Socio-economic factors often determine the extent to which different global regions have been invaded by non-native plant species, yet few studies examine whether such variables are similarly important for native species richness. In contrast to previous studies that have assembled regional floras for comparison, we examine global patterns of non-native and native plant species richness within a single, globally distributed ecosystem: cultivated wheat fields. Native species richness increased with the historic length of wheat cultivation, total wheat harvested area and arable land area in a country. In contrast, non-native weed species richness declined with increasing length of wheat cultivation, higher fertilizer inputs, and lower absolute latitude. The low percentage of native species in countries with a short history of wheat cultivation suggests that the invasion of crop fields by native plants takes longer than the migration of non-native weed species pre-adapted to crop fields in other parts of the world. Seed contaminants in grain imports are probably one of the main sources of non-native weeds, especially for regions with more recent wheat cultivation histories. To date there has been a tendency for research on agricultural weeds to be dissociated from that of plant invasions, yet we show how cultivation history has shaped the weed flora and why non-native weeds are perceived as such a problem in New World agriculture, as well as confirming at a global scale the role of crop husbandry and agricultural extent on weed floras.