Arable field margins are an important semi-natural habitat providing multiple functions in agroecosystems. Despite three decades of research, analyses of species assemblage and functional traits are lacking. Leveraging a national monitoring network in metropolitan France, we aim to provide a comprehensive taxonomic and functional synthesis of field margin flora, outline main field margin types and explore their associations with management practices, climate, and agricultural landscapes. We analysed data from 532 field margins surveyed between 2013 and 2017, using both uni- and multivariate analyses. Field margins exhibited great diversity with 711 distinct taxa (12% of all flora in mainland France) at the national scale and an average of 16 species per 10 m2 locally. While field margins contained few species of conservation value, they offered a refuge for many declining species as well as rare arable weed species. We identified seven main field margin types, each linked to distinct conditions of climate, soil, landscape and agricultural practices. Mediterranean field margins notably differed from all others. In the main cluster, vineyard margins also stood out as distinct from annual crop margins. Additionally, field margins in landscapes with a high proportion of grassland differed from those within intensively cultivated field crop plains in conventional agriculture. Overall, our study highlights the high botanical diversity of field margins and their interest for plant conservation in agricultural landscapes. Promoting the installation and/or maintenance of field margins through agri-environmental schemes should thus favour biodiversity conservation and associated ecosystem provision.