Madroño 13: 15. 1955.
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.