Japanese knotweed is recognised by the World Conservation Union as one of the most invasive species due to its rapid growth, and resilience to cutting and crowding out of other species.

A recent Japanese knotweed study carried out by researchers at the University of Leeds, the National University of Ireland Galway (NUI Galway) and the consulting firm AECOM, found that fully drying the plant out in a laboratory environment can prevent its regeneration when re-planted.

This is major news in the knotweed world and can help reduce the many problems and stigmas attached to the properties of Japanese Knotweed.

An unwelcome visitor

It has for decades been a nemesis of gardeners and landscape planners, with infestations known to damage walls, buildings and pavements and even impact house buyers’ mortgage applications.

Invasive Japanese Knotweed is the bane of homeowners and their properties. According to scientists, Japanese Knotweed might be eradicated by drying it off, which prevents even the slightest part from regrowing again.

Japanese knotweed is a notorious non-native species in the UK, and elsewhere in Europe and North America. 

In the UK, Japanese knotweed is widely believed to pose a significant risk of damage to buildings that are within seven metres of the above-ground portions of the plant – the so-called ‘seven-metre rule’ – due to its underground shoots, known as rhizomes. 

Japanese knotweed can regrow even from half a gram, according to new research, and can grow up to 10cm a day to a height of two to three metres.

If left alone, Japanese knotweed can dominate an area to the exclusion of other plants and can even grow through cracks in concrete.

The stigma

The plant thought to be “impossible to kill” can result in mortgages being refused and chemical treatments, that don’t always work, can cost £6,000.

When identified in homebuyers’ surveys, mortgage lenders often require evidence that a treatment programme is in place to control Japanese knotweed, entailing significant expense for sellers.

The stigma associated with the plant means that property values can be affected, even after action is taken to control it.

As well as setting out to test the accuracy of the seven-metre rule, researchers examined the risk from multiple lines of evidence and all came to the same conclusion.

Automatically refusing mortgages on properties where Japanese knotweed is found is out of proportion to the risk posed by this invasive species, new research has found.

Japanese knotweed study into its permanent eradication
Japanese knotweed study into its permanent eradication

The Japanese Knotweed study

Ecologists from global infrastructure services firm AECOM and the University of Leeds have carried out the most extensive research to date.

They assessed the potential of Japanese knotweed (Fallopia japonica) to cause structural damage compared to other plants. 

The scientists tested the drying theory at three sites in the North of England on knotweed plant mass which had proved to be resistant to herbicide treatment.

It worked by taking different parts of the plant – namely crowns (underground mass from which stems and shoots emerge) and rhizomes (underground root-like structures) – and trying to regrow them at the sites in Leeds, Huddersfield and Lancashire.

The Japanese knotweed study found the smallest initial fragment weight to regenerate and survive was 0.5 grams, but the team also found the larger the fragment the quicker and larger the regeneration.

The Japanese knotweed was subjected to air-drying for sitting on a lab bench for 38 days and then replanted in soil, provided with the same nutrients, water and light conditions as the growth phase.

Japanese knotweed study findings

The findings discovered the removal of moisture was a valuable strategy for small to medium-scale infestations of Japanese knotweed without resorting to expensive chemical treatments.

Dr Mark Smith, Associate Professor in water research from the University of Leeds’ School of Geography, said: “This study provides evidence that while Japanese knotweed rhizome fragments as small as 0.5 g can regenerate and survive, the growth rates and growth success vary substantially based on the initial size of those fragments.

“Smaller fragments resulted in much smaller regrown plants and may thus be less problematic.

“While herbicide treatment is an important control strategy, it is only effective if done correctly.

Dr Mark Smith

“Here we found no significant difference in the size of regrown plants from sites that had undergone two years of herbicide treatment and those with no history of treatment, suggesting that the herbicide treatment had not been applied correctly.”

Dr Karen Bacon, an active ecologist and researcher for over 10 years, from the National University of Ireland Galway, said: “The findings of this study show clearly that the size of the plant fragment is critical to the initial regrowth, with smaller fragments producing much smaller regrown plants.

“Additionally, if there are no nodes, there is no regeneration, which may suggest potential management strategies in the future.

“This also highlights that small infestations and plants should not be viewed with the same concern as larger ones and that rapid management should be a goal of tackling this problematic species.”

Dr Bacon added: “Our finding that the removal of moisture has a 100% success rate on killing Japanese knotweed plants and preventing regrowth after they were replanted also raises an important potential means of management for smaller infestations that are common in urban environments.”

It also showed that if there are no nodes – the area on the stem where buds are located – attached to the root structures, there would be no regeneration.

The good news is ecologists can find no evidence Japanese knotweed causes significant structural damage.

In conclusion

While research has shown that the wrong use of herbicide treatment does not prevent the knotweed from regrowing and regenerating, the drying process did, scientists have confirmed.

Ultimately, more research is required beyond this Japanese knotweed study, but this is a promising start to hopefully gain back control of this invasive weed.

It might be some time before there is a commercial solution, but it’s comforting to know that progress is being made and it is only a question of time.

In the meantime, certainty around properties becoming devalued has now been resolved and acknowledgement of effective treatment is still recognised as the most effective means to eliminate this weed.


A Q&A with Dr Karen Bacon from the University of Leeds on Japanese Knotweed – The Property Buying Company

Ecologists find no evidence Japanese knotweed causes significant structural damage – AECOM

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