Monogr. Rumex, 56, plate 1, fig. 1. 1819.
Herbs, annual; taprooted. Stems prostrate to erect, glabrous. Leaves deciduous, cauline, alternate, petiolate; ocrea often deciduous, chartaceous; blade subhastate, triangular, or ovate to ovate-oblong, margins entire to obscurely crenulate or dentate, sometimes undulate. Inflorescences terminal and axillary, racemelike, pedunculate. Pedicels present on staminate flowers, absent or nearly so on pistillate flowers. Flowers unisexual, all plants having both staminate and pistillate flowers, the two types segregated in different inflorescences, base not stipelike; perianth accrescent (pistillate) or nonaccrescent (staminate), greenish, campanulate, glabrous; tepals (5–)6, sepaloid, dimorphic, especially in pistillate flowers. Staminate flowers 1–8 per ocreate fascicle, terminal and axillary; tepals distinct, subequal; stamens 4–6; filaments distinct, free, glabrous; anthers yellowish to reddish, elliptic to ovate. Pistillate flowers 1–7 per ocreate fascicle, axillary; tepals connate basally, outer 3 each with apex ending in a stout spine in fruit, inner 3 erect, tuberculate; styles 3, erect, distinct; stigmas penicillate. Achenes included in indurate perianth, brown, not winged, 3-gonous, glabrous. Seeds: embryo curved. x = 10.
Introduced; tropical to subtropical or temperate regions, South America, Eurasia, Africa, Atlantic Islands, Pacific Islands.
Species 2 (2 in the flora).
Both species of Emex are on the United States federal noxious weed list. Emex australis appears to be a more serious threat to agriculture and livestock production than does E. spinosa, based on experience in other parts of the world. The former is a serious agricultural pest in southern Australia, where it was introduced in the 1830s. Emex spinosa also was introduced there, apparently in the mid-1900s. The two species were formerly geographically isolated, but hybrids between them have recently been reported in Australia (E. Putievsky et al. 1980). Hybrids exhibit irregular meiosis and high sterility when self-pollinated; backcrosses with either parent often yield viable seeds.