Japanese knotweed (Fallopia japonica [Houtt.] Ronse Decraene) is considered one of the world’s most invasive species. Riparian systems provide numerous services such as flood regulation, erosion control or temperature regulation (Palmer et al. 2009) and they are particularly affected by knotweed invasion. The invasive ability of F. japonica is still poorly understood but in addition to competitive superiority (Siemens and Blossey 2007) and niche construction (Dassonville et al. 2011), allelopathy is suspected to be partly responsible for its invasion success (Fan et al. 2010; Murrell et al. 2011). We examined the allelopathic effect of F. japonica on growth of tree cuttings that may be used in restoration of river bank ecosystems (Salix viminalis and Populus nigra). We tested the allelopathic potential of F. japonica by an original semi-controlled experiment which allows discriminating allelopathy from resource competition (Fig. 1). Leachate of donor pots containing either F. japonica or bare soil irrigated target pots containing a cutting. Total polyphenol concentration was measured all along the experiment as well as soil nitrogen in target pots at the end of the experiment. Fig. 1. Experimental design Survival of tree cuttings was not affected. S. viminalis growth was reduced under F. japonica which was not due to a difference in soil nitrogen. P. nigra was not significantly affected by allelopathy (Fig. 2). Fig. 2. Mean (±s.e, n=10) of the aboveground, below-ground and total biomass of Salix viminalis and Populus nigra cuttings, placed under bare soil or Fallopia japonica donor pots. Treatments marked with the same letter for each biomass type are not significantly different. Our results show that polyphenols could be partly responsible for the success of F. japonica, but not equally harming all species. These findings alert on the importance to work with adapted and resistant species to restore successfully functional riparian systems.