Amaranthus retroflexus
Amaranthus crassipes
common amaranth, pigweed amaranth, red-root amaranth, red-root pigweed, rough pigweed, wild-beet amaranth
clubfoot amaranth, spreading amaranth, tropical spreading 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.
prostrate or weakly ascending, erect when young, branched mostly from base, 0.1–0.6 m. Leaves: petiole nearly 1/2 as long as blade;
blade broadly elliptic, obovate, orbiculate, or oblanceolate, (0.5–)1–3(–4.5) × 0.3–2(–2.5) cm, base cuneate, margins entire, plane to undulate, apex broadly rounded or 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.
lanceolate to subulate, (2.5–)3.5–5(–6) mm, exceeding tepals, apex acuminate with excurrent midrib.
of pistillate flowers keeled (only A. scleropoides and A. crassipes have keeled bracts), deltate, 0.5–0.9 mm.
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 borne from base to apex, axes much thickened, appearing inflated, becoming indurate at maturity (only in A. scleropoides and A. crassipes).
few at tips of inflorescences;
tepals 5;
stamens (3–)4–5.
intermixed with pistillate;
tepals 5, membranaceous;
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 (4–)5, narrowly spatulate, clawed, with small expanded blade, equal or subequal, 2–3 mm, apex rounded and apiculate;
claws becoming indurate and scarious at maturity;
style branches spreading;
stigmas usually 2(–3), almost sessile.
black to dark reddish brown, lenticular to subglobose-lenticular, 1–1.3 mm, smooth, shiny.
dark brownish or reddish black to black, compressed-ovoid to broadly lenticular, 1–1.4 mm diam., 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 compressed-obovoid, 1.4–2 mm, shorter than tepals, prominently muricate (especially in distal 1/2), indehiscent.
Amaranthus retroflexus
Amaranthus crassipes
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.)
Two varieties have been distinguished by J. Henrickson (1999) within Amaranthus crassipes: var. crassipes with leaf blades nearly orbiculate to ovate (obovate) and conspicuous veins on the green to yellow-green abaxial leaf surface; and var. warnockii (I. M. Johnston) Henrickson (= A. warnockii I. M. Johnston) with narrower oblanceolate grayish leaves with less distinct venation on the abaxial leaf surfaces. In Henrickson’s treatment, var. crassipes occurs mostly in the coastal areas (and is also reported for Arizona), while var. warnockii is reported from inland southwestern Texas and the Chihuahuan Desert region (Mexico: Coahuila, Chihuahua). We have not seen material of the new varieties and cannot evaluate their validity.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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