Amaranthus blitoides S. Wats.

The name Amaranthus graecizans often has been misapplied to both A. blitoides and A. albus in older North American floras and manuals.

Amaranthus blitoides was probably originally native to central and partly eastern United States, but now it is widely and successfully naturalized almost everywhere in temperate North America and in many subtropical to warm-temperate regions. It has not been reported from Mississippi or North Carolina but since it is found in all other conterminous United States it can be expected to occur in these two as well.

Varieties have been described within Amaranthus blitoides; most of them are of no taxonomic significance, being mostly ecologic forms or local morphologic variants. Among the infraspecific taxa, the most constant is var. reverchonii Uline & W. L. Bray, with narrower, more elongated leaves.

Plants annual, glabrous. Stems prostrate or ascending (very rarely suberect), much-branched (usually from base), (0.1-)0.2-0.6(-1) m. Leaves: petiole ± ¹/2 as long as blade; blade obovate, elliptic, or spatulate, 1-2(-4) × 0.5-1(-1.5) cm, base cuneate and tapering, margins usually entire, plane, rarely slightly undulate, apex obtuse, rounded, mucronulate. Inflorescences axillary glomerules, green. Bracts of pistillate flowers narrow, thin, 1.5-5 mm, ± equaling or slightly exceeding tepals. Pistillate flowers: tepals (3-)4-5, narrowly ovate to broadly linear, unequal or subequal, 1.5-3 mm, thin, apex acute or acuminate; style branches spreading; stigmas 3. Staminate flowers intermixed with pistillate; tepals 3(-4); stamens 3. Utricles broadly ovoid, 1.7-2.5 mm, equaling tepals, mostly smooth (slightly verrucose or rugose in dry plants), dehiscence regularly circumscissile. Seeds black, lenticular to broadly plumply lenticular, 1.3-1.6 mm diam., rather dull.

Stem greenish white, 15-50 cm tall, divaricately branched from base, glabrous. Leaves dense; petiole 0.5-1.5 cm; leaf blade obovate or spatulate to oblong-oblanceolate, 0.5-2.5 cm × 3-10 mm, base cuneate, margin entire, apex obtuse or acute, with a mucro. Flowers clustered at axils, shorter than petioles. Bracts and bracteoles lanceolate, ca. 3 mm, apex acute. Tepals 4(or 5), green, ovate-lanceolate to oblong-lanceolate, 1-2.5 mm, apex acuminate and pointed. Stigmas 3. Utricles ellipsoid, longer than longest perianth segment, ca. 2 mm, circumscissile. Seeds black, slightly shiny, ovoid, ca. 1.5 mm in diam. Fl. Aug-Sep, fr. Sep-Oct. 2n = 32.

Alta., B.C., Man., Ont., 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., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; introduced and often completely naturalized in South America, Eurasia, and other regions.

Flowering summer-fall.

Disturbed habitats: roadsides, riverbanks, railroads, fields, waste places, sandy flats; 0-2200m.

Fields, roadsides. Beijing, Liaoning, Nei Mongol [native to North America].

Amaranthus blitoides S. Wats. Proc. Am. Acad. 12: 273. 1877
Amaranthus blitoides densifolius UHne & Bray, Bot. Gaz. 19: 315. 1894. Amaranthus blitoides Reverchoni UHne & Bray, Bot. Gaz. 19: 315. 1894. Amaranthus aragonensis Sennen, Bull. Geogr. Bot. IV. 21: 123. 1911. Galliaria blitoides Nieuwl. Am. Midi. Nat. 3: 278. 1914. Amaranthus blitoides crassior Jepson, FL Calif. 449. 1914.
Stems stout, prostrate, much branched, 1.5-6 dm. long, glabrous orsparsely pubescent, pale-green or whitish, rarely tinged with red; leaves usually numerous, often crowded, especially near the ends of the branches, the petioles rather stout, 2-20 mm. long, the blades obovate to oval, spatulate, or elliptic, 0.8-4 cm. long, rounded to acutish at the apex, broadly cuneate to attenuate at the base, pale-green, glabrous, plane, prominently veined, the veins whitish beneath, the smaller leaves' often white-margined; flowers monoecious, in dense axillary clusters, these usually shorter but sometimes longer than the petioles ; bracts oblong to lanceolate, equaling or slightly exceeding the sepals, erect, attenuate at the apex .to a short spinose tip, green; sepals 4 or 5, those of the staminate flowers scarious, oblong, acute, those of the pistillate flowers oblong or narrowly oblong, 2.5-3 mm. long, acuminate, 1-nerved, green, white-margined; stamens 3; style-branches 3; utricle subglobose, equaling or slightly longer than the sepals, smooth or nearly so, circumscissile, sometimes tinged with red; seed rotund, 1.3-1.5 mm. in diameter, black, rather dull.
Type locality: Not definitely stated.
Distribution: Dry or cultivated ground, Washington to Wyoming, Kansas, Texas, and Chihuahua; established in waste ground in many places in the eastern United States and southern Canada; adventive in southern Europe.

Amaranthus blitoides, commonly called mat amaranth, prostrate pigweed, procumbent pigweed, prostrate amaranth, or matweed, is a glabrous annual plants species. It usually grows up to 0.6 m, though it may grow up to 1 m (3 feet). It flowers in the summer to fall.

It is believed to have been a native of the central and possibly eastern United States, but it has naturalized in almost all of temperate North America. It has also naturalized in South America and Eurasia. Some authorities list it as an invasive species.