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How to identify Japanese Knotweed

When it comes to buying or selling a home, the presence of Japanese Knotweed is – in almost every case – incredibly significant. This hardy, vastly destructive invasive species can infiltrate basements, foundations, surfacing and get directly into houses.

It takes years to be entirely rid of it. Spotting it early is absolutely vital for both homebuyers and sellers. A buyer knows to avoid the property at all costs; a would-be seller can get started on a treatment plan as soon as possible.

So, how can you identify Japanese Knotweed? What are the signs of its presence, and why is it so crucial to avoid?

What is Japanese Knotweed?

Japanese Knotweed, also known by its Latin names of Fallopia japonicaReynoutria japonica and Polygonum cuspidatum (all different names for the same plant), is a perennial weed native to the Japanese islands, China, Taiwan and Korea. Originally discovered by a European in the 1700s (Dutchman Maarten Houttuyn), it was then ‘rediscovered’ in the 1800s by Philip von Siebold. Much later, European botanists realised that the two men had discovered the same plant (despite both giving it a different name).

In winter, the plant’s shoots and leaves die back to ground level. The growing season for Japanese Knotweed generally starts around March, when rhubarb-like red/purple shoots spring up from the ground. The hollow bamboo-like stems grow incredibly rapidly and can grow from the soil to heights of three metres by June.

In spring, the leaves are dark green or red and rolled up, growing in a zig-zag pattern on the stem. Once summer comes around, the leaves unfold – they can be up to 20cm long and are generally heart-shaped. In September, small cream-coloured flowers bud, and the stems begin to turn brown.

As the environmental temperature cools, the leaves turn brown and drop off. The stem also turns completely brown and appears dead.

Under the ground, the root and rhizome systems spread incredibly quickly. This is why it’s declared to be an invasive species and how it causes so much damage.

Why is Japanese Knotweed a problem?

Like many proverbial issues, the problem lies under the surface. During winter, the rhizome network continues to thrive, easily allowing the plant to survive through our now relatively mild cold snaps.

As its roots and rhizomes expand (which they do very quickly), this causes damage to foundations and flood defences. It’s also very capable of eliminating all competing vegetation in an area and can completely dominate a garden or outdoor park space.

In its native lands, Reynoutria japonica is often found on the side of volcanoes. Since the environment can be much more hostile (fungi and insects), its growth and natural spread are checked. However, in Britain and much of Europe, the conditions are much kinder, allowing for its rapid expansion. The government declared the species an invasive threat in 1981.

It’s very tricky to get rid of Japanese Knotweed because it grows from both the original stem and the spreading rhizome network. If even 1cm of rhizome is left untreated, a new plant can grow and the problem starts all over again.

If a property is found to have an active case of Japanese Knotweed, it will be almost impossible to sell. Even if the homeowner managed to find a willing buyer and a generous mortgage company, a significant percentage of the building’s value would be stripped away.

Japanese Knotweed can damage foundations, causing a whole multitude of bigger issues. It’s particularly dangerous because it’s so hard to kill.

It makes much more sense to deal with the root of the problem (no pun intended) before buying or selling a house.

When did Japanese Knotweed come to the UK?

It was first introduced to Britain in the mid-1800s (exactly 1850, to be precise) by Philip von Siebold to help fund his expedition. The German botanist studied over 1,000 plant species during the time he spent in Japan.

During the Georgian and Victorian eras, owning and cultivating mysterious foreign trees, shrubs, flowers, and vegetation was the height of fashion. Perhaps it represented one’s multicultural connections and worldwide influence. Either way, they would bring people from near and far to look at these curious new plants.

Japanese Knotweed was shipped to Kew Gardens in London by Siebold. It was to be grown and sold to affluent members of Victorian society. Before long, cuttings were being sold across the country, and the plant began spreading naturally, too.

Despite the speed with which it grew, the Victorians believed they had the spread of Japanese Knotweed under control, planting it in various locations and sometimes even using it to stabilise soil. They thought this because only a female plant had been brought to the country (this is why UK examples don’t produce any seeds). In fact, all Japanese Knotweed plants today are direct clones of that first fateful specimen from 1850.

Unfortunately, Japanese Knotweed can also spread through its underground rhizome networks, growing into entirely new plants. Combined with how the English population willingly spread it across the country, we’re now facing a relatively severe nationwide issue.

Where are the worst UK spots for Japanese Knotweed?

Japanese Knotweed is especially present in and around big cities and industrial areas like London, Birmingham, Bristol and Nottingham.

Statistically, some of the worst areas in the UK for Japanese Knotweed outbreaks are Bolton, Rotherham, Swansea and Glasgow.

If you live anywhere in the northwest, south-east Wales or London, your area might be at higher risk of Japanese Knotweed. As a result, it makes sense to be extra vigilant when buying or preparing to sell a house.

What should I do if I find Japanese Knotweed?

If you or a specific Japanese Knotweed Survey finds the plant’s presence, the only realistic long-term option is to get started on a treatment plan. Of course, if you’re buying a house, we’d strongly recommend avoiding the purchase altogether.

Treatment plans usually take three years minimum – often longer. You’ll need to hire a specialist and licensed individual or company to chemically spray the plant, regularly returning to check and respray it.

Even once Japanese Knotweed has been killed off, it’ll take another few years before you can receive a certificate officially stating that your property is free of the species.

Treatment costs depend on the contractor, but prices for a five-year plan for residential properties vary from £1,000 (for a small patch) to £5,000+ (covering a larger area).

Here is some specific government advice you should follow regarding Japanese Knotweed.

What can GB Home Surveys do for you?

Here at GB Home Surveys, our chartered surveyors check property for Japanese Knotweed with a tailored home survey. It’s one of the most critical parts of surveying, as the plant’s presence knocks a significant chunk of value off a house and is usually a deal-breaker.

If you’re unsure what to look out for when you view a house, consider instructing a surveyor to spot the signs for you. The cost of a RICS Level 2 or Level 3 Home Survey will be more than covered by the potential savings you’d make on treatment plans.

For more information about Japanese Knotweed, please feel free to get in touch with us (contact@gbhomesurveys.co.uk). If you’d prefer a voice-to-voice conversation, phone us on 03333 600 685, and we’ll be more than happy to answer any questions you might have.

Looking for a quote, or maybe you have a question? Get in touch today.

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