Controlling an Invasive Thug

By Bengt Miller SFEG Stewardship Coordinator

 

If you are familiar with the term Polygonum then you deserve a pat on the back and a gold star. Polygonum is the name for a group of plants known collectively as the buckwheats. Within this family there is also a genus called Polygonum. These plant species are known collectively as knotweeds. The various species: Japanese Knotweed, Giant Knotweed, Bohemian Knotweed (a hybrid between Japanese and Giant) and Himalayan Knotweed are all listed as noxious weeds in Skagit County. All are required for control, except Bohemian Knotweed.

 

The Washington State Department of Agriculture has recognized the negative impacts knotweed has on riparian areas and has set aside funds specifically for the control of this invasive species. Knotweed poses the biggest threat to riparian areas. These stream corridors provide the perfect habitat for knotweed: the constant moisture provides a great growing medium, regardless of the soil type. In addition, knotweed propagates by fragmentation, meaning parts that are broken off the parent plant readily sprout into new plants.  Streams provide an ideal mode of transport, and thus one plant that is fragmented by beaver or weedwhacking can form a whole new colony. Once established knotweed forms a dense monotypic stand, reaching up to 13 feet tall, that outcompetes native understory vegetation. This is exceptionally troublesome because studies have shown that streams with large knotweed infestations have up to 40% fewer macroinvertebrates, due to the lower nutrient levels of knotweed leaves versus native species. These macroinvertebrates are the primary food source for juvenile salmon. Hence there is an inverse relationship between knotweed density and the number of juvenile salmon in a stream.

 

SFEG has been working since 2010 to control knotweed in the Skagit Watershed. The Upper Skagit Knotweed Control Program has the mandate to control knotweed anywhere in the Skagit Watershed upstream of Rockport. We survey for and treat knotweed on public and private land. Because SFEG is not a state agency we do not have any regulatory authority and depend on the willingness of private landowners to participate in the program. We are able to treat knotweed at no cost to the landowner because of grants from the United States Fish and Wildlife Service, National Fish and Wildlife Foundation, Mount-Baker Snoqualmie National Forest and Washington Department of Agriculture. Their support of the program has been vital.

 

Since the project began, the effort has identified over 2,000 patches of knotweed and has achieved 81% control in the floodplain and 70% control of knotweed in the entire Upper Skagit Watershed. In 2017, SFEG and the Washington Conservation Corps (WCC) surveyed within the 100-year floodplain along 34.5 mainstem river miles of the Skagit and Sauk Rivers, plus 5.2 miles of various tributaries, traversing approximately 3,269 acres (5.1 mi²) in which 3,087 individual knotweed stems in 137 patches were treated. The average size of each patch treated was 215 square feet. The average of less than one stem per acres is unheard of in infested watersheds.

 

This program is recognized throughout the state as a success. You can paddle, hike or drive along the banks of the Skagit or Sauk Rivers upstream of Rockport and with the exception of a few spots you will not see knotweed without a map showing where it is located. This is a stark contrast to the lower portions of the Skagit River, where knotweed continues to proliferate, sometimes being the only established vegetation on riverside gravel bars, and is a tribute to the quick and continued action taken by organizations in the Skagit Coordinated Weed Management Area.

 

If you would like to learn more about knotweed or are a Skagit landowner with property upstream of Rockport and would like a free knotweed survey conducted on your property contact Bengt Miller with the Skagit Fisheries Enhancement Group at bmiller@skagitfisheries.org.

 

Foraging tip: young, tender knotweed shoots can be collected and cooked similar to asparagus. Apparently they taste similar to rhubarb.