Can Japanese Knotweed Be Eradicated With Herbicide?

A research paper called 'Optimising physiochemical control of invasive Japanese knotweed' was published in February 2018. A team of research scientists, led by Dan Jones, conducted the most extensive field-based assessment to date of the effectiveness of established control methods on Japanese knotweed. Three test sites were divided into fifty-eight 225m² plots of mature knotweed and subjected to either herbicide treatment, physical control or a combined methodology.

The study concluded "Though no control treatment delivered complete eradication of [Japanese knotweed] within 3 years … glyphosate … was found to be the most effective control treatment".

In April 2018, a number of national media outlets published stories on this report. Sadly, the desire for a good story and a bold headline took precedence over the truth. "The world's largest field trial on the control of Japanese knotweed … has found that eradicating the plant is not possible," screamed BBC News Online. Other news outlets, including The Telegraph and the Daily Mail, took the same line.


This was a gross misrepresentation of the facts, amounting to a significant attack on consumer confidence. "Why should I pay for a treatment programme if it won't work", clients began asking, before embarking on their own methods to deal with their knotweed. By encouraging clients to adopt home-grown methodology, such as indiscriminate herbicide applications, pulling, cutting, mowing or attempts to dig up the knotweed, the media had shown itself to be wholly irresponsible. At best, these control attempts can prove ineffective and at worst can lead to the knotweed being spread to new locations.

The published paper quite clearly states the project ran for a period of three years – which is absolutely crucial when putting the results of the research into context. This would be the absolute minimum duration any reputable contractor would assign to an herbicide treatment programme to combat knotweed – and even then, only for an area that was not very extensive. Killing off knotweed is all about the rhizomes, which are to be found beneath the ground. A well-established knotweed stand can have an extensive rhizome mass extending several metres beyond and beneath visible growth. It takes time to get herbicide into these rhizomes, which is why herbicide treatment programmes take a number of years.

The three research sites each contained thousands of square metres of mature Japanese knotweed. That's a lot of rhizomes. To kill off the knotweed with herbicide on these sites would take over a decade, so it is unsurprising that the three-year project saw no eradication in any of the test plots.

Eradication of knotweed by use of herbicides is possible, but it will take several years. The more established the knotweed is, the longer period of time it will take to kill it (there is no ‘one size fits all’ option with knotweed when it comes to planning an herbicide programme). Bottom line? Don’t believe everything you read in a newspaper!



Jim Glaister Regional Manager at The Knotweed Company Limited