In order to clarify the ambivalence, with regard to ecosystem’ services, of urban wastelands, reservoirs of both native biodiversity and alien and invasive species, we studied plant diversity of urban wastelands in two medium-sized cities in France. Our results show that urban wastelands host a great species’ diversity, both common and rare or patrimonial. The overall rate of invasive species is similar to the one observed in large cities’ wastelands, while their local rate, within wastelands, is much lower. We discuss of these results with regard to differences in cities’ size and wastelands’ starting point.