Oak regeneration in temperate forests often fails in the presence of understorey grass. Competition by resource exploitation between plants has been extensively studied. By contrast, competition by interference, especially chemical interference (allelopathy), has been much less thoroughly examined and its relative importance remains unclear. We investigated the influence of allelopathic interaction on plant performance (biomass production) in a pot experiment with sessile oak (Quercus petraea) and purple moor grass (Molinia caerulea), either sole- or mixed-grown. Plants were watered with either Quercus root exudates or Molinia root exudates. After 6 months of growth, oak biomass increment was significantly lowered by Molinia root exudates. The oak’s root system was more strongly affected than its aerial part. Quercus root exudates favoured oak growth but did not affect moor grass. Conversely, Molinia root exudates had a small depressive effect on its own growth, but its biomass was favoured by the presence of oak grown in the same pot. Resource exploitation had a more detrimental effect than allelopathy and both processes together decreasing oak biomass by 50%. Although untargeted metabolomic analysis by UHPLC failed to identify any potentially allelopathic substances involved, our study demonstrates a lower but critical contribution of chemical interference on oak seedling-moor grass competition compared to exploitation processes. To ensure oak regeneration, management of forest ecosystems should thus first focus on reducing moor grass close to oak seedlings to help decrease its allelopathic effect and ease resource competition.