Hedgerows are among the most stable refugia for biodiversity in agricultural landscapes, providing food and shelter to many living organisms. However, the destruction and alteration of hedgerow ground vegetation compromise their potential for biodiversity conservation. The purpose of this study was to find local and landscape-scale drivers that promote plant diversity in hedgerows and prevent their colonization by troublesome weeds. Using a functional approach, we assessed the effects of hedgerow features, adjacent farming systems (conventional vs organic) and landscape context (bocage, semi-natural habitat cover, organic farming cover) on the diversity and composition of plant communities in 40 hedgerows, in Brittany (France). Hedgerow features had no effect on species diversity, but influenced functional diversity measured as a standardized effect size (SES), i.e. independent of species diversity. Organic farming at local scale was the main driver increasing both species and functional diversity (SES), doubling the cover of insect-pollinated forbs. High organic farming cover in the landscape increased species diversity, but not functional diversity (SES), of hedgerows adjoining conventional farming systems. Besides, high cover of semi-natural habitats and organic farming in the landscape prevented colonization of hedgerows by troublesome weeds. Promoting extensive management at both local and landscape scale is thus necessary for successful restoration of hedgerow ground vegetation, which should favour biodiversity conservation and ecosystem service provision.