Most plant species are rare and only a few are common, but whether species success is associated with functional traits is debated. We measured five root traits and seed mass on 242 Central European grassland species, and extracted their specific leaf area, height and bud-bank size from databases. Then we tested if trait values are associated with commonness at seven spatial scales from 16m² grassland plots to worldwide naturalization success. At every scale, success was associated with at least four traits, and they explained the highest proportions of variance for naturalization success (41%) and abundance in grassland plots (37%). Low root tissue density characterized successful species at every scale, whereas other traits showed directional changes depending on the scale. Across scales, belowground traits explained overall more variance in species success (18.1%) than aboveground traits (12.5%). So, belowground traits are at least as important as aboveground traits for species success.