The Ecosystem Guild/Knotweed Control Experiments

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Broad-scale distribution of knotweed in the Skykomish Watershed has prompted efforts by County Staff and collaborators to begin a landscape-scale control effort. This page is being used to gather information and evidence about this effort in the Skykomish Basin, as well as knowledge gleaned from other efforts. in an attempt to better understand the scale, rate, and effectiveness of efforts. This page is used to support work by the Ecosystem Guild at Biocultural Restoration Field Stations


  • Polygonum cuspidatum and several other species and hybrids are listed as Class B Designate in Snohomish County, meaning that because of its wide distribution, state regulators have left control decisions in the hands of Snohomish County Weed Board, and the board has decided to attempt control.
  • Based on three separate inventories there are around 600 acres of knotweed mapped in the Skykomish River Basin. Based on field observations at the Reiner Farm compared to county surveys, this dramatically underestimates the area of large patches as well as distributed colonies under forest canopy.
    • What is the current rate of knotweed treatment? What is the current rate (acres/year) at which sites are designated "clear" and not revisited? Do we have those sites mapped?
  • Snohomish County has a Knotweed Control Page with limited information.
  • Urgenson et al 2009 giant knotweed implications indicates that Knotweed has very rich leaves (12:1, C:N) but is over twice as efficient at recovering nitrogen compared to native species, resulting in low nitrogen litter (50:1).
  • Grimsby et al 2007 knotweed sexual or clonal observed that within six different contiguous populations they detected 26 genotypes, suggesting that there is sexual reproduction occurring within and among patches. clonal networks are either male or female among Fallopia japonica

Control Notes

  • File:Soll 2004 controlling knotweed in pacific northwest.pdf - published following a knotweed control workshop. It describes the weaknesses of mechanical control, and is the origin of the claim that knotweed must be cut every two weeks. It does suggest that mechanical control can support more efficient and effective chemical control.
  • King County Knotweed Video Page
  • Pennsylvania State University Summary
  • Friends of Alington's Great Meadows completed several knotweed control trials -
    • The first trial was nicknamed "Cut-Dig-Cover". In spring knotweed was cut to the ground, rhizome crowns were excavated and removed, and the site covered in black sheet plastic. At the end of the following growing season, the plastic was removed, and meadow seed sown. Knotweed sprouts returned. As a site where the plastic was left for three more growing seasons, there has been no knotweed return, suggesting that three to five seasons of cover are required. It is unclear if the dryness created by plastic encouraged rhizomes to go dormant.
    • The second trial named "Cut-Cut-Cut" the knotweed was cut to the ground two to three times per growing season for two years. At that time remaining plants were diminished and rhizomes were more easily pulled out of the ground. The author believed that this approach held greater promise.
    • It seems that based on this approach, Initial volunteer efforts could result in greatly reduced and more efficient herbicide application, with more complete control.
  • Rootwave is a English company experimenting with electrocution as a weed control method.

List of Claims and Evidence

The following hypotheses describing program presumptions were gathered in conversation with county staff and in reviewing various fact sheets. Many fact sheets repeated information suggesting that fact sheet authors may be reiterating information. I could find no citation of research or field experiments. Fact sheets tend to focus on what the plant is capable of (reproducing from 0.7g of stem tissue) rather than what it does under field conditions.

  1. CLAIM - Chemical control (typically glyphosate or imazapyr) is the only feasible method for the long-term control of knotweed. It is also the most cost effective method.
    • Several sources indicate that carbohydrate starvation is effective.
    • What is the cost/acre for complete control using pesticide? At what point do we walk away from a site and stop monitoring?
    • This quote was found on a Thurston CD pamphlet "“There have been no field trials that reported adequate control though repeated cutting. One research paper cites that cutting knotweed tops can reduce root biomass, but these roots were in small pots in a greenhouse, and may not be applicable to a field population where root expansion is not limited.” Tim Miller, Ph.D., Associate Professor of Weed Science at Washington State University.
    • This only indicates that Miller is unaware of any field trials and that lab experiments weakly support carbohydrate starvation.
  2. CLAIM - Carbohydrate starvation requires mowing every two weeks during the growing season.
    • This was found in one TNC pamphlet. What is the basis for this? It was also a claim repeated on a King County video.
    • A British Source makes the following claim: Cutting may be effective for small populations if repeated several times a year with constant monitoring. Cutting should be repeated until root reserves are depleted (usually several years). Cutting is most effective when followed up with herbicide application.
  3. CLAIM - Any control method that involves cutting green stems creates a significant risk of dispersal through re-rooting of stems. Because of this, on-site disposal is actively discouraged in favor of bagging and landfill disposal. As an alternative on upland sites, stems should be stockpiled on tarps until dry, then composted or burned.
    • Supported by this claim "Because woody knotweeds reproduce readily from very small pieces of roots and stems, mechanical control is ineffective." (TCD)
    • Are stems able to root at all times of year?
    • It a tarp necessary if stockpiles are burned?
    • Why is composting or burning necessary if stems are dead?
  4. CLAIM - Use of volunteer labor for manual knotweed control is not effective, and therefore not an efficient allocation of volunteer labor.
    • If frequent volunteer tending of sites supports carbohydrate starvation, while providing other benefits for planting survival and growth, while increasing location and treatment of new clones within a management area, while providing additional benefits to volunteers, in a way that cannot be replicated by contract crews, would that be a useful component of volunteer effort?

Additional Hypotheses

  1. Does shading from dense rapid canopy create the potential for increasing the efficiency of root starvation? Knotweed is describes and preferring moist soil and full or partial sun. I have observed Knotweed in full deciduous shade (Pcereghino (talk)).
  2. Does a pre-treatment of cutting, pulling, and or grazing increase herbicide effectiveness or reduce herbicide application volume?
  3. What are reliable methods for on-site disposal of roots?
  4. How does and herbicide specifications affect translocation of herbicide to roots?
  5. How does timing of growth or regrowth affect effectiveness of herbicide application?

Reiner Farm Experiments

A group from the Ecosystem Guild started a knotweed control experiment at a site on the Reiner Farm.

  • The site was selected in the floodplain but is protected from river flows (outside "fast and deep" zones.)
  • The area is intended to be a "mother garden" where intensive cultivation of native plants provides wildcrafting and production of cuttings and seeds for use on other sites, and is located at a field station location that will receive frequent work and oversight over a long period of time. This position near routine intensive work supports experimentation.
  • On Sept 29, areas of knotweed and blackberry in full green leaf (amongst old dead stems) were cleared. Windrows were placed at approximately 8-10 foot spacing, to accommodate scythe mowing between windrows. Biomass is placed in windrows and roughly chopped with a machete to reduce fragment size. This reduction is presumed to 1) increase mulch benefits, 2) increase the ease of planting into slash (manageability), and 3) increase the speed of mortality.

In one area, knotweed root crowns were pulled and placed on top of slash piles to desiccate.

  • On Nov 9 slash piles were examined. No living stems or rooted stems were observed. Undamaged root crowns in soil were alive, with turgid buds awaiting spring. Uncut knotweed was in late senescence. Unearthed root crows were not examined closely. There was no regrowth of new shoots following cutting.

Current Draft Plan

  • The goal is to starve the root system using stems as mulch or food.
  • A scythe with a 2.5' brush blade will be used to mow alleys, with the swath cleared to the windrow. Leaves will be used to maintain mulch in strips.
  • Regrowth in the mulch strips will be grubbed and placed on top to maintain mulch. Rooting will be observed, particularly during spring harvest.
  • Fast-growing deciduous trees (Alnus, and Populus) will be planted at high density into the slash piles to provide shade and mulch, with the intention to thin in 5-10 years for an early harvest of mushroom logs or poles, while creating gaps for diversity once weed species are controlled.
  • If control is not robust, we may choose to switch to spot spraying, but with a much much lower volume of spray required for control.


  • A large stand of green stems were cut late in the season and left in windrows of slash (September 29, 2022). Observation a little over a month later, showed no signs of life on any stems and no rooting (November 9, 2022). (Pcereghino (talk) 22:40, 10 November 2022 (UTC))