More than weeds: Spontaneous vegetation in streets as a neglected element of urban biodiversity

•The ecological value of built elements has largely been ignored in cities.•We assessed the ability of pavements to promote urban spontaneous flora.•Pavement flora was highly diverse at city scale (>300 species).•Plant assemblages were much more diversified on sandy than asphalt pavements.•The effects of weeding frequency and landscape context on pavement flora were low.