Stems erect, branched, stout to robust, usually 1.5–3 m (occasionally to 9 m!) × 30 cm. Leaves: petiole 1/3–2/3 length of blade; blade narrowly lanceolate to narrowly ovate, 10–20 × 1–4 cm, base cuneate, margins entire, plane, apex acute or long-attenuate to acuminate. Inflorescences mostly terminal, linear spikes to panicles, usually interrupted. Bracts: of pistillate flowers 1.5–2 mm; of staminate flowers with moderately heavy midribs, 1.5–2 mm. Pistillate flowers: tepals absent; style branches spreading; stigmas 3–5. Staminate flowers: tepals 5, inner tepals with moderately prominent, excurrent midribs, equal, 2–2.5 mm, apex subacute to mucronulate; stamens 5. Utricles stramineous to brown, with 3(–5) longitudinal ridges corresponding to 3–5 style branches, elliptic or obovoid, 1.5–2.5 mm, slightly fleshy, smooth (slightly rugose in herbarium specimens). Seeds reddish brown to dark brown, 1–1.2 mm diam., shiny.
Phenology: Flowering summer–fall.
Habitat: Freshwater and brackish wetland habitats, coastal marshes, swamps, riverbanks, bayous, canals, ditches, estuaries, lakeshores, hammocks
Elevation: 0-100 m
Ala., Ark., Fla., Ga., La., Miss., N.C., Tenn., Tex., Va., e Mexico, West Indies, n South America.
Plants of Amaranthus australis, a herbaceous annual, can be amazingly tall, with a single hollow main stem, up to 9 m, and the stem base can reach 30 cm in diameter. Large plants may somewhat resemble young trees of Taxodium distichum var. imbricarium, pondcypress.