Eradicating Knotweed in the Upper Skagit Watershed | Project 14 of 30

By Bengt Miller, Stewardship Coordinator

If you have ever seen knotweed in the Upper Skagit Watershed (upstream from Rockport) you are one of a small, select group. This invasive species has been wreaking havoc on watersheds throughout the state. Knotweed has made the list of ‘100 worst invasive species’ list compiled by the International Union for the Conservation of Nature. Skagit Fisheries has been working to eradicate knotweed from the Upper Skagit Watershed since 2010. While progress has been made the fight continues.  Without the efforts of Skagit Fisheries the Upper Skagit would look like adjoining watersheds, with knotweed infestations lining the banks.

While the upper Skagit Watershed is fairly clean the lower Skagit Watershed has detrimental levels of knotweed. This is due to the physiology of the plant. Knotweed spreads most often by plant fragmentation.  This describes the process by which part of a plant becomes disconnected from the parent plant and is then capable of forming an entirely separate plant.  Knotweed fragments as small as ½ inch are capable of growing into new plants. This process happens most often along stream and river banks where floods transport knotweed pieces downstream. If these pieces come to rest someplace on the ground, they begin to grow roots and form a new colony.  This is why Skagit Fisheries has taken the approach of treating knotweed in the upper watershed, since treating downstream without getting the source is like continually chasing your tail.

 

Knotweed looks like a broad leaved bamboo, with alternate leaves and a segmented stem. For the uninitiated it looks like a slightly taller than average understory shrub. For the initiated it is a harbinger of doom. One study found that streams with large knotweed infestations have 40% fewer macroinvertebrates compared to streams banks without knotweed.  These aquatic bugs are the main food source for juvenile salmon in fresh water, so more knotweed= fewer macroinvertebrates= fewer salmon. 

In addition to negative ecological impacts knotweed also changes natural forest succession. The typical floodplain forest in western Washington is an alder and cottonwood canopy with a deciduous understory. While it appears that these understory plants are growing in an impenetrable thicket there is actually enough light reaching the ground that shade tolerant conifers can germinate and survive, albeit slowly. Knotweed on the other had grows so densely that no light penetrates to the forest floor meaning that nothing germinates, and there is no second generation of trees. Alders and cottonwoods are fairly short lived species, so when the mature trees die, there is nothing growing to replace them.  All that remains is a knotweed patch.

The third reason knotweed is insidious is that after the first frost in the fall the above ground stems of the plant die. This can look like the knotweed has died, but this is a false flag. Over the course of the summer the knotweed has been using the sunlight to create energy, which it has been transferring down into its rhizomes. Here the stored energy rides out the winter, while the top of the plants appears dead.  As mentioned earlier nothing is capable of growing under knotweed. This 

becomes especially problematic in the winter. When the above ground growth dies it exposes the bare ground underneath, just in time for the winter rains. When the rain falls on the unprotected ground it causes lots of erosion which washes sediments into streams increasing turbidity and affecting water quality.  Once spring arrives the knotweed plant uses the energy stored in its roots to create new stems, which can grow over an inch a day when emerging.

Skagit Fisheries has been on the front lines of the battle against knotweed since 2010. Every year a crew thrashes through the watershed looking for and treating knotweed.  Initial efforts focused on the upland parts of the watershed, since eliminating these infestations would prevent downstream infestations. Once those patches were eliminated focus was shifted to the floodplain. This is where we have been working ever since. Floodplains are very dynamic and with constantly shifting river courses and high water events knotweed is continually getting redistributed.

While all the negative impacts of knotweed make this seem like a doom and gloom story there are bright spots.  It is a slow and labor intensive project, but we have seen progress. In the traditional project area we have seen the amount of knotweed treated each year continue to decline.

 

 

 

 

 

Figure 1  Knotweed patches in Upper Skagit Watershed treated between 2010 and 2020.

In 2020 Skagit Fisheries was able to expand the treatment area downstream for the first time. This was possible because we have gotten a handle on knotweed in the Upper Skagit Watershed.  Expanding into this previously untreated area added 8 river miles to the program. It was jarring to see how much knotweed was in this new section. It provided a visual representation of how the years spent in the upper watershed had prevented knotweed from spreading.  We will continue to survey for and treat knotweed in the upper watershed to keep the minor infestation from spreading, but the main brunt of our efforts will focus on the river between Concrete and Rockport. The hope is that eventually we will be able to get this section of river to look like the areas upstream from Rockport.

Much of the land in this expanded reach is private. We are very fortunate that we have a good reputation in the Skagit Valley. Because Skagit Fisheries is not a government agency and has no enforcement authority we work solely with willing landowners. Education and outreach provide us the opportunity to reach private landowners. When the negative effects of knotweed are explained to people they are generally wiling to allow knotweed treatment. The Skagit Valley is a unique and beautiful place and most landowners want to be good stewards of the land. Not everyone is aware of threats to the landscape, but when they are made aware most participate in the Upper Skagit Knotweed Control Program. 

Over the decade of the program Skagit Fisheries has been committed to keeping the program funded. We have been fortunate that we have been able to acquire grants to maintain continual operation. Over the years we have received funding from:

• Washington Department of Agriculture
• Ecotrust
• US Fish and Wildlife Service
• US Forest Service through Resource Advisory Committee of the Mount Baker-Snoqualmie National Forest
• National Fish and Wildlife Foundation
• Pulling Together Partnership (funds donated by Phillips 66)
• 5 Star and Urban Waters Fund (funds donated by Shell)