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.

Flowering summer-fall. Freshwater and brackish wetland habitats, coastal marshes, swamps, riverbanks, bayous, canals, ditches, estuaries, lakeshores, hammocks; 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.