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Study suggests Japanese knotwood can’t be controlled with herbicides

Japanese knotweed

Following extensive trials, a team of bioscientists at Swansea University have developed an effective way to tackle Japanese knotweed, listed by the World Conservation Union as one of the world’s worst invasive species, and an issue of huge concern to householders in the UK.

In 2016, the first phase of the world’s largest Japanese knotweed field trial was completed. After five years of intensive research Professor Dan Eastwood and Dr Dan Jones have defined a patent on one aspect of potential Japanese knotweed control treatment known as ‘The 4-Stage Model’.

Based on their research, they say eradication isn't possible in the short term. The methods used were physical, chemical and an integrated approach.

Professor Eastwood said: “We began focusing on knotweed at a time when there was a great deal of hysteria surrounding it. At the time, most information for people affected by the plant was largely based on anecdote. This led to the prospect of unscrupulous companies offering expensive and ineffective treatment solutions.

"Over five years, we tested all 19 of the main physical, chemical and integrated methods of controlling Japanese knotweed in the UK, working with the biology of the plant to inform our treatment approaches. Our research tested more methods of control than any other invasive species trial ever conducted, allowing us to replace out-dated guidance based on short-term experiments and anecdotal information.”

Control and management, not eradication 

Claims made by companies stating that they could eradicate Japanese knotweed using herbicides in short spaces of time have now been proven to be false, based on the experiments. Furthermore, they have shown that applying the wrong herbicides at the wrong time of the year, leads to greater herbicide use and environmental impacts.

"The 4-Stage Model developed links herbicide selection and application with the seasonal targeting of knotweed delivering better results and uses lower doses of herbicide across the whole treatment lifecycle. For the public and land managers, this means more affordable treatment that is also more environmentally friendly than traditional, blanket herbicide application.”

Dr Jones said: “The 4-Stage Model is the starting point for developing more effective and efficient knotweed control. Our experimental benchmarking of all practical knotweed control methods does, however, ensure that methods that don't work can be discontinued in the future.”

Tips to stop the spread of Japanese knotweed

The team has some advice for anyone worried about the spread of knotweed:

  • Ignore the hysteria - your house will not fall down. Knotweed can be controlled using glyphosate at the right time of year, though this can usually take three to five years. However, we don't recommend you do this yourself – we've seen mortgages declined because of botched treatment undertaken by homeowners.
  • Knotweed is a resilient weed that cannot be controlled by one herbicide treatment in a single year, a claim frequently made by unscrupulous companies. Any treatment strategy should be long-term and target both the above-ground and extensive below-ground parts of the plant.
  • Effective treatment centres on working with the biology of the plant and targeting the correct herbicide when the plant is vulnerable to its effects, from summer into late autumn (depending on the weather).
  • Calling out a weed control company to control knotweed is not the same as calling out a plumber. We would expect the plumber to get the work done soon, if not immediately. If you call out a company in spring to control knotweed, quite rightly, you will need to wait until later in the year to get the best results – this will save you time, money and hassle in the long term.
  • Once knotweed has been effectively controlled using herbicide, don't disturb this land by digging, for example, as it is likely to come back; even if the above-ground parts of the plant are dead, the below-ground rhizome system probably isn’t. Again however, don’t panic, if it comes back, call the contractors to regain control.
  • Don't try digging out the knotweed yourself, it's easy to miss parts of the root and spread it and you can't dispose of this plant material along with your garden waste – in fact, it's illegal to do this.

For most people, Japanese knotweed represents a nightmare, particularly if they are trying to sell their home.

Further information

Swansea University scientists lead the way in tackling Japanese knotweed

How to identify Japanese knotweed