Germination of the invasive water primrose Ludwigia grandiflora in Belgium and potential implications for management

Invasive Alien Aquatic Plants (IAAPs) are recognised as one of the major threats to aquatic biodiversity globally. Because of their high growth rate and ability to form dense mats, these plants strongly impact biodiversity and the conservation status of aquatic ecosystems. In Belgium, the invasive water primroses Ludwigia spp. are listed on the Invasive Alien Species watchlist, and conventional management actions (manual or mechanical removal) have been carried out, but successful control stories are underreported. Although seed production has not been reported for L. grandiflora in Belgium, local managers suspect it has occurred over the last five to ten years. We studied seed germination of 18 populations within Belgium by growing collected seeds in Petri dishes under a 12|12 h light regime and 14|24°C temperature regime in growth chambers. Results showed that 14 L. grandiflora populations produced germinating seeds, with final germination percentages ranging from 4 % to 77 % at the end of the study. Time to reach 50 % of germination ranged from 12 to 27 days. These results stress the need to implement control measures for seed production of L. grandiflora populations in the management schemes, especially considering the fast emergence and the high number of seeds produced for some populations. The potential germination could hinder management actions by increasing follow-up control efforts and its associated costs. It is thus in the best interest to prevent seeds ripening by removing the vegetative biomass early in new infestations, and by managing the potential soil seed bank in already established ones.