Plants annual or biennial. Stems 4-8(-12) dm, sparingly prickly. Leaf blades: abaxial surface scattered-prickly on main veins, adaxial surface unarmed; proximal lobed 2/3 distance to midrib; distal clasping. Inflorescences: buds ellipsoid-oblong, body 15-22 × 10-15 mm, sparingly prickly; sepal horns terete, 6-10(-15) mm, usually unarmed. Flowers 7-10 cm broad, usually closely subtended by 1-2 foliaceous bracts; petals white, very rarely lavender; stamens 150 or more; filaments lemon yellow; pistil 3-4-carpellate. Capsules narrowly to broadly ellipsoid, 35-50 × 10-17 mm (including stigma and excluding prickles), prickly, surface clearly visible, prickles widely spaced, longest 4-10(-12) mm, interspersed with a few shorter ones. Seeds ca. 2 mm. 2 n = 28. Flowering spring-summer; fruiting late spring-summer. Prairies, foothills and mesas; 300-2300 m; Colo., Kans., Mont., Nebr., N.Mex., N.Dak., Okla., S.Dak., Tex., Utah, Wyo. Argemone polyanthemos is introduced in Utah.