Societal Impact StatementThe plants sold in nurseries directly influence urban landscapes by providing habitat that supports humans and other organisms and indirectly influence the natural landscape by acting as a major source of invasive species. We analyzed the ornamental plants sold in US nurseries from 1719 to 1946 and found broad patterns of increasing diversity occurring in step with an increase in species relatedness, a switch from native to alien species, and a decrease in regional distinctness. Patterns like these, which have been observed recently in urban areas, thus have a basis in cultivation/horticultural history.Summary Cultivated plants are foundational species in urban areas, where they simultaneously provide essential ecosystem services while posing a risk of invasion into natural communities. We sought to characterize the spatial and temporal patterns of taxonomic and phylogenetic diversity of cultivated plants using historical data to better understand the origins of the patterns of diversity observed in present‐day human‐dominated landscapes. Using records of 5098 ornamental vascular plant species available in 319 US nursery catalogs published from 1719 to 1946, we quantified the taxonomic and phylogenetic ‐diversity, phylogenetic structure, and ‐diversity of the historical US nursery flora in cultural regions, biomes, latitudinal bands, and longitudinal zones. We showed a peaked increase in taxonomic and phylogenetic diversity of the US nursery flora over time, concurrent with the proliferation of nurseries and the addition of new species to nursery offerings. However, standardized phylogenetic diversity showed a convex pattern, with a trough during a time of peak growth and connectivity in the nursery industry, and diversity actually decreased at the end of the time series. We also showed a pattern of increasing homogenization of regional nursery floras, as well as a shift from offering predominately native to predominately alien species. Echos of the history of the US nursery industry can be found today in patterns with great ecological relevance. Previous reports of high alien plant diversity in urban areas, phylogenetic clustering and overdispersion among naturalized alien plants, and homogenization of urban plant communities are all consistent with the historical availability of these plants in the nursery flora.