Redroot Amaranth

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).

One of us (Clemants) does not recognize the following varieties.

Annual herb, erect or with ascending branches, (6-) 15-80 (-100) cm, simple or branched (especially from the base to about the middle of the stem). Stem stout, sub-terete to angled, densely furnished with multicellular hairs. Leaves furnished with multicellular hairs along the lower surface of the primary venation and often the lower margins, long-petiolate (petioles up to c. 6 cm, in robust plants not rarely equalling the lamina), lamina ovate to rhomboid or oblong-ovate, (1-) 5-11 x (0.6-) 3-6 cm, obtuse to subacute at the mucronulate tip, shortly cuneate or attenuate into the petiole. Flowers in greenish or rarely somewhat pink-suffused, stout, axillary and terminal spikes, which are usually shortly branched to give a lobed appearance, more rarely with longer branches, the terminal inflorescence paniculate, very variable in size, male and female flowers intermixed, the latter generally much more plentiful except sometimes at the tip of the spikes. Bracts and bracteoles lanceolate-subulate, pale-membranous with a prominent green midrib excurrent into a stiff, colourless arista, longer bracteoles subequalling to twice as long as the perianth. Perianth segments 5, those of the male flowers 1.75-2.25 mm, lanceolate-oblong, blunt to subacute, those of the female flowers 2-3 mm, narrowly oblong-spathulate to spathulate, obtuse or emarginate, ± green-vittate along the midrib, which ceases below the apex or is excurrent in a short mucro. Stigmas 2-3, patent-flexuose or erect, c. 1 mm. Capsule subglobose, c. 2 mm, usually shorter than the perianth, circumscissile, with an indistinct neck, rugose below the lid. Seed black and shining, compressed, c. 1 mm, almost smooth centrally, faintly reticulate around the margins.

Plants densely to moderately pubescent, especially distal parts of stem and branches. Stems 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 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. Inflorescences 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. Bracts lanceolate to subulate, (2.5-)3.5-5(-6) mm, exceeding tepals, apex acuminate with excurrent midrib. Pistillate flowers: 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. Staminate flowers few at tips of inflorescences; tepals 5; stamens (3-)4-5. Utricles 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. Seeds black to dark reddish brown, lenticular to subglobose-lenticular, 1-1.3 mm, smooth, shiny.

Stem erect, light green, 20-80 cm tall, stout, branched or not, slightly obtusely angulate, densely pubescent. Petiole light green, 1.5-5.5 cm, hairy; leaf blade ovate-rhombic or elliptic, 5-12 × 2-5 cm, both surfaces shortly hairy, but densely hairy abaxially, base cuneate, margin entire and undulate, apex acute or notched, with a mucro. Complex thyrsoid structures terminal and axillary, erect, 2-4 cm in diam., including many spikes; terminal spikes longer than lateral ones. Bracts and bracteoles white, subulate, 4-6 mm, apex slenderly long pointed. Tepals white, oblong or oblong-obovate, 2-2.5 mm, membranous, with a green midvein, apex acute or notched, with a mucro. Stamens slightly longer than perianth. Stigmas 3, rarely 2. Utricles light green, ovoid, compressed, shorter than perianth, circumscissile. Seeds brown or black, subglobose, ca. 1 mm in diam., obtuse at margin. Fl. Jul-Aug, fr. Aug-Sep. 2n = 32*, 34*, 102*.

Distribution: A native of N. America south to N. Mexico; introduced into the Old World as a weed, but in more temperate regions than many of its allies, occurring in S. and C. Europe, Mediterranean N. Africa and temperate Asia from Cyprus and Turkey to Iran, Siberia, Middle Asia, Mongolia, China and Japan. Also adventive in Australia and S. America (Bolivia etc.) and probably elsewhere. Generally occurs as a weed of cultivation, occurring as such in the cooler parts of Pakistan, ascending to 1820 m or more.

St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask.; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; introduced and naturalized nearly worldwide.

Flowering summer-fall.

Banks of rivers, lakes, and streams, disturbed habitats, agricultural fields, railroads, roadsides, waste areas; 0-2500m.

Waste places, field margins, roadsides. Gansu, Hebei, Heilongjiang, Jilin, Liaoning, Nei Mongol, Ningxia, Shaanxi, Shandong, Shanxi, Xinjiang, Zhejiang [native origin uncertain; now cosmopolitan].

Amaranthus retroflexus var. salicifolius I. M. Johnston