Argemone mexicana is an erect spiny annual or biennial herb, up to 1 m tall, with a slightly branched taproot. Its stem is branched and very prickly; it exudes a yellow juice when cut. The leaves are thistle-like and alternate, without leaf stalks, toothed and with spiny margins. The grey-white veins stand out against the bluish-green upper leaf surface. Showy solitary flowers of 2.5-5 cm diameter appear at the tips of the branches ; their six rounded petals are bright yellow. The fruit is a prickly oblong or egg-shaped capsule that releases numerous small black seeds.