Puget SoundCorps Helps Stomp out Knotweed

Puget SoundCorps members regroup after a morning filled with knotweed.

Each year, the King County Noxious Weed Control Program gets a great boost of support from the Puget SoundCorps, a branch of the Washington Conservation Corps’ AmeriCorps Program. Just when we need it most, the Washington State Department of Natural Resources provides our program with the time of six hardworking young adults who help us tackle some of the county’s toughest noxious weeds.

Knotweed grows on the banks of…
and even in the Snoqualmie River.

This summer, the crew is spending fourteen days battling knotweed on three sites along the Snoqualmie River* under the guidance of Justin Brooks, riparian team lead for the Snoqualmie River. I met up with the corps members at the last of the three sites, on the South Fork Snoqualmie River just north of downtown North Bend. (Our crew calls this site “the Lost Forest” because it’s easy to lose your way out there.) This year’s team came to us from EarthCorps, a Seattle-based organization that joins AmeriCorps members and international young adults to work on conservation projects. By the time I arrived, most of the crew was already out controlling knotweed. Crew Leader Hannah Supplee was waiting for me at her truck. Fording the river, we hiked downstream to join the rest of the team.

Crew member Nora Landri works on one of the area’s larger knotweed patches.

The crew had split into two groups, each of which was methodically controlling knotweed on the South Fork Snoqualmie’s riverbanks. The area had been partially treated for the last three years, and the infestation was spotty. Corps members told me the earlier control had made their job a lot easier. Still, it was hard work.

Crew member Adnane Sounni, of Morocco, takes notes while controlling knotweed.

Hannah’s team had come from all over the country—and world—including Illinois, Georgia, Virginia, India, and Morocco. They’ve swiftly become a tight-knit group. When I ask them the best part of their job, they’re quick to answer: “Getting to hang out with the crew.”

The hardest part? “Having to make trails. Once make it, it gets a lot easier.”

Many thanks to the Puget SoundCorps crew for all its help!

Crew member Ellen Hughes controls young knotweed plants.
Crew members head back to the truck midday.
Hannah and her crew take a well-earned break.

Have a question about this article, battling your own knotweed, or anything else noxious-weed-related? Feel free to call us at 206-477-WEED (206-477-9333) or email noxious.weeds@kingcounty.gov.

*Specifically, the three sites are:

  1. River bank right of the main stem Snoqualmie above Snoqualmie Falls, between the Hwy 202 bridge and the Meadowbrook bridge. This is river miles 38.7 through 40.3
  2. River bank right of the South Fork Snoqualmie between river miles 0.9 and 1.3.
  3. River bank left of the South Fork Snoqualmie between river miles 0.2 and 1.5.


    • Hi Diane,

      Thanks for your question. Below is a response from Sayward Glise with our Riparian Projects Team, which oversaw the Puget SoundCorps project:

      The crew is doing a foliar treatment (spray) on the knotweed in the Upper Snoqualmie River project. We have two chemicals that are effective to use in aquatic habitats to treat knotweed. One is an aquatic form of glyphosate (brand name Aqua Neat) at a 4% solution, the other is an aquatic formulation of imazapyr (brand name Polaris) at 1%. We primarily use imazapyr on our projects, because it has such a low effective treatment rate, but we will use glyphosate at times to switch up the mode of action or if native plant installation will be happening the following season (glyphosate has a short half life, imazapyr has a residual effect to help sites not get colonized by other weeds).

      The herbicide Puget Sound Corps crew was using is aquatic imazapyr (Polaris) at a 1% solution in water. An aquatic rated surfactant is also added to the solution to help the product stick to the leaves (Agri-Dex at 1%) and a blue vegetable dye indicator so the crew and others can see where they have treated.

      For the next few weeks KCNWCP will be taking a break from treating the Snoqualmie River Confluence to avoid flowering plants and pollinators. We will resume treatment in that area after flowering.

      We do occasionally use injection as a tool for knotweed treatment on rainy days or if a stand of knotweed is in flower and we can’t reschedule treatment for another date. Knotweed injection provides a very targeted herbicide application, but it uses full strength aquatic glyphosate at 3mL injected into each cane. That amount of herbicide adds up very quickly on a landscape scale, so we try to use injection as little as possible on our projects.

      Thanks again!

  1. Thank you so much for your thorough reply. I really appreciate it. I didn’t know that injection was considered more environmentally harmful than a foliar spray. And yes, 3 milliliters in each cane could indeed add up quickly in a landscape.

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