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African amaranth, muricate amaranth

common amaranth, pigweed amaranth, red-root amaranth, red-root pigweed, rough pigweed, wild-beet amaranth

Habit Plants annual or short-lived perennial, glabrous or slightly pubescent near tips. Plants densely to moderately pubescent, especially distal parts of stem and branches.
Stems

ascending or prostrate, much-branched from stout rootstock, 0.1–0.4 m.

erect, reddish near base, branched in distal part to simple 0.2–1.5(–2) m; underdeveloped or damaged plants rarely ascending to nearly prostrate.

Leaves

petiole to 1/2 as long as blade;

blade linear to narrowly lanceolate, 1.5–8 × 0.2–0.5(–1) mm, base tapering, margins entire, plane to undulate, apex obtuse and often emarginate.

petiole 1/2 to equaling blade;

blade ovate to rhombic-ovate, 2–15 × 1–7 cm, base cuneate to rounded-cuneate, margins entire, plane or slightly undulate, apex acute, obtuse, or slightly emarginate, with terminal mucro.

Bracts

of pistillate flowers linear, 0.7–1.2 mm, 1/2–2/3 as long as tepals.

lanceolate to subulate, (2.5–)3.5–5(–6) mm, exceeding tepals, apex acuminate with excurrent midrib.

Inflorescences

terminal, compact pyramidal panicles and axillary glomerules, erect or reflexed, green, leafless at least distally.

terminal and axillary, erect or reflexed at tip, green or silvery green, often with reddish or yellowish tint, branched, leafless at least distally, usually short and thick.

Staminate flowers

intermixed with pistillate or at tips of inflorescences;

tepals 5;

stamens 5.

few at tips of inflorescences;

tepals 5;

stamens (3–)4–5.

Pistillate flowers

tepals 5, narrowly oblanceolate, not clawed, equal, 1.5–2 mm, apex obtuse or subacute;

style branches erect;

stigmas 3.

tepals 5, spatulate-obovate, lanceolate-spatulate, not clawed, subequal or unequal, (2–)2.5–3.5(–4) mm, membranaceous, apex emarginate or obtuse, with mucro;

style branches erect or slightly spreading,;

stigmas 3.

Seeds

black, lenticular, 1–1.2 mm diam., semiglossy.

black to dark reddish brown, lenticular to subglobose-lenticular, 1–1.3 mm, smooth, shiny.

Utricles

compressed, subglobose, 1.7–2 mm, ± equaling or slightly exceeding tepals, muricate, indehiscent.

broadly obovoid to broadly elliptic, 1.5–2.5 mm, shorter than or subequal to tepals, smooth or slightly rugose, especially near base and in distal part, dehiscence regularly circumscissile.

Amaranthus muricatus

Amaranthus retroflexus

Phenology Flowering summer–fall. Flowering summer–fall.
Habitat Waste places, on ballast Banks of rivers, lakes, and streams, disturbed habitats, agricultural fields, railroads, roadsides, waste areas
Elevation 0 m (0 ft) 0-2500 m (0-8200 ft)
Distribution
from FNA
AL; s South America (Argentina, Paraguay, Uruguay) [Introduced in North America; introduced in s Europe, s Africa, Australia, and other regions]
[BONAP county map]
from FNA
AK; AL; AR; AZ; CA; CO; CT; DC; DE; FL; GA; IA; ID; IL; IN; KS; KY; LA; MA; MD; ME; MI; MN; MO; MS; MT; NC; ND; NE; NH; NJ; NM; NV; NY; OH; OK; OR; PA; RI; SC; SD; TN; TX; UT; VA; VT; WA; WI; WV; WY; AB; BC; MB; NB; NL; NS; NT; NU; ON; PE; QC; SK; SPM [Introduced and naturalized nearly worldwide]
[WildflowerSearch map]
[BONAP county map]
Discussion

The vernacular name “African amaranth” is sometimes used for this species; it is a misnomer; the species is native to South America and naturalized in Africa.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Amaranthus retroflexus, native to central and eastern North America, is a successful invasive species and has effectively colonized a wide range of habitats on all inhabited continents. Its variability is extremely wide; usually the species is easily recognized and its identification causes no specific problems. Infraspecific entities described within A. retroflexus are mostly ecologic variants of little or no taxonomic value. Two varieties are more easily recognized: the common var. retroflexus, with bracts about 1.5–2 times as long as tepals, and a more rare var. delilei (Richter & Loret) Thellung (= A. delilei Richter & Loret), with bracts 1–1.5 times as long as tepals.

Occasional forms morphologically intermediate between Amaranthus retroflexus and taxa of the A. hybridus aggregate (e.g., A. powellii and A. hybridus, in the strict sense) are known both in the Americas and the Old World. Usually such plants are treated as hybrids; in many cases they are probably just extremes of the natural variability of A. retroflexus. Putative hybrids of A. retroflexus were described from Europe as A. ×ozanonii Thellung (A. hybridus × A. retroflexus) and A. ×soproniensis Priszter & Karpáti (A. powellii × A. retroflexus) (see A. Thellung 1914–1919; S. Priszter 1958; P. Aellen 1959; F. Grüll and S. Priszter 1973).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 4, p. 431. FNA vol. 4.
Parent taxa Amaranthaceae > Amaranthus > subg. Albersia Amaranthaceae > Amaranthus > subg. Amaranthus
Sibling taxa
A. acanthochiton, A. albus, A. arenicola, A. australis, A. blitoides, A. blitum, A. californicus, A. cannabinus, A. caudatus, A. crassipes, A. crispus, A. cruentus, A. deflexus, A. dubius, A. fimbriatus, A. floridanus, A. graecizans, A. greggii, A. hybridus, A. hypochondriacus, A. obcordatus, A. palmeri, A. polygonoides, A. powellii, A. pumilus, A. retroflexus, A. scleropoides, A. spinosus, A. tamaulipensis, A. thunbergii, A. torreyi, A. tricolor, A. tuberculatus, A. viridis, A. viscidulus, A. watsonii, A. wrightii
A. acanthochiton, A. albus, A. arenicola, A. australis, A. blitoides, A. blitum, A. californicus, A. cannabinus, A. caudatus, A. crassipes, A. crispus, A. cruentus, A. deflexus, A. dubius, A. fimbriatus, A. floridanus, A. graecizans, A. greggii, A. hybridus, A. hypochondriacus, A. muricatus, A. obcordatus, A. palmeri, A. polygonoides, A. powellii, A. pumilus, A. scleropoides, A. spinosus, A. tamaulipensis, A. thunbergii, A. torreyi, A. tricolor, A. tuberculatus, A. viridis, A. viscidulus, A. watsonii, A. wrightii
Synonyms Euxolus muricatus A. retroflexus var. salicifolius
Name authority (Moquin-Tandon) Hieronymus: Pl. Diaph. Fl. Argent., 227. (1882) Linnaeus: Sp. Pl. 2: 991. (1753)
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