Summary

Japanese knotweed, giant knotweed, and their hybrids (Reynoutria spp.) are recognized as some of the most invasive and damaging plants across North America. Their ability to spread easily and widely along waterways, displacing native species, degrading riparian habitat, and accelerating streambank erosion, make them a significant challenge for land managers and watershed communities. 

Knotweed spreads primarily through the dispersal of rhizomes and broken plant fragments, a process facilitated by both human activity and extreme weather events. Under climate change, extreme rainfall and flooding events become more frequent and severe. Understanding how these disturbances influence knotweed dispersal is critical for prevention, early detection, and effective management. 

This study examines the impact of two major flooding events along the Vedder-Chilliwack River in British Columbia on knotweed dispersal and population growth between 2022 and 2025. 

Key Focus Areas: 

  • Impacts of extreme flooding events on the distribution of knotweed along the Vedder-Chilliwack River in British Columbia. 
  • Knotweed establishment and recovery in disturbed, post-flood environments. 
  • The role of flood-driven changes in river dynamics and channel structure in creating new invasion pathways. 
  • Differences in knotweed population age and structure between the main river and its tributaries. 

Take-home points:

  • Knotweed patches increased substantially following both the 2021 and 2024 flooding events, with a five-fold increase after 2021 and a near doubling of patches following the 2024 flood. 

  • The age structure and spatial distribution of new patches indicate that floodwaters spread knotweed primarily through rhizome fragmentation and downstream transport.

  • Flood-driven erosion and sediment redistribution created newly disturbed, vegetation-free sites, allowing knotweed to establish ahead of native riparian plants.  

  • Knotweed populations in tributaries were typically larger, older, and more established than those in the main river, indicating that tributaries act as important source populations for downstream spread. 

  • The study highlights how climate change and invasive species interact synergistically: flooding accelerates knotweed spread, while knotweed further destabilizes riverbanks, increasing erosion during future flood events. 

Management Implications:

  • Post-flood monitoring along major waterways is critical for early detection of newly established knotweed patches. 

  • Targeting juvenile and newly established plants is the most effective and cost-efficient management approach, making rapid response after flooding events imperative. 

  • Control of knotweed along tributaries is particularly important for reducing reinfestation and downstream spread. 

  • Climate change and flood-driven dispersal should be considered in invasive species management planning. Knotweed control and riverbank restoration can help create riparian habitats that are more resilient to future extreme flooding events.