How To Identify Japanese Knotweed

How To Identify Japanese Knotweed
Discovering a property has Japanese knotweed is every homeowner’s and house hunter’s nightmare. But spotting it quickly is vital, and that’s why it’s really useful to know how to identify Japanese knotweed.

Once you know the signs to look out for you can approach buying a new property with much more confidence.

It can also save you vital time and money as you won’t need to get a survey done before you realise what that strange plant growing near the house is.

But what exactly is it and how do you go about Japanese knotweed identification? Let’s take a look…

What Is Japanese Knotweed?

Japanese knotweed, or Asian knotweed as it is sometimes also known, is a large, herbaceous perennial plant of the knotweed and buckwheat family Polygonaceae.

Its scientific name is Fallopia japonica and it’s a plant that’s native to East Asia but one that has also successfully established itself in the UK and Europe as an invasive species.

It looks similar to the bamboo plant but is not actually related to it. While it may not be desired near a property, the plant does have a number of uses.

Some beekeepers regard it as a source of nectar for honeybees, while young leaves and shoots are boiled and eaten in some parts of the world.

Japanese Knotweed Damage – Why Is It So Bad?

Japanese Knotweed Damage

The main problem with Japanese knotweed is the strength of its roots. Once growing in close proximity to any kind of structure, it is likely to begin damaging the foundations.

That’s because it’s roots are very similar to tree roots; thick, woody, and strong. The root system can grow up to 3 metres deep and 7 metres sideways meaning the plant doesn’t have to be right up against a property to cause damage.

The main issue is the rhizome part of the root which will easily break through concrete, including foundations, should any weakness be found.

Once Japanese knotweed starts growing into the foundations of a wall or building, it won’t take long for the building to show signs of subsidence or cracking that could lead to serious structural issues.

That’s why Japanese knotweed is so bad for homeowners, if left untreated, it could literally break away the foundations of your home!

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Where Did Japanese Knotweed Come From?

We have European adventurer Philipp Franz von Siebold to thank for Japanese knotweed in the UK, at least partly. It was he who is first thought to have introduced it into Europe, bringing it to Holland from a Japanese volcano.

By 1850, a specimen from the plant in Holland was added to the Royal Botanic Gardens, Kew. Gardeners of the time loved the plant as it looked like the exotic bamboo plant and was easy to grow almost anywhere!

Since then, the plant has been somewhat unstoppable and is now found across the UK. In fact, it’s reported that it cost £70m to eradicate knotweed from 10 acres of the London 2012 Olympic Games velodrome and aquatic centre!

The costs for homeowners will clearly not reach such heights but it can still be expensive to deal with as it does need specialist attention.

How To Identify Japanese Knotweed

Japanese Knotweed Flowers

Japanese knotweed will first show itself in spring when small shoots will appear that look similar to asparagus but are red or purple. The shoots will quickly turn into green stems that look similar to bamboo and grow very quickly, up to around 3m in height.

The leaves of Japanese knotweed are bright green and heart-shaped with a smooth, rounded edge, flattened base and pointed tip. As the leaves grow on alternate sides along the branches, you’ll notice a distinct ‘zig-zag’ growth of the branch.

Once the leaves are fully grown they will remain a vivid green colour until they die off later in the year. The leaves reach a maximum of 120mm in length. If the leaves are longer than this, you probably do not have Japanese knotweed.

By early summer, the Japanese knotweed plant is usually fully grown and will begin flowering by late summer.

Japanese knotweed flowers are creamy-white in colour and develop in small clusters. Each plant can have dozens of these clusters but the seeds are rarely viable in the UK.

In autumn, the leaves will fall to the ground and the stems will become dark brown in colour. Japanese knotweed remains dormant over winter will be come back with a vengeance in Spring if left untreated.

Japanese Knotweed Look-Alikes

There are a number of Japanese knotweed look-alikes that often get people unnecessarily worried. One of the most common is bamboo, since the stems bear a resemblance to the stems of Japanese knotweed.

However, unlike Japanese knotweed, bamboo shoots are hard and cannot easily be snapped and the leaves are very slender and long.

Woody shrubs and trees such as lilac and dogweed can also sometimes be mistaken for Japanese knotweed. However, leaves often grow opposite each other on such plants and so you don’t get the distinctive zig-zagging of branches like you do with knotweed.

Houttunyia is another plant commonly mistaken as Japanese knotweed. However, these plants will only reach 30cm in height so can soon be discounted once they stop growing.

In addition, red bistort, lesser knotweed, Himalayan balsam, broadleaved dock, bindweed, Himalayan honeysuckle, and Russian vine can also be mistaken for knotweed if you are not familiar with how to identify Japanese knotweed.

Japanese Knotweed Removal & Treatment

Japanese Knotweed Removal
Image: Peter O’Connor / CC BY-SA 2.0 – Image has been cropped

If you are unfortunate enough to have found Japanese knotweed near your property or one you are thinking of buying, you’ll need to consult with a Japanese knotweed specialist.

The plant will need to be completely eradicated in order to avoid causing any further problems. This is achieved through the correct choice of herbicide as you’ll need one that can travel right through the roots to kill the plant completely.

If you catch it early, it may be possible to pull up the entire plant and all of its roots. However, keep in mind that knotweed is classed as controlled waste in the UK so you can’t just put it in a bin.

Because it’s such a serious issue, we don’t advise trying to tackle Japanese knotweed removal yourself, far better to bring in the experts to ensure it’s dealt with properly at the first time of asking.

When To Repair The Damage Caused By Japanese Knotweed

Keep in mind there is no point in repairing the damage Japanese knotweed has caused until you are sure it has been completely removed. That’s because as it dies off, there may be more movement in the foundations and things re-settle.

If you’ve already repaired cracks, you may find they open up again, so, fix the cause of the problem before you repair the damage.

If you have any doubts at all about how to identify Japanese knotweed, make sure you get a proper survey done on a property before putting in an offer. A chartered surveyor will quickly pick up on any issues Japanese knotweed may cause on a property.

Don’t forget, we can help you find and compare chartered surveyors to assess potential Japanese knotweed issues.

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