Amaranthus retroflexus
Amaranthus blitum
common amaranth, pigweed amaranth, red-root amaranth, red-root pigweed, rough pigweed, wild-beet amaranth
Guernsey pigweed, livid amaranth, pale amaranth, purple 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.
ascending to prostrate, sometimes erect, simple or branched, sometimes radiating from base and forming mats, 0.1–0.6 m. Leaves: petiole usually equaling to 2 times as long as blade;
blade ovate or obovate, 1–6 × 0.5–4 cm, base tapering or cuneate, margins entire, plane, apex distinctly emarginate to almost bilobate, mucronate.
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.
of pistillate flowers lanceolate, inconspicuous, 0.5 mm, shorter 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.
slender terminal spikes or panicles and also axillary clusters, in some forms only axillary clusters are present;
spikes erect or sometimes reflexed, green, leafless at least distally.
few at tips of inflorescences;
tepals 5;
stamens (3–)4–5.
clustered at tips of spikes;
tepals 3;
stamens 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.
tepals 3, elliptic or spatulate, not clawed, equal or subequal, 0.8–1.5 mm, margins entire, apex broadly acute;
style branches erect;
stigmas 3.
black to dark reddish brown, lenticular to subglobose-lenticular, 1–1.3 mm, smooth, shiny.
black or dark reddish brown, subglobose or broadly lenticular, (0.8–)1–1.8 mm diam., smooth, shiny, filling fruit almost completely.
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.
compressed, subglobose to obovate, 1.2–2.5(–3) mm, exceeding tepals, smooth or faintly rugose, indehiscent.
Amaranthus retroflexus
Amaranthus blitum
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 lividus has been widely used for A. blitum, in addition to other Linnaean names (see F. Fillias et al. 1980; J. P. M. Brenan and C. C. Townsend 1980; R. K. Brummitt 1984). Amaranthus blitum is of tropical origin and not common in temperate regions. It has been cultivated in Europe as a minor leaf-vegetable crop, but now it is declining and its range is becoming progressively smaller. In many temperate countries (in particular in Europe), A. blitum persists mostly as an uncommon and sporadic weed in greenhouses, ornamental gardens, and flower beds.
In Europe, it may be possible to distinguish two or three subspecies within Amaranthus blitum. The occurrence and distribution of infraspecific taxa of the A. blitum complex is insufficiently known in North America and requires additional floristic and taxonomic studies, although it appears that the most common is subsp. polygonoides (Moquin-Tandon) Cattetero. Some literature records of A. blitum from southern regions of North America are misidentifications of A. viridis, and vice versa; because of that the distributions of these two species in the flora area require critical reassessment.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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