Seed germination trials of the invasive plant Crassula helmsii from nature restoration sites in Belgium

Background and aims – Crassula helmsii is a highly invasive aquatic plant in Europe, subject to international policy measures (EU Regulation 1143/2014). While its vegetative reproduction is well-documented, the role of sexual reproduction and seed ecology remains poorly understood. This study aimed to determine the germination capacity of C. helmsii seeds and revisit its potential for dormancy and seed bank formation. Material and methods – We conducted in vitro germination trials using plant material collected from ten populations in Belgium. Following cold storage (winter conditions), seeds were sown on agar, and germination was tracked. In a comparative trial, flowers from the same populations were sown on a soil substrate, with germination estimated based on the number of seeds per flower—the only method previously used to study the species. Key results – Crassula helmsii populations exhibited high germination rates, with an estimated overall mean of 68.5%. When excluding seeds that proved non-viable, germination was nearly complete (95.7%). The flower-based trial yielded lower germination (13.8%), consistent with earlier results. This lower emergence is likely real, not an artifact, and suggests environmental constraints that are relevant under more realistic germination conditions. Conclusion – Our study demonstrates that C. helmsii populations in Belgium produce numerous viable seeds that germinate readily. Considering additional evidence from the Crassulaceae family, we hypothesize that C. helmsii is self-compatible, positively photoblastic, exhibits physiological dormancy broken by chilling, and is capable of forming persistent seed banks. The species can thus combine asexual (clonal growth and dispersal) and sexual strategies (seed dispersal, dormancy, and seed bank formation). These insights have important implications for the management of this invasive species.