Road verges can harbour a great diversity of species belonging to different organism groups and are thus increasingly considered as habitats with high potential for conservation value. However, species richness differs between verges, and to improve management and construction of roadsides that contribute to biodiversity conservation, we require a quantitative understanding of the different factors that influence species in this distinct type of habitat. We analysed a data set on vascular plants in roadsides of four locations in Sweden to provide insights on how different environmental factors affected plant species richness in roadside locations. Specifically, we tested for effects of vegetation and litter height, solar radiation, proportion of bare soil, and relevant interactions between these variables. Effects differed across locations, in both significance and magnitude. Vegetation height had negative effects on species richness in two of the four locations. Litter height had no significant effects on species richness. Solar radiation significantly affected species richness in two of the locations, through interactions with proportion of bare soil and vegetation height, respectively. Proportion of bare soil had positive effects on species richness, but only in one location. Our results highlight the complexity of the question of “what makes a road verge species rich”. Indeed, species composition results from a combination of factors, including the species pool in the surrounding landscape and the age of the road. In addition, what factors are important may change at different stages of vegetation succession. We conclude that to date, roadside habitats have probably been treated rather poorly in frameworks for habitat classification, and that it is essential to understand the ecology behind different roadside habitats.