Investigating the rapid spread of invasive knotweed in a riparian setting (Washington)

  • Published source details Holman M.L., Dunwiddie P.W. & Carey R.G. (2007) Investigating the rapid spread of invasive knotweed in a riparian setting (Washington). Ecological Restoration, 25, 140-141.


The Skagit is one of the largest rivers in the western United States, and a portion of its reach has been designated as a Wild and Scenic River. The Skagit River System is a landscape that, if conserved, promises to ensure long-term survival of its characteristic flora and fauna. It provides habitat for a myriad of species, including bald eagle Haliaeetus leucocephlaus, threatened Chinook salmon Oncorhynchus tshawytscha, listed bull trout Salvelinus confluentus, and six other species of anadromous fish. These are keystone species that are integral to the function of the river and their continuing decline is of great concern. Invasive species have been identified as one of the primary threats to the Upper Skagit ecosystem through a site conservation assessment by The Nature Conservancy. In particular, three species of knotweed (Japanese Polygonum cuspidatum, giant P.sachalinensis, and Bohemian Polygonum Ă— bohemicum), hereafter collectively referred to as knotweed, pose a significant threat to the Skagit river-floodplain system. Knotweed is widely recognized as an aggressive invasive that can significantly alter the composition and structure of habitats in which they become established. Knotweed can be particularly problematic in riparian settings where it may alter important ecological processes involving stream bank stability, sediment deposition and nutrient budgets.

This project is part of a long-term, landscape-scale, community approach to knotweed control in the Upper Skagit Basin that has been advanced by the Skagit Knotweed Working Group (SKWG) since 2001. The SKWG is leading the effort to control knotweed in the Upper Skagit River corridor in an on-going public-private effort. By treating invasive knotweeds and allowing a multi-tiered, diverse vegetation structure (including native trees) to reestablish, future riparian corridors may better serve natural functions.

Study area: In 2001 survey and treatment was begun in the Upper Skagit Knotweed Management Area (Fig. 1), approximately 2,960 square miles (7,666 square km) in size. The Skagit, situated in Washington State, north-western USA, is one of the largest rivers in the western United States, and a portion of its reach, in the upper section, has been designated as a Wild and Scenic River. The project area contains 175 miles (282 km) of stream reaches, where survey and treatment efforts have occurred over a number of years (Fig. 2).

Treatments: Knotweed patches are treated following a prioritized list that ranks patches according to several factors, including location in the watershed (e.g., riparian vs. upland; upriver vs. downriver), size, years of previous treatment, and ownership. Treatments follow integrated pest management methods including:

i) foliar spraying a mix of 1% imazapyr and 1% Agridex with 1.5-litre hand sprayers or 4-gallon backpack sprayers (except for patches on the National Forest, which are sprayed with 5% glyphosate and 1% Agridex);

ii) manual bending of stems in early summer followed by foliar spraying later in the summer;

iii) injection with 100% glyphosate sometimes followed by spray treatments (this method was used vary rarely in 2007).

Manual treatments take place from May through mid-July and are undertaken on easily accessible patches greater than 50 stems. Nearly all patches were bent manually without breaking the stalks rather than cut with brush cutters or other means, because this method greatly reduces the chance that (otherwise) cut knotweed fragments will resprout on site or be moved off site by equipment or water thus potentially be spread to new, uninfested areas.

Most herbicide treatments begin in late July, with the exception of patches located in the upper Sauk floodplain, which are treated in late June when water levels allowed access by raft. Herbicide treatments continue through late-October.

Monitoring: Treated knotweed patches were revisited and average stem-size category (recorded prior to treatment) was again recorded as a representative of patch size change.

After seven years of survey and six years of treatment, a total of 1,377 knotweed patches have been located, 665 of which are still living. Of these patches 46% of those treated at least once have been eradicated, and 52% have been eliminated from the project area either as a result of treatments, flooding or some other means. Average stem-size category has decreased by 45% (Table 1).

The ability to efficiently and effectively treat knotweed continues to be a struggle. The chemical treatment procedure was changed in 2006 (from 5% glyphosate spray to a mixture of glyphosate and imazapyr) after discovering that 11% of patches without aboveground growth in 2004 had resprouted by 2005. In 2006, continuing high resprouting rate (22%) was observed as well as a notable portion of previously treated knotweed plants with mutated growth that is symptomatic of treatment with glyphosate. Because of these observations, the Conservancy essentially eliminated glyphosate from the treatment regime in 2007, using only imazapyr for spray treatments.

From an analysis of four years of knotweed treatment data, conducted in 2006, the following became apparent:

i) Integrating manual and chemical control increased mortality by 16% (Fig. 3.)

ii) Mortality was 46% lower for larger patches (>50 stems).

iii) Regardless of patch size, mortality rates decrease as years of treatment increase.

iv) An average of 16.5% of patches that appeared dead for one or more years experienced re-growth in subsequent years, and this was more likely to occur in large patches (>100 stems).

v) Foliar spray with glyphosate may cause mutated growth (small leaves with little surface area that often grows close to the ground) that is seen in a significant portion of regrowing plants, making subsequent treatment more problematic.

As a result of this analysis and subsequent observations, treatments and survey procedures continue to be refined in an attempt to more efficiently and effectively control knotweed. This continues to be a struggle with mortality rates hovering just over 40% and with 391 new patches found in 2006 and 199 in 2007. However, on a positive note, in 2007 the resprouting rate dropped from 16.5% to 8%, and in 2007 we also eliminated 62% more patches than we found (323 vs. 199), whereas in 2006 we only eliminated 20% as many patches as we found (81 vs 397) (Fig. 4).

Perhaps the most illustrative lesson learnt occurred following a 2006 survey of the Upper Sauk River (a major tributary of the Skagit River), along eight linear miles of floodplain from the Bedal Campground downstream to the White Chuck River confluence. During this survey, 200 new knotweed patches were found where a survey in 2002, prior to extensive flooding in 2003, had identified only 20 patches. The rapid spread of knotweed in the upper Sauk River illustrates how quickly a few plants, if left uncontrolled, can serve as the nucleus for extensive new infestations that are more difficult to detect and much more costly to control. The delay in eradicating these few patches resulted in an enormous increase in the number of patches as well as the geographic extent of the infestation in only a couple of years (Holman et al. 2007).

These experiences with rapid knotweed spread, knotweed persistence and resurrections, and results of extensive surveys, underscore three fundamental tenets that have been frequently advocated in controlling invasive species - the importance of early detection of infestations, rapid action to control all occurrences, and consistent follow up for several years to ensure that actions were effective and thorough. Further, given the potential of one individual to spawn many more and the discovery of knotweed in previously un-surveyed or under-surveyed habitats, we emphasize that it is critical that repeated, intensive and exhaustive surveys are conducted to locate (and treat) every single individual in the source area.

For a more complete description of methods and results contact: Melisa Holman (

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at:

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