Requirements for dormancy break and embryo growth were determined for seeds of the western North American species, Osmorhiza depauperata. Seeds were collected in August 2001 from Sandia Crest (3200 m elevation) and Las Huertas (2300 m), New Mexico (USA). Embryos in fresh seeds were c. 0.6 mm long, and they had to grow to c. 9–10 mm before the radicle emerged from the mericarp. Embryo growth occurred at low temperatures (1 and 5°C), and seeds germinated to high percentages at 1°C during 32 weeks of incubation in the light. No seeds germinated at 5, 15/6, 20/10, 25/15 or 30/15°C during 32 weeks of incubation. Although a 4–18 week warm-temperature (25/15°C) pretreatment increased germination rates at 1°C, it was unnecessary for a high percentage of seeds to germinate. Gibberellic acid (GA3, 10–1000 mg l–1) did not substitute for cold stratification. Seeds from the low-elevation population contained larger embryos and required less time to germinate than those from the high-elevation population. O. depauperata seeds have deep complex morphophysiological dormancy (MPD), which is similar to two other western North American congeners and an Asian congener, but different from two eastern North American congeners. Results from this study suggest that: (1) phylogenetic niche conservatism has played a role in the persistence of deep complex MPD in the three western North American species of Osmorhiza; and (2) the stimulatory effect from a warm pretreatment in species needing only cold stratification for dormancy break is a preadaptation that initiated the development of an absolute warm requirement in species needing both warm and cold stratification.