Why Spray Knotweed?

In response to a query from Area D Director, Andreas Tize, about knotweed spraying, we recently received this very thorough letter from Crystal Chadburn, Senior Invasive Plant Specialist with FLNRORD. 

Knotweed growing on the beach in Elphinstone.

As you likely know, invasive knotweeds are extremely aggressive non-native invasive plants that require herbicide for control due to their extensive root systems. Knotweeds pose a serious environmental risk, and very negatively impact fish and fish habitat, as well as a number of other important values. Mechanical control of knotweeds such as smothering, cutting or mowing stimulate the roots and the plants to spread further and more aggressively.

We have had over a decade of all sorts of trials on control of knotweed, from Haida Gwaii, down to the South Coast, which have shown that herbicides are required for successful control for well established sites.  This is also consistent with the literature on knotweed control across North America and in Europe. Here is a good reference for more information about invasive knotweeds if you are interested:  https://www.bcinvasives.ca/documents/ISCBC-Factsheet-Knotweeds-180216-WEB.pdf

While stem injection is an option for application of glyphosate for the knotweeds due to their hollow stems, our preference and treatment standard is to use selective application with backpack sprayers for the following reasons:

  1. Stem injection uses undiluted product and actually results in a lot more active ingredient within the treatment area as compared to what is used with foliar application of a diluted herbicide solution.
  2. Stem injection requires waiting until later in the year to conduct the treatment as the stems have to be large enough for the stem injection “gun” to be used, whereas foliar application can be conducted as soon as the plants reach 1m in height in the spring/early summer. This delay in treatment can not only require additional herbicide product to be used, it can result in increased risk of spread by waiting longer to treat the plant.
  3. Selectively applied foliar application of herbicide (ie. spraying) using backpack sprayers has been shown to be just as targeted as stem injection, and is recognized by the Province, including the Integrated Pest Management Branch of the Ministry of Environment and Climate Change as an equally selective application method as spraying. We consistently see native and/or desirable vegetation returning shortly after treatment of knotweed using foliar application of glyphosate.
  4. Stem injection takes significantly longer than foliar application, which means if we used it widely, we wouldn’t be able to manage as many sites overall, which of course is contradictory to our goal of landscape level control and containment which requires treating as many sites as possible within an area. In addition, knotweeds are not the only high risk invasive plant we need to manage to protect environmental and economic values in the sunshine coast, and there are always more sites that resources available to treat. Therefore, choosing the most effective and efficient (and in this case the one that uses the least herbicide) option is always our priority.

If you are interested in a bit more information about knotweed control, please have a look at our Knotweed Treatment Prescriptions for Provincial Public (Crown) lands available here:  https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/invasive-species/pest-management/2019_herbicide_summary_for_control_of_knotweeds_on_crown_lands.pdf

Further, all our herbicide applications are conducted under our Invasive Plant Pest Management Plan, in accordance with the BC Integrated Pest Management Act and Regulation, which includes a ton of detail about the set backs from water and other requirements that are followed to protect human and environmental health while using herbicides.

Knotweed smothering Charman Creek in Gibsons. Since this photo was taken in 2016, community volunteers have started cutting the knotweed back by hand. Herbicides cannot be used right on top of creeks. Manual eradication of small knotweed patches is doable, but it requires constant attention for years.

Posted by Donna