Plants to 25 dm. Stems simple or branching distally. Leaves 10-30 cm long and wide; petiole 2-15(-20) cm; blade broadly ovate-cordate, 7-9-lobed; margins irregularly and coarsely toothed; abaxial surface usually whitish and densely short-pubescent, adaxial glabrous. Inflorescences to 30 cm or more. Flowers: pedicels 4-10 mm; sepals white to cream colored, spatulate, 5-10 mm; style to ca. 1 mm. Capsules oblanceoloid, strongly compressed, 15-20 mm, glabrous. Seeds dark brown, reticulate. 2 n = 20.

Flowering summer. Deciduous woods, thickets, old fields, ditches, roadsides, pond margins, and along watercourses; 0-800 m; introduced; Ont., Que.; Ala., Conn., Ill., Ind., Maine, Mass., Mich., N.J., N.Y., N.C., Ohio, Pa., Va., W.Va.; e Asia.

A garden perennial esteemed for its vegetative features and large, plumelike inflorescence, Macleaya cordata occasionally escapes from cultivation, persisting and reproducing fairly successfully. It might be found almost anywhere in temperate North America east of the Mississippi River at elevations below 1000 m.