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Why is Japanese knotweed such a problem?

Japanese knotweed is not native to Europe and was introduced to the UK without its natural enemies. In Japan, the plant generally grows in harmony with the environment and is not considered to be invasive. 

One could say it has an unfair advantage over native species because the controlling influence of the many insects and fungi that attack the plant in its native range has been removed. Our native species have not taken a shine to knotweed and very few if any insects or fungi can be found on the plant even after almost 200 years.

An estimate put the cost of control, were it to be attempted UK-wide, at over £1.56 billion. Such control methods, relying mainly on chemicals, are widely considered to be unsustainable and so a longer-term and cost effective solution to the problem is required.

Japanese knotweed growing through concrete    Japanese knotweed infested river    Japanese knotweed in a car park

Biodiversity – Knotweed affects ecosystems by crowding out native vegetation and limiting plant and animal species diversity. Recent studies led by the CABI team in Switzerland have proved that knotweed areas suffer reduced species diversity. They have even found evidence of allelopathy (the release of chemicals that suppress the growth of other plants). It has been described as having the biodiversity value of concrete! 

Water quality and flood risk – Aquatic organisms are less able to process knotweed leaf litter compared with the native vegetation it displaces and this has the potential to alter food chains. Dense summer foliage causes heavy shading of small streams, which reduces aquatic plant communities. Profuse knotweed canes also reduce the capacity of river channels to carry floodwater, as well as blocking sluices and grids. In winter, the riverbanks become exposed when the knotweed dies back, increasing erosion and silting fish spawning gravels.

Recreation – Knotweed is a nuisance to anglers, boaters and other river users as it impedes access. It is visually unappealing and can block panoramic viewpoints.

Infrastructure – Knotweed’s stout rhizomes (underground stems) are notorious for pushing through asphalt, building foundations, concrete retaining walls and even drains, causing significant damage. This can add huge costs to development and regeneration schemes. Contaminated soil should be treated as controlled waste. 

Housing devaluation – Knotweed found on or close to a property can have an impact on its actual and perceived value. An increasing number of mortgage providers are refusing applications for properties where knotweed is revealed to be present (RICS).

Safety – Knotweed is capable of obscuring railway signals and road signs as well as causing trip hazards in paving.

Costs – Japanese knotweed costs Great Britain an estimated £165m every year (Williams et al 2010) and the cost of eradication, were it to be attempted UK-wide, could be more than  £1.56 billion. Such control methods, relying mainly on chemicals, are widely considered to be unsustainable and so a longer-term and cost effective and more environmentally friendly solution to the problem is required.

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