Plants 10-25 dm. Leaves 5-20 cm. Flowers: calyx distinctly beaked at bud apex, glabrous; petals 6-10 cm. Capsules 3-4 cm. Seeds dark brown, papillose. 2n = 38.
Phenology: Flowering spring–summer (Apr–Aug).
Habitat: Dry washes, canyons
Elevation: less than 1200 m
Romneya coulteri shares with Hibiscus lasiocarpos the distinction of having the largest flowers of any plant native to California.