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Apiaceae subfamily Apioideae is characterised by intra-and interfloral dichogamy. There are two distinct patterns: multicycle protandry and multicycle protogyny. Protogyny is almost completely restricted to New World genera. We determined the form of floral dichogamy for 198 species representing 36 of the 49 genera of Apioideae native to North America north of Mexico. We also examined variation in sex expression (proportions of perfect and staminate flowers) within and among plants in populations of 16 species. The form of dichogamy was constant among congeners, with the exception of a few adichogamous selfers. The 17 protandrous genera included 11 native to both the Old and New Worlds and six endemic to the Americas. The protandrous genera were characterised by white flowers, the presence of stylopodia, decreasing proportions of staminate flowers in sequentially blooming umbels, wetter habitats, later flowering times, and eastern or transcontinental distributions. The 19 protogynous genera were all New World endemics characterised by mostly yellow or purple flowers, the absence of stylopodia, increasing proportions of staminate flowers in sequentially blooming umbels, drier habitats, earlier flowering times, and western distributions. The transition from protandry to protogyny probably occurred in the late Pliocene, coincident with adaptation to newly forming dry grassland and cold desert habitats. The evolutionary forces involved may have included selection for reproductive assurance, pollinator specialisation, or both. The distinctive patterns of within plant variation in sex expression associated with protandrous and protogynous apioids probably evolved in response to floral dichogamy.