Combining species with contrasting traits has been shown to increase resource use efficiency by plant communities. However, knowledge about mixture effects on root growth is still very limited. We assessed the effects of mixing two grasses (perennial ryegrass, timothy), two legumes (white and red clover), and two herbs (common chicory, ribwort plantain) on aboveground biomass till three harvests and belowground (root) biomass at the third harvest (0-30 cm depth). Red clover achieved the highest aboveground biomass among the monocultures. The six-species equi-proportional mixture revealed significant positive mixing effects resulting in an aboveground biomass similar to that of red clover. Perennial ryegrass had the largest root biomass among the monocultures, and there were no beneficial mixing effects on root biomass of the six-species equi-proportional mixture. Correspondingly, the root biomass of the six-species mixture was significantly lower than that of perennial ryegrass. As a consequence, community root biomass did not correlate with aboveground biomass. We conclude that in mixtures combining grasses, legumes, and herbs in temporary grasslands (≤ 1 year after sowing), species interactions resulting in positive mixture effects on aboveground biomass do not relay on mixture effects on root biomass.