• Background and Aims: Root system plasticity contributes to plant performance by optimizing belowground resource foraging. As resource availability, soil compaction and mechanical obstacles significantly affect root system architecture (RSA) and plant productivity. In poplar plantations, as in crops, poor rooting reduces aboveground growth and stress resistance, leading to economic losses. Here we investigated the impact of mechanical obstacles on the early development of the poplar adventitious root system. • Methods: Poplar cuttings (P. deltoides x P. nigra cv. Flevo) were planted in rhizoboxes filled with an artificial substrate supplemented with either graded fine gravel or medium gravel. RSA response to treatments was characterised by aggregating the traits measured on the individual roots. Split-root design was used to test for any systemic control of RSA. • Results: A trade-off in biomass allocation appeared between the flank and the basal root compartments. Fine gravel significantly reduced the growth of adventitious and lateral roots but medium gravel did not. When fine gravel was confined to one half of the root system, compensatory growth did not occur. However, specific root length was higher in the gravel-free side and lateral root density was decreased in the fine gravel side compared to uniform treatment. In the medium gravel split-root treatment, lateral root density was decreased in the gravel-free side compared to uniform treatment. • Conclusion: The fine gravel substrate reduced root growth but not the medium gravel one, probably due to differences in root path availability. The split-root design revealed that lateral root density was systemically regulated.