Japanese knotweed (Raynoutaria japonica)
Native to Japan, China, Korea, and Taiwan, Japanese Knotweed was introduced to North America in the late 19th century as an ornamental plant, for livestock forage, and for erosion control1, 2. Japanese Knotweed is an invasive herbaceous perennial that grows in a variety of soil types and is highly adaptable to extreme temperatures, salinity, droughts, and floods3. It grows very quickly and forms dense thickets, crowding and shading out native plants and grasses. Japanese knotweed is found in isolated patches throughout the Credit River Watershed. It is listed as a restricted species under the Invasive Species Act.
Japanese Knotweed has hollow bamboo-like stems which grow up to four meters tall and are ridged, jointed and light green to tan or reddish in colour4. It has tear-drop shaped leaves that are sharp-pointed and flattened at the base. They alternate up the stem and are dark green with reddish veins. Japanese Knotweed has small light green to white flowers that bloom from August to September1.
Japanese Knotweed stem showing reddish joints
Japanese Knotweed is an aggressive, semi-shade tolerant plant that prefers moist to wet soils and grows well in or near wetlands, watercourses, and roadside ditches3. Its root system or rhizomes, can spread 15-18 feet and are so strong that they can even grow though pavement4.
To remove Japanese Knotweed, mowing or cutting the large stalks at least 3 times per season (every two weeks is recommended) can help drain the energy stores in the rhizomes. For younger plants hand pulling may work, but if all the roots are not completely removed, new shoots will re-sprout from the broken rhizomes. A foliar herbicide application is the most effective method for controlling Japanese Knotweed, however, to apply the herbicide, a licensed pesticide applicator is required4.
*CVC is interested in any locations of Japanese Knotweed and is tracking the population within our watershed. If you think you have located Japanese Knotweed, please contact CVC via email or to report sightings call the Ontario Federation for Anglers and Hunters Invasive Species Hotline at 1-800-563-7711 and add the sighting to the EDDMaps Ontario website.
Links for further Information:
Ontario Invasive Plant Council’s “Japanese Knotweed (Falllopia japonica): Best Management Practices in Ontario”: https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/06/OIPC_BMP_JapaneseKnotweed.pdf
Ontario Invasive Plant Council’s “Japanese Knotweed (Reynoutria japonica): Best Management Practice Technical Document for Land Managers”: https://www.ontarioinvasiveplants.ca/wp-content/uploads/2017/05/OIPC_TechnicalBMP_JapaneseKnotweed_Apr282017_D6_WEB.pdf
- Remaley, T. 2005. “Fact Sheet: Japanese Knotweed.” Plant Conservation Alliance’s Alien Plant Working Group. Web. 16 August 2018. https://www.invasive.org/alien/fact/pdf/faja1.pdf
- Anderson, H. 2012. Invasive Japanese Knotweed (Fallopia japonica (Houtt.)) Best Management Practices in Ontario. Ontario Invasive Plant Council, Peterborough, ON. Web. 16 August 2018. https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/06/OIPC_BMP_JapaneseKnotweed.pdf
- Boiche, A., Chauvet, E., Dobson, M., Lecerf, A., Patfield, D., and Riipinen, M.P. 2007. “Stream ecosystems respond to riparian invasion by Japanese Knotweed (Fallopia japonica).” Canadian Journal of Fisheries and Aquatic Sciences 64: 1273-1283. Web. 16 August 2018. http://www.nrcresearchpress.com/doi/pdf/10.1139/f07-092
- Pridham, D. 2009. “The Landowner’s guide to Controlling Invasive Woodland Plants.” Ontario Invasive Plant Council. Web. 16 August 2018. https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/GuideControlInvasiveWoodPlantsWEB.pdf