Background and Aims <p>Growing cover crops can increase soil carbon sequestration through their root and shoot biomass carbon inputs to the soil. However, data on the root biomass and root:shoot ratios of cover crops as affected by species or mixtures, sowing date and environmental conditions is still limited due to the large sampling effort. Our study aims to address these knowledge gaps.</p> Methods <p>We sampled cover crop root and shoot biomass in a field trial in northern Germany, where ten species in monoculture and eight commercially available species mixtures were sown on three dates (early August, mid-August and mid-September) in a year with a wet summer (2021) and a year with a dry summer (2022) compared to long-term average.</p> Results <p>The average dry shoot biomass was 3.27 (± 0.17) Mg ha⁻¹ in 2021 and 1.44 (± 0.15) Mg ha⁻¹ in 2022. The average dry root biomass of all species and mixtures was 0.48 (± 0.04) Mg ha⁻¹ in 2021 and 0.31 (± 0.04) Mg ha⁻¹ in 2022. Year and sowing date strongly affected root and shoot biomass. Species differed strongly in 2022, but not in 2021. Mixtures mostly tended to overyield in root and shoot biomass compared to a weighted average of component monocultures and were more stable between years. Root biomass was more strongly affected by sowing date than shoot biomass.</p> Conclusion <p>Sowing date is the most important management factor to maximise cover crop carbon inputs to the soil. Species mixtures can help to maintain cover crop productivity under drought conditions.</p>