus retroflexus as a weed (worldwide references) may be available from the Global Compendium of Weeds (GCW).

Taxonomic information about Amaranthus retroflexus may be available from the Germplasm Resources Information Network (GRIN).

References:

Belov, Michail. 2011. Chileflora (online resource).

Holm, Leroy/Doll, Jerry/Holm, Eric/Pancho, Jaun/Herberger, James. 1997. World weeds: natural histories and distribution. John Wiley & Sons. 1129 pp.

Li-ying, Li/Ren, Wang/Waterhouse, D. F. 1997. The distribution and importance of arthropod pests and weeds of agriculture and forestry plantations in southern China. ACIAR, Canberra, Australia. 185 pp.

Mito, Toshikazu/Uesugi, Tetsuro. 2004. Invasive alien species in Japan: the status quo and the new regulation for prevention of their adverse effects. Global Environmental Research 8(2)/2004: 171-191.

U. S. Government. 2011. Integrated Taxonomic Information System (ITIS) (on-line resource).

U.S. Dept. Agr., Agr. Res. Serv. 2011. National Genetic Resources Program. Germplasm Resources Information Network (GRIN). Online searchable database.

Wagner, Warren L./Shannon, Robynn K./Herbst, Derral R. 1997. Contributions to the flora of Hawai‘i. In: Evenhuis, Neal L. and Miller, Scott, E., eds. Records of the Hawaii Biological Survey for 1996. Part 1: Articles. Bishop Museum Occasional Papers. 48:51-65.

Webb, C. J./Sykes, W. R./Garnock-Jones, P. J. 1988. Flora of New Zealand, Volume IV: Naturalised pteridophytes, gymnosperms, dicotyledons. Botany Division, DSIR, Christchurch. 1365 pp.

Zhengyi, Wu/Raven, Peter H./Deyuan, Hong. 2011. Flora of China (online resource).