Knotweed

Knotweeds are native to Asia and were introduced to British Columbia as an ornamental plant. In British Columbia, they are found in the following regions: Vancouver Island, Central Coast, Sunshine Coast, North Coast (Haida Gwaii), Lower Mainland, Nechako, Cariboo, Thompson-Okanagan and the Kootenays. They thrive on freshly disturbed soil in moist locations, such as roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. They are also found in riparian areas, along stream banks, and in other areas with high soil moisture. Additional plants may exist in many gardens in communities across B.C. 

• Knotweeds threaten biodiversity and disrupt the food chain by reducing available habitat and increasing soil erosion potential. Stream banks are at particular risk as exposed knotweed roots break off and float downstream to form new infestations. 
• Knotweeds can reduce or eliminate access to water bodies for recreation activities including fishing, swimming, boating, canoeing, and kayaking.  
• Knotweeds can also grow through concrete and asphalt, damaging infrastructure. This results in significant control, management, and repair costs, and even a reduction in property value.  

The most effective way to ensure that your lands do not become infested with knotweeds is by prevention. Here are some recommendations to prevent invasion on your property: 

• Report new infestations of knotweed to your regional invasive species committee, or report online here. 
• Do not purchase, trade, or grow knotweed. Instead, grow regional native plants as they are naturally adapted to the local environment and are non-invasive.  Alternatives include Redosier Dogwood, Peegee Hydrangea, False Soloman’s Seal, and Goat’s Beard.  
• Regularly patrol your property for knotweed plants and immediately treat new patches before seed set. 
• Maintain your crops and natural lands in a healthy, vigorous condition to ensure a competitive plant community. 
• Remove plants, plant parts, and seeds from personal gear, clothing, pets, vehicles, and equipment before leaving the infested area.
• Cooperate with adjacent landowners and encourage them to prevent knotweed spread. 
• Ensure soil, gravel, and other fill material are not contaminated with knotweed. Do not move contaminated soils to a new area. 
• Take special care when controlling knotweed near streams, or ditch lines, to prevent the movement of plant parts downstream.  
• Immediately re-vegetate disturbed, bare soils with a suitable seed mixture that provides dense, early colonization to prevent weed invasion. 

Treatment  

• Chemical control with a systemic herbicide is the recommended treatment strategy for knotweeds due to their extensive root structure and aggressive growth and reproduction. This treatment method is the easiest, most cost effective, and successful treatment method.  
• Knotweeds typically require treatment with herbicide for 3-5 years. Before applying herbicides, read the label for full use and precautionary instructions.  
• For further information on the selection and application of chemicals to protect your crop, contact AgriService BC at 1-888-221-7141 or email AgriServiceBC@gov.bc.ca. 
• Mechanical control is only recommended under specific circumstances and should be carried out with extreme caution due to the likelihood of spread through root and stem fragments. 
• Mowing can deplete root reserves over time.  
• Digging can be effective on very small and recently establishing populations is all root and shoot tissue are successfully removed.  
• Grazing may result in short term reduction.  
• Cutting may be effective for small populations if repeated several times a year with constant monitoring. Cutting should be repeated until root reserves are depleted (usually several years). Cutting is most effective when followed up with herbicide application.  
• Burning is not recommended as the plants contain high water content and all plant tissue may not burn.  
• A sap sucker psyllid, Aphalara itadori, has been studied as a potential biological control in the Pacific north-west. The agent has been permitted for release and Agriculture and Agri-Food Canada is at a very early research stage of trying to identify the conditions required for establishment. 

Disposal

• Chemically treated knotweed canes can be left on site to compost. If not chemically treated, do not compost as it is likely to increase the spread of this species. 
• Manually removed knotweed plants, plant parts and seeds must be bagged or tarped before transporting to a designated disposal site (e.g. landfill or transfer station). 
• Soil contaminated with knotweed plant material or seed should be handled carefully and either under go deep burial or disposed of at a suitable disposal site.  
• Disposal sites should be far enough away from water and drinking wells to enable herbicide treatment. Disposal sites should be monitored and treated as needed.  

Primary source for information: Knotweeds- Invasive Species Council of BC. See here.