The legumes white melilot (Melilotus albus ʻMebaʼ) and white clover (Trifolium repens ʻJuraʼ) were tested for their suitability as intercrops for fast growing poplar hybrids grown as short rotation coppice (Populus nigra × P. maximowiczii ʻMax-4ʼ, P. maximowiczii × P. trichocarpa ʻNE-42ʼ and P. trichocarpa × P. koreanaʻP-468ʼ). Legumes were sown in the established poplar plantation after its first harvest (5 years after the establishment of the plantation) and their effect on soil nitrogen, vegetation undergrowth and poplar growth was monitored for two years. All three parameters were influenced by the legume intercrops but the effect depended on the poplar clone, legume species and their interaction. In P-468 white clover lowered inter-year decline of soil nitrogen content, increased the Ellenberg indicator value for nutrients and promoted height increment. Such an effect was not detected in the other two poplar hybrids probably due to insufficient establishment of clover. Plots sown with legumes and subjected to weeding exhibited profound differences of the understorey vegetation compared to controls. The difference persisted even during the second year after legume growth had largely receded. Our results indicate a short-term potential of white clover to contribute to poplar growth, and N supply in poplar plantations. Further studies should explore the benefits of clover intercropping from a long-term perspective, with additional poplar clones and under different climate-soil conditions. •Co-cultivation of white clover (Trifolium repens ʻJura’) with hybrid poplar P. trichocarpa × P. koreana ʻP-468′ improved soil nitrogen and poplar growth.•Though transient the presence of white clover had high impact on poplar plantation understorey composition.•N-fixing white clover significantly promoted nitrophilous species as indicated by the Ellenberg indicator value for nutrients.