Plants annual, usually monoecious (in some species, staminate flowers rare). Stems usually erect, not fleshy. Inflorescences terminal and axillary, ± long, cylindric spikes or panicles; axillary inflorescences similar to terminal ones, but shorter. Pistillate flowers: tepals usually (3-)5. Utricles transversely dehiscent (except A. bouchonii Thellung and some forms of A. spinosus and A. obcordatus).

Species ca. 20 (14 in the flora): North America, Central America, South America, one species perhaps native to Europe, introduced into other regions.