Invasive Japanese knotweed (Fallopia japonica) is posing a huge threat in the Squamish area and is renowned as one of the "world's worst species." Japanese knotweed can be identified by its tall (less than 3 metres) bamboo-like stem structure, large leaves and small white flowers that bloom in late summer. It is a perennial plant that dies back every winter so, amazingly, its height is achieved in just one growing season. This plant is already established in over 100 Squamish locations including Cleveland Avenue across from the Brew Pub, on Finch Drive across from the police station, and at the entry to the Squamish Estuary on 3rd Avenue. Yellow signs on the Sea to Sky Highway also indicate infestations of knotweed.
Knotweed was introduced to B.C. as a garden ornamental. In its native habitat of eastern Asia, knotweed lives in harsh volcanic slopes where it plays an important role as a colonizing species. But here in coastal B.C. - where there is an absence of its predators and diseases, combined with its incredible reproductive capabilities - it possesses the ability to thrive.
Invasive knotweed usually spreads when roots and stems are moved by waterways or human activities. These activities include soil movement containing knotweed plant material, the mowing or cutting of knotweed, or discarding knotweed plant materials in receptive habitats. In river corridors, Japanese knotweed can reproduce from fragments that travel downstream during high-water events, affecting the gravel bars and riparian forests of entire river systems. As little as 0.6 of a gram of root material (from roots that have been known to extend up to 20 metres) and stem material can produce a new plant in as few as six days. As a result, one patch can be the source of many downstream infestations. The rate of spread of populations is exponential and the size of infestations will likely double every five years.
Riparian systems (streams and rivers) are most threatened by knotweed. Knotweed's extensive root system lacks the well-developed root hairs necessary to bind and hold in place stream bank soil, especially during peak winter rains. Furthermore, each winter the entire plant collapses on itself (and other surrounding plants), leaving a sparse vegetative and bare soil that is vulnerable to erosion. During peak flow events, banks can erode resulting in sedimentation that can negatively impact human water quality and fish populations. Flood events are catalysts that spread knotweed stem and root material further downstream, where they eventually lodge, establish and perpetuate their aggressive growth cycle.
Biodiversity and wildlife are also affected. Knotweed forms dense monocultures displacing native plant communities, as well as rare and endangered species. Although minor insect grazing has been observed on invasive knotweeds, no wildlife species here are known to feed on it, therefore reducing wildlife habitat.
Finally, knotweed also impacts safety, infrastructure and amenity values. As a safety issue, knotweed reduces sightlines due to rapid growth of roadside populations. Regarding infrastructure, hydrological changes caused by knotweed can also lead to over-widening stream channels, undercutting existing adjacent roads and highways. Knotweed can also grow through small cracks in pavement, concrete or drainage structures, reducing structural integrity of public infrastructure - a huge potential burden to tax payers. Finally, land values will be negatively impacted if a given area becomes knotweed infested since there is significant cost associated with treatment and disposal of knotweed.
Unfortunately, there is no "silver bullet" or single best control strategy for knotweed - each site is different. Using integrated approaches, it will likely take three to five years to be successful. Due to its dispersal method, control projects require co-ordination with multiple land owners and across jurisdictions.
Kristina Swerhun is co-ordinator of the Sea to Sky Invasive Species Council. For more information on getting rid of knotweed, please contact your local municipality or the Sea to Sky Invasive Species Council at [email protected] or (604) 935-7665. For information, visit www.ssisc.info
