Roadsides in urban areas are often seeded with turfgrass mixtures to provide ground cover and reduce weed abundance. Designing mixtures to withstand exposure to biotic and abiotic stress is challenging. Research from managed and natural ecosystems have shown that increasing plant species richness and diversity can increase groundcover and suppress weed cover, but it is unclear whether such relationships hold in roadside environments. Our objective was to determine the effect of seeded turfgrass species richness on ground cover and weed suppression alongside roadsides in diverse regions in Minnesota. We tested six turfgrass species in monocultures, two-way mixtures, some three-way mixtures, and a single six-way mixture at seven sites seeded in the fall of 2018, and seven sites seeded in the fall of 2019. Seeded turfgrass, weed, and bare soil coverage was measured at each site over two growing seasons. There was a positive relationship between turfgrass species richness and turfgrass cover, and this interaction effect increased over time. We found that increasing turfgrass species richness reduced bare soil coverage. Turfgrass cover was also more consistent across research sites (i.e., greater spatial stability) with increasing species richness. Our results show that positive relationships between plant species richness and groundcover hold in highly disturbed and managed roadside environments. These findings can improve the design of seed mixtures for roadsides and in other ecological contexts where vegetative cover is important.