KNOTWEED

METHODS

Optimising physiochemical control of invasive Japanese knotweed
2018

ABSTRACT

Japanese knotweed, Fallopia japonica var. japonica, causes significant disruption to natural and managed habitats, and provides a model for the control of invasive rhizome-forming species. The socioeconomic impacts of the management of, or failure to manage, Japanese knotweed are enormous, annually costing hundreds of millions of pounds sterling (GBP£) in the UK alone. Our study describes the most extensive field-based assessment of F. japonica control treatments undertaken, testing the largest number of physical and/or chemical control treatments (19 in total) in replicated 225 m2 plots over 3 years. Treatments focused on phenology, resource allocation and rhizome source–sink relationships to reduce the ecological impacts of controlling F. japonica. While no treatment completely eradicated F. japonica, a multiple-stage glyphosate-based treatment approach provided greatest control. Increasing herbicide dose did not improve knotweed control, but treatments that maximised glyphosate coverage, e.g., spraying versus stem injection, and exploited phenological changes in rhizome source–sink relationships caused the greatest reduction of basal cover and stem density after 3 years. When designing management strategies, effective control of F. japonica may be achieved by biannual (summer and autumn) foliar glyphosate applications at 2.16 kg AE ha−1, or by annual application of glyphosate in autumn using stem injection at 65.00 kg AE ha−1 or foliar spray at 3.60 kg AE ha−1. Addition of other herbicides or physical treatment methods does not improve control. This work demonstrates that considering phenology, resource allocation and rhizome source–sink relationships is critical for the control of invasive, rhizome forming species.

CONCLUSION POINTS

DOI Link

PDF Link

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CITATION

Jones, D., Bruce, G., Fowler, M. S., Law-Cooper, R., Graham, I., Abel, A., Street-Perrott, F. A., & Eastwood, D. (2018). Optimising physiochemical control of invasive Japanese knotweed. Biological Invasions, 20(8), 2091–2105. https://doi.org/10.1007/s10530-018-1684-5

Japanese knotweed BMPs (Best Management Practices)

DO NOT disturb knotweed roots or cut them into small pieces, which can encourage spread. A single pea-sized fragment of rhizome or lower stem node carried downstream or transferred via soil or equipment can lead to a whole new stand.


EDDR (Early Detection Rapid Response): Smaller single knotweed stems growing from propagules in new areas along stream banks can be very carefully hand-cut and dug out and disposed of.


Established stands: Careful cutting regimes, at least 4x a season, repeated for at least 4 years, may eventually deprive rhizomes of carbohydrates. Cut stalks can be dried on site, or disposed of according to local regulations. Never compost freshly cut knotweed or leave stems in contact with ground.


Ask your local conservation orgs about chemical regulations, and talk with your neighbors. Glyphosate spray at 2% solution will effectively translocate to rhizomes only if applied to leaves in Fall window after flowering but before frost. Earlier applications will not translocate, and stronger solutions will only kill visible parts without affecting underground reserves. Tall and hard to reach thickets can be cut in June to prepare for Fall treatment of regrowth. Stem injection at higher concentrations may also be used in sensitive areas.


Other removal methods like mowing, tarping, smothering, solarizing, burning, digging and excavating, and other chemical treatments have been shown to be ineffective, costly, and sometimes counterproductive.


Remember: Knotweed can go into dormancy below ground for many years, and management is a long term project that requires patience and commitment, as well as careful monitoring of new growth.


ALWAYS have a plan before you begin for what to plant to replace and compete with knotweed. Fast-growing willow trees are easy to grow and propagate, can help in streambank stabilization, flood mitigation, and carbon sequestration, and can be sustainably harvested and used in a variety of ways. Also consider native grasses and other pollinator plants. And have a plan for stewardship: who—besides you, with you, or after you—will help care for this place?