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Premise of the Study: In response to climate warming, plant species may shift their distribution northward, but such a process is slow and hard to detect. Alternatively, phenological changes (earlier flowering) are expected as first adaptations for populations located near their distribution limit. That could be the case for the invasive Japanese knotweed (Fallopia japonica s.l., including the hybrid Bohemian knotweed F. ×bohemica). We hypothesized that climate warming now allows the species to produce viable seeds in the northernmost populations., Methods: Seeds were collected along a 550 km long transect in Quebec, Canada, and tested for germination. The genetic diversity of a population was determined using polymorphic microsatellite markers to verify whether the species is actually producing new individuals through sexual reproduction., Key Results: Japanese knotweed produces, in Quebec, a large number of seeds with a high germination rate (up to 93%). The geographical limit for viable seed production in North America has been extended to Quebec City, about 500 km north of the formerly reported limit. Bohemian knotweeds are genetically diverse, while true Japanese knotweeds all share a common multilocus genotype. This suggests that Bohemian knotweed stands mostly arose from seed, while true Japanese knotweeds result only from the propagation of rhizome or stem fragments., Conclusions: The effect of climate change is already palpable on the phenology of invasive plant species at their northern distribution limit. Bohemian knotweed, which until recently was rare in Quebec, could rapidly spread in the near future with the help of an additional diaspore type (seeds).