Amaranthus retroflexus
Amaranthus torreyi
common amaranth, pigweed amaranth, red-root amaranth, red-root pigweed, rough pigweed, wild-beet amaranth
Bigelow's amaranth, sandhill amaranth, Torrey's amaranth, Torrey's amaranthus
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.
erect or ascending proximally, much-branched especially near base, 0.1–0.7 m. Leaves: petiole less than 1/2 as long as blade;
blade oblanceolate or lanceolate to ovate-lanceolate, (1.2–)1.5–5(–7) × 0.3–2 cm, base narrowly cuneate to cuneate, margins entire, plane or slightly undulate, apex acute to subobtuse, mucronulate.
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.
lanceolate to subulate, (2.5–)3.5–5(–6) mm, exceeding tepals, apex acuminate with excurrent midrib.
lanceolate to linear-subulate, 1.3–3.5 mm, slightly longer than tepals.
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.
axillary clusters, toward apex aggregated in spikes (rarely spicate panicles), axes, leafy (occasionally almost leafless distally).
few at tips of inflorescences;
tepals 5;
stamens (3–)4–5.
mostly at tips of inflorescences;
tepals 5, equal or subequal;
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 5, spatulate to narrowly spatulate, clawed, equal or subequal, 1.5–2.5 mm, margins entire, apex obtuse, rounded, or slightly emarginate;
style branches erect;
stigmas 3.
black to dark reddish brown, lenticular to subglobose-lenticular, 1–1.3 mm, smooth, shiny.
black, subglobose to broadly lenticular, 1 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.
obovoid to subglobose-obovoid, 1.5–2 mm, nearly equaling or slightly shorter than tepals, smooth, dehiscence regularly circumscissile.
Amaranthus retroflexus
Amaranthus torreyi
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.)
The name Amaranthus torreyi has been widely misapplied to at least two other species, 7. A. arenicola and 6. A. watsonii. Because of that, the name A. bigelovii was used for A. torreyi by J. D. Sauer (1955) and in some recent floras. Sometimes A. torreyi (California and Arizona) and A. bigelovii (New Mexico and Texas) are recognized as separate species. The nomenclature and taxonomic relationships in this group should be critically reviewed based on type specimens and additional experimental and field studies.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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