Scientists test 8 methods to control Japanese Knotweed

Struggling to deal with Japanese Knotweed in YOUR garden? Scientists put 8 management methods to the test to reveal the most effective way to control the weed

  • Glyphosate herbicides are the cheapest and most effective in controlling weeds
  • Ground covers were most expensive and most damaging to the environment 
  • Invasive Japanese Knotweed is a known killer of other plant species in the UK

It has been growing uncontrollably across Britain since being introduced nearly 200 years ago.

But scientists reveal that it’s possible to reign in Japanese Knotweed after testing the most effective ways to manage the rogue plant.

In the assessment of eight different methods, experts found that Glyphosate-based herbicides were the cheapest, most sustainable and most effective when tackling the alien species.

On the flip side, use of ground cover sheeting to starve the weeds was the worst method tested, with the highest price and most damaging impact on the environment. 

These findings come at a time when there are more than 50,000 known Japanese Knotweed infestations throughout the UK, dominating gardens, railway lines and waste grounds.  

Invasive Japanese Knotweed (pictured) is a known killer of other plant species across the UK

Japanese Knotweed is a known killer of other plants, releasing chemicals that stifle their growth and blocking sunlight from reaching them.


Japanese Knotweed is known to be a huge killer of other plants across the UK.

To spot this invasive species, look out for some key signs:

Source: Japanese Knotweed Ltd

The weed can grow to a soil depth of nearly 40 inches and live for up to 20 years, according to pest controllers Japanese Knotweed Ltd.

To spot it, gardeners should look out for its identifiable zigzag stem structure and white flowers that bloom in the summer. 

On its website, Japanese Knotweed Ltd said: ‘When allowed to spread, or disturbed Japanese Knotweed can grow under footpaths, and buildings causing structural damage where it finds a weakness within the structure.’

As part of the latest research, numerous herbicides were used to spray either plants or surrounding soil, including brands such as Glyfos ProActive, Depitox, and Picloram.

Each of the eight methods took a different stance on how these herbicides or geomembrane covers were used, alternating usage in different seasons of the year.

Scientists also changed up the herbicide quantities each time with Glyphosate for instance ranging between 37lbs and 47lbs (16.9 and 21.62kg) per ha.

Other, more physical techniques complemented some approaches too, including the removal of knotweeds by hand as well as digging and turning each underground stem (rhizome).

Glyphosate herbicides are the cheapest and most effective in controlling weeds (file image)

Overall, the most effective methods saw Glyphosate spay being applied either just in autumn or both summer and autumn. 

Not only was this deemed most cost-effective, but it was also far less time consuming than other methods.

It also had the least detrimental environmental impact with its low concentration of 37lbs (16.9kg) and 40lbs (18.43kg) per ha.

Geomembranes were said to be most damaging due to numerous factors including the plastic needed to manufacture them.

Researcher Sophie Hocking, of Swansea University, wrote in The Conversation: ‘We found geomembrane covering to be the most damaging. That was due to the production of the plastics needed to manufacture the geomembranes, as well as the ground preparation needed to install them. 

‘And we also found that using digging as part of a knotweed management programme was also less sustainable due to the carbon emissions produced from using machinery.

There are over 50,000 known Japanese Knotweed infestations throughout the UK and plant is notorious for its ability to spread and cause damage to building structures

‘In light of the current climate crisis, minimising greenhouse gas emissions is vital. As we are aiming to achieve carbon net zero by 2050 in the UK, we need to think carefully about the sustainability of the approaches we use for managing Japanese knotweed and other problematic invasive plants.’

Controlling Japanese Knotweed has now become a legal requirement in the UK with costs of managing it estimated to be £165million each year, according to a 2010 study. 

Despite its presence here, the plant can also survive in extreme environments – whether it be on the side of volcanoes or in freezing conditions. 

Japanese Knotweed Ltd added: ‘Knotweed can re-emerge and re-grow of its own accord any time, but especially if the contaminated ground is disturbed.

‘When developing a site affected by Japanese knotweed, if the necessary due diligence and control of Japanese knotweed has not been undertaken, the developers risk legal action for Professional Negligence. 

‘Ignoring knotweed can result in regrowth during or after construction, appearing through hard and soft landscape areas and even within the fabric of buildings themselves.’

Top tips for protecting your property

Like it or not, knotweed can put any property at risk across the country. See the map to find out if you live in an area at risk of the invasive plant. 

Simply enter your postcode to check for the number of verified infestations within a 4km radius. The worst affected locations are highlighted in red, orange and so on. 

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The legal implications surrounding Japanese knotweed

Since 2013, property sellers are required to state whether Japanese knotweed is present on their property. 

Sellers: It is the sellers responsibility to check the garden for Japanese knotweed. Sellers must complete a TA6 form, used for conveyancing, which asks for confirmation as to whether the property is affected by Japanese knotweed and, where it is, to provide a plan for its removal by a professional company

Buyers: If the property has Japanese knotweed, it will be stated in the responses to the TA6 form. It may result in the mortage lender requiring assurances that the plant will be eradicated before agreeing to funds. A plan by a professional eradication company tends to be enough assurance. 

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