peer reviewed ; † Background and Aims The knotweed complex, Fallopia spp. (Polygonaceae), belongs to the most troublesome invasive species in Europe and North America. Vegetative regeneration is widely recognized as the main mode of reproduction in the adventive regions. However, the contribution of sexual reproduction to the success of these invasive species has only been detailed for the British Isles. An examination was made as to how hybridization may influence the sexual reproduction of the complex in Belgium and to determine how it may contribute to the dispersal of the species. † Methods Studies were made of floral biology, reproductive success, seed rain, seed bank, germination capacity, seedling survival and dispersal capacity in order to characterize the reproductive biology of the species. Moreover, chromosome counts and flow cytometry were used to assess the hybrid status of seedlings produced by sexual reproduction. † Key Results In the area investigated, extensive sexual reproduction by hybridization within the complex, including one horticultural species, was demonstrated. A small percentage of seeds may be dispersed outside the maternal clone (.16 m) allowing the formation of genetically differentiated individuals. Seed germination was possible even after a winter cold period. †Conclusions The extensive sexual reproduction by hybridization could further contribute to the dramatic invasive success of knotweeds in Belgium and should not be underestimated when considering control and management measures.