Stems erect, branched, stout, usually 1-3 m. Leaves: petiole 1/2 length of blade; blade lanceolate or narrowly lanceolate to linear, to 20 × 4 cm, base narrowly cuneate, margins entire, plane to slightly undulate, apex acute to acuminate. Inflorescences mostly terminal, narrow spikes or thyrses, usually interrupted, linear. Bracts: of pistillate flowers 1.5 mm; of staminate flowers with midribs not prominent, scarcely excurrent, shorter than 1 mm. Pistillate flowers: tepals absent or 1-2 and rudimentary; style branches ± erect; stigmas 3-5. Staminate flowers: tepals 5, without prominent midribs, equal, 2.5-3 mm, apex obtuse to indistinctly mucronulate in outer tepals; stamens 3-5. Utricles often black, with 3(-5) longitudinal ridges corresponding to 3-5 style branches, ovoid or obovoid, 2.5-4 mm (exceeding tepals, when present), somewhat fleshy, smooth (slightly rugose, especially in herbarium specimens). Seeds reddish brown to dark brown, 2-3 mm diam., shiny. Flowering summer-fall. Coastal salt or brackish marshes, edges of sloughs, tidal riverbanks, tidal flats, rarely freshwater semiaquatic habitats; 0-50 m; Ala., Conn., Del., D.C., Fla., Ga., Maine, Md., Mass., N.H., N.J., N.Y., N.C., Pa., R.I., S.C., Vt., Va. Reports of Amaranthus cannabinus from southern Florida and the Gulf Coastal Plain are the result of misidentifications of A. australis. According to J. D. Sauer (1955), plants of A. cannabinus from northern coastal Atlantic Florida are atypical and probably represent hybrid populations transitional toward A. australis.
Dioecious; stout, erect, 1-2.5 m, lvs narrowly lanceolate, to 15 נ4 cm, the upper shorter and narrower, all acuminate to an obtuse tip; fls in large terminal panicles composed of numerous slender, spike-like thyrses, the staminate commonly naked, the pistillate usually leafy; bracts 1-2 mm, the midrib slender in males, heavier in females, scarcely excurrent; male fls with 5 subequal sep 2.5-3 mm, the outer acute, the inner emarginate, the midvein not excurrent; female fls usually without perianth, rarely with 1 or 2 irregular, vestigial sep less than 1 mm; fr 2.5-4 mm, indehiscent, fleshy, 3-5-ribbed, often rugose; style-branches 3-5; seed 2-3 mm, very flat, commonly obovate, dark reddish-brown. Salt and brackish marshes along the coast; Me. to Fla. (Acnida c.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.