Invasive Species: The Japanese Knotweed
By Maggie Trask, NSEN Journalism Volunteer
About the Japanese Knotweed
The Japanese knotweed (Fallopia japonica) is a perennial species of plant native to Asia. This plant made its way into Canada and America during the 1800s. Due to the knotweed's attractive foliage, this non-native species was introduced around North American homes. Since then, the Japanese knotweed has escaped lawns and began thriving on riversides, roadsides and ditches among other spaces. There are four species of knotweed that have taken root in Canada; aside from the Japanese knotweed, the Giant, Bohemian, and Himalayan knotweeds have all begun to pop up in several provinces (Wilson 2007). All of these variations are invasive to Nova Scotia. However, Japanese knotweed is by far the most common variant in the province.
A large Japanese knotweed plant located within Dartmouth, Nova Scotia (Maggie Trask/Author).
Why is Japanese Knotweed so successful?
Once a Japanese knotweed takes root, its growth is rapid. The success of this plant is largely due to the capabilities of the rhizome. The rhizome is essentially a horizontal continuation of the plants stem underneath the soil. This structure sprouts new growth up through the soil, and increases the above ground growth of a Japanese knotweed. A section of rhizome that weighs only 16 g can give rise to an entirely new Japanese knotweed plant (Francois-Marie Martin et al, 2020).
This makes it extremely difficult to manually remove Japanese knotweed. Mowing a patch of knotweed will only damage the above ground growth of the plant. Leaving the rhizome and root systems intact almost guarantees regrowth. Similarly, it would not be advantageous to simply pull out the Japanese knotweed. The root and rhizome systems are dense and complex. It would be nearly impossible to ensure that 100% of the plant's underground growth has been removed, therefore risking potential regrowth.
Why is Japanese Knotweed harmful?
Japanese knotweed can cause negative effects on local biodiversity and ecosystem services. This includes native plant species, soil biodiversity and some animal species (Lavoie 2017). The impressive above ground growth of the Japanese knotweed produces shade and leaf litter which can have detrimental affects to the existing native ground cover (Gover et al, 2005).
Not only does this species have negative impacts on the natural environment, but it also causes several problems for built infrastructure as well. Japanese knotweeds that exist on the shoulders of roads can cause limited vision for drivers. The shoots are also strong enough to puncture through asphalt causing an expensive issue (Gover et al, 2005).
How to identify Japanese Knotweed
Japanese knotweeds present themselves in clusters most often found in cities along infrastructure and roadsides. Outside of the city, Japanese knotweeds favour riparian areas such as riverbanks. Some defining characteristics are as follows:
Stalks of this plant are woody and look similar to bamboo shoots. They can grow to be about 2.5 m tall by the end of May.
The flowers are small and exist in clusters. They can be either white or green in colour and bloom through late August.
Leaves can be up to 10 cm in length with a flat bottom rounding into a pointy tip.
How you can help
Educate and Observe
Learn how to properly identify a Japanese knotweed
Keep an eye out! Chances are Japanese knotweeds exist in all sorts of familiar places. Once you start to see them, they seem to pop up everywhere.
Point them out to family members and friends. The more people educated about this invasive species, the better.
Report
The City of Halifax is aware Japanese knotweeds are dangerous and spreading quickly. Residents are urged to call 311 if any Japanese knotweeds are spotted on municipal property.
It is also possible to report a sighting on the Nova Scotia Invasive Species Council website (https://nsinvasives.ca/report-an-invasive-species/).
References
Gover, A., Johnson, J., & Kuhns, L. (2005). Managing Japanese knotweed and giant knotweed on roadsides. Roadside Vegetation Management Factsheet, 5.
Lavoie, C. (2017). The impact of invasive knotweed species (Reynoutria spp.) on the environment: review and research perspectives. Biological Invasions, 19(8), 2319-2337.
Martin, F. M., Dommanget, F., & Evette, A. (2020). Improving the management of Japanese knotweed sl: A response to Jones and colleagues. NeoBiota, 63, 147-153.
Wilson, L. M. (2007). Key to identification of invasive knotweeds in British Columbia. Ministry of Forests and Range, Forest Practices Branch, Invasive Alien Plant Program.