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To understand the potential for northward expansion of knotweeds into currently uninvaded regions, it is crucial to assess the potential for sexual reproduction alongside the ongoing vegetative spread. For that reason, we tested viability in Reynoutria japonica seeds in south Sweden, a region where viable seeds had not been previously reported. Knotweed in northernmost Europe has no genetic variation, indicating lack of fertile seeds, and northward colonization is considered limited by temperature and a short growing season. Observations of profuse flowering and seed production in recent years, however, prompted us to test seed germination. Seeds from seven localities in Scania and 20 localities in Halland, south Sweden, were collected in October and November 2021 and 2023 and germinated under laboratory conditions. In Scania, seeds from six out of seven stands germinated with an average germination of 57.7 ± 32.1%. A second trial 10 months after seed collection showed 48.0 ± 14.2% germination on seeds from two of the sites. In Halland, seeds collected in November 2023 germinated with an average germination of 78.13 ± 20.71%, and germination in soil was 15% ± 16.1. Field inventories detected seedlings that were determined by metabarcoding to be R. japonica × Fallopia baldschuanica hybrids. Collected Reynoutria × bohemica × F. baldschuanica seeds from one site were fertile and the seedlings survived cool conditions for two winters. We established that R. japonica produces viable seed in south Sweden, which is further north than previously demonstrated in Europe. The observation that F. baldschuanica was a common pollen donor concurs with studies from other parts of Europe. Conclusion: the presence of fertile seeds and seedlings has implications for the northward dispersal of knotweeds in Europe. Russian vine F. baldschuanica was identified as the pollen donor for R. japonica stands and one putative R. × bohemica stand in south Sweden.