Implications of extreme flooding events modifying fluvial geomorphology for dispersal of knotweed (Reynoutria spp.) in the wake of climate change

The knotweed species complex ( Reynoutria spp.) constitutes one of the most invasive plant taxa globally. Knotweed dispersal mechanisms facilitate rapid spread along waterways. The 100-year Pacific Northwest flood of November 2021 impacted British Columbia’s Vedder-Chilliwack River and its biota both through historically high rates of discharge and altered fluvial morphology including formation of new islands in the river and removal of existing vegetation. According to our field surveys, a five-fold increase of knotweed patches was observed in 2022 along the Vedder-Chilliwack River as compared to 2019. Much of the knotweed population increase occurred in newly available disturbed areas created by the flooding. Knotweed patches in the river’s tributaries tended to be well-established whereas the age distribution in the mainstem was skewed towards younger age intervals, indicating tributaries may act as a major source of population growth if not controlled. Knotweed in densely concentrated areas along the river was resurveyed in subsequent summers. Knotweed continued to expand, with 21% more patches in 2024 than in 2022 followed by a 97% increase in 2025 relative to 2024. The near doubling of the knotweed population in 2024 was likely connected to a significant rainfall event in October 2024. Both the 2021 and 2024 flooding events are consistent with the global increase in flooding frequency and intensity due to climate change. Such flooding events can significantly contribute to the expansion of invasive plant populations in waterways, highlighting the crucial need for targeted management in river systems to mitigate further spread across watersheds.