Whilst not an unattractive plant, Japanese Knotweed is an aggressive plant that has some particular impacts upon the building ownership and construction industry. If you suspect your property has Japanese Knotweed, or a property you are interested in purchasing may have it you should seek expert professional advice. (e.g. not me – this is a ‘for interest’ article)
It was introduced into the UK in 1825 by horticulturists and found its way into large homes and manors. Since then the spread has been rapid, accelerated by human interaction. Japanese Knotweed is a very rapid growing invasive plant that not unlike bamboo, spreads through it’s underground root system but can grow through and damage parts of buildings and landscaping. In spring surface shoots can grow so fast you can practically see it, growing as much as 20cm per day, reaching 3m high by the middle of summer.
You are not obliged to remove or treat Japanese knotweed, however you must not allow it to spread to adjacent land, (neighbouring owners can take legal action) or plant or encourage the spread of it outside your property. Note: This can include removal of contaminated soil incorrectly or without licence. In the UK Japanese Knotweed spreads through the root system, called Rhizome, (rather than seeds) as thankfully there are no or very little male plants. However, breaking, cutting or disturbing the root system will cause more plants to grow. Very small pieces, about the size of a fingernail, can produce a new plant. If you suspect a plant is Japanese Knotweed, to curtail spread, it is vital that you do not disturb the soil around it, as this will simply encourage more growth. The root system commonly spreads 3m underground away from the surface shoots, and can reach as far as up to 7m away.
Pieces of plant or rhizome root can be transported by rivers, tracked vehicles, fly-tipping and moving of soil (including bringing new soil into your site! – ‘cheap’ soil won’t be such a bargain if contaminated with knotweed pieces) Chemical treatment is effective at controlling spread, but it requires persistent and prolonged treatment over several growing seasons, eg years of treatment, it is not a quick fix. Shock single-application chemical treatment can make the plant become dormant, lying “dead” until the ground is disturbed, when new growth will reoccur, and should not be undertaken. It is possible to remove the plant and root system altogether, however it requires specialist knowledge and experience.
A typical plant could occupy a volume of several lorries full of soil, which can be extremely expensive to remove. Japanese Knotweed is a controlled waste requiring permits and authorised landfill sites to dispose of it. Specialist companies such as Environet have methods of filtration/treatment on site, that significantly reduces the amount of soil having to be removed off-site. (Imagine a volume of soil 2m deep over an area of 5m x 5m – this is not an unrealistic sized established Knotweed plant)
Crucially it has been recognised as an issue during house purchasing and forms a part of the Law Society Property Information Form questionnaire when selling a home. It is also a significant issue to lending as many mortgage companies will not lend to purchase a property known to have Japanese Knotweed. As a result of this values of properties fall with Japanese knotweed nearby. Insurance companies are beginning to offer insurance from discovery of knotweed following house purchase, and you should check that any historic treatments come with an insurance-backed guarantee to cover future regrowth.
It is a difficult subject as the strict regulations and vicious spread, encourage less-scrupulous people to try to hide it or dump it, making the national problem much worse. If in any doubt or concern, seek expert advice before disturbing the ground. Many companies will provide free identification services.Japaneseknotweed.co.uk has some good identification images also.