Amaranthus retroflexus
Amaranthus powellii
common amaranth, pigweed amaranth, red-root amaranth, red-root pigweed, rough pigweed, wild-beet amaranth
green amaranth, green pigweed, Powell's amaranth, Powell's smooth amaranth
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.
usually erect, green or sometimes reddish purple, branched, mainly in inflorescences, to nearly simple, 0.3–1.5(–2) m, stiff.
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.
petiole mostly equaling or longer than blade;
blade rhombic-ovate to broadly lanceolate, 4–8 × 2–3 cm, occasionally larger in robust plants, base cuneate to broadly cuneate, margins entire, apex cuneate to obtuse or indistinctly emarginate, with mucro.
lanceolate to subulate, (2.5–)3.5–5(–6) mm, exceeding tepals, apex acuminate with excurrent midrib.
lanceolate to linear-subulate, 4–7 mm, 2–3 times as long as tepals, rigid.
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.
mostly terminal, usually with spikes at distal axils, erect and rigid, green to silvery green, occasionally tinged red, leafless at least distally.
few at tips of inflorescences;
tepals 5;
stamens (3–)4–5.
clustered at tips of inflorescence branches;
tepals 3–5;
stamens 3–5.
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.
tepals usually 3–5, not clawed, unequal;
outer tepals narrowly ovate-elliptic or elliptic, 1.5–3.5 mm, apex aristate;
style branches spreading, shorter than body of fruit;
stigmas 3.
black to dark reddish brown, lenticular to subglobose-lenticular, 1–1.3 mm, smooth, shiny.
black, subglobose to lenticular, 1–1.4 mm diam., smooth, shiny.
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.
subglobose or compressed-ovoid, 2–3 mm, equaling or shorter than tepals, smooth or lid slightly rugose or minutely verrucose, dehiscence regularly circumscissile.
Amaranthus retroflexus
Amaranthus powellii
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.)
Amaranthus powellii is originally native to southwestern United States and adjacent regions of Mexico; now, it is widely naturalized almost everywhere in temperate regions of North America. The distribution of A. powellii is probably underestimated both in North America and the Old World, and literature references are somewhat confusing, because A. powellii has been commonly confused with A. hybridus.
Forms of Amaranthus powellii with indehiscent or occasionally irregularly dehiscent utricles were described from Europe (southwestern France, the Gironde estuary) as A. bouchonii Thellung. Similar forms occasionally occur in North America. According to J. M. Tucker and J. D. Sauer (1958) and J. D. Sauer (1967b, 1972b), they are mostly “mutant or aberrant forms” of A. powellii, or hybrids of A. powellii and/or A. hybridus with other species. Recent comparative studies of morphology and isozymes of A. bouchonii (P. Wilkin 1992) indicated that that taxon, whatever its origin was, now differs from its presumably parental species and probably deserves recognition, at least as a separate subspecies. It seems that in North America, the situation with indehiscent-fruited forms is much more complicated than in Europe, and multiple entities are involved, including deviate forms of A. powellii and also partly sterile hybrids of dioecious taxa with species belonging to the A. hybridus group. The formal recognition of A. bouchonii in North American material would be premature.
The names Amaranthus hybridus, A. chlorostachys Willdenow, and A. hybridus subsp. chlorostachys (Willdenow) Hejný were occasionally misapplied to A. powellii in North America and Europe.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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