How do you identify Japanese knotweed?

The most easily identifiable trait of Japanese knotweed is the leaves which are heart or shovel-shaped. The plant, however, looks different depending on the time of the year. In the spring, when it’s first beginning to grow, the shoots have a red or purple colour. Light green leaves will start to develop fairly early on.

In the summer, the plant will grow quite quickly and can take over parts of the garden. The stems will start to resemble bamboo shoots and you may see small purple specks. The leaves will grow bigger and have distinctive ribs and veins. You’ll also see small, cream-coloured flowers developing towards the end of summer.

In the autumn, the leaves will start to go yellow and wilt as winter approaches. The plant can grow to about two or three metres if left unattended. The stems will change to a darker brown before the plant becomes dormant in winter.

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What does Japanese knotweed look like in April?

It can be difficult to recognise Japanese knotweed in spring or April as this is when the plant first starts to grow. If you have an existing infestation that has been dormant over the winter, you’ll easily be able to spot the brown, bamboo-like stems sticking out of the ground.

In April, new Japanese knotweed appears as asparagus-like shoots. These start off as reddish knotweed crowns and can grow at a rate of a couple of centimetres a day. They often outgrow surrounding plants. The more mature plant can grow at a rate of 20cm a day.

As the fleshy shoots grow some more, they are likely to start sprouting pale green leaves with purplish or pink veins that are quite distinctive. As we move from April into May and June, the stems gradually develop into bamboo-like structures with a reddish-brown colour and bigger leaves. By the end of the summer, the Japanese knotweed can grow to two or three metres.

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What can be mistaken for Japanese knotweed?

Plants that people often mistake for Japanese knotweed include bindweed, Himalayan balsam, Russian vine, broadleaf dock and some lilac and woody shrubs. Japanese knotweed can cause a great deal of damage to properties. Identifying the plant is not always simple and it’s easy to get confused.

Bindweed, for instance, has heart-shaped leaves that look almost the same as Japanese knotweed. The difference is that this is a climbing plant so it will tend to be wrapped around garden structures and up walls rather than growing straight up out of the ground.

Himalayan balsam differs in how the leaves are arranged on the stem and the slightly pink ribbing. The broadleaf dock comes from the same family of plants so look similar too – the difference is in the stems which are shorter and fluted.

It’s important to get a proper identification for Japanese knotweed and ensure that it is removed from your property.

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Does Japanese knotweed have pink flowers?

Some species such as dwarf Japanese knotweed can have pink flowers but these are less invasive and their incidence in the UK is lower. The Japanese knotweed we find in our gardens and on business properties have small clusters of flowers that are creamy white.

They generally appear towards the end of the summer and into Autumn, just before the plant becomes dormant and ‘closes down’ for the winter.

Making the right identification when it comes to Japanese knotweed is difficult if you don’t have experience of it. Many plants such as bindweed and broadleaf dock have similarities in leaf shape and growth and often get mixed up. If you do find Japanese knotweed on your property, it’s important to get a professional team in to handle its removal.

They will be able to use a mix of digging and chemical control to ensure the plant doesn’t return or do any damage to your property.

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Does Japanese knotweed have red stems?

At certain stages of its lifecycle, Japanese knotweed will have red or reddish-brown stems that look similar to bamboo. Even when it is first growing and shoots are just emerging, you will be able to see a red/purple tinge in the asparagus-like tips.

As the spring fades and we move into summer, the stems of the Japanese knotweed will become thicker and start to resemble bamboo. One key characteristic is that you will notice little purple speckles on the surface of the stem.

As the plant moves into autumn, you’ll see the leaves begin to yellow. The stems will switch from a reddish-brown to a deeper hue of brown as it prepares for the dormancy of winter. During winter, all you are really left with are the broken, bamboo-like stems and nothing else which can make it difficult to identify.

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What do the flowers of Japanese knotweed look like?

Japanese knotweed flowers are fairly distinctive. They form in creamy clusters and are small in size. They normally start to appear during the late summer and early autumn. Identifying the flowers is important but it usually means that the plant has established itself quite strongly and may be difficult to remove.

Other, less prevalent types such as dwarf Japanese knotweed have pinkish leaves but these are not so invasive in the UK. Himalayan knotweed can have white or pale pink flowers.

Most people have trouble identifying whether they have Japanese knotweed at all. That’s why it’s a good idea to have it checked by a specialist.

Ideally, you want to catch the plant in its early development in the spring or the beginning of summer. Waiting too long, particularly until the Japanese knotweed flowers appear in late summer, can mean that you are more prone to property damage.

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What does Japanese knotweed do to a house?

The reason that Japanese knotweed is so problematic is that it can cause structural damage to properties. It is able to push through areas like cavity walls, drains and anywhere there is a weakness such as a crack or a fissure.

Once it finds its way into infrastructure, Japanese knotweed can cause more damage as it grows, widening gaps and causing mayhem along the way. If you find Japanese knotweed in your garden, it’s imperative that you do something about it as soon as possible.

The problem with knotweed is that its roots can grow as deep as 3m and spread out across 7m. Infestations are quick to take hold and if the plant gets near to your house you can quickly find many problems with structural damage.

The best way to get rid of Japanese knotweed on your property is to use a mix of digging and chemical control to ensure that the plant does not re-establish itself.

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What is the difference between bindweed and Japanese knotweed?

Bindweed and Japanese knotweed can often be mistaken for each other. Both have large, heart-shaped leaves and can grow quickly, getting out control in a short time. The main difference between the two, however, is that bindweed is a climbing plant and will tend to wrap around garden structures or grow up the wall.

Japanese knotweed is a freestanding plant and doesn’t need any support. Both plants start to take hold in the springtime and can appear even more similar at this stage, thought the shoots for Japanese knotweed have a red/purple colour and resemble asparagus tips.

The other way to differentiate the two is the flowers. Bindweed has largish white or pink trumpet flowers while knotweed has clusters or clumps of small creamy flowers. Knotweed flowers appear towards the end of summer and autumn compared to late spring-early summer for bindweed.

Both plants can be a nuisance but Japanese knotweed is by far the most invasive and likely to cause damage to property.

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Can Japanese knotweed grow through concrete?

The simple answer to this question is no. When people first find Japanese knotweed on their property it often leads to a sense of panic and an attempt to get rid of it. One of the stories that we often see about this invasive weed is that it can grow through concrete but this is actually a myth.

Japanese knotweed is a highly invasive, fast-growing plant and it can cause significant structural damage which is why it is so important to get rid of quickly. It causes damage, however, by taking advantage of structural weaknesses such as cracks and gaps.

No plant can actually get through solid concrete but it will seek out cracks to try and eventually breakthrough. When the plant starts to grow more, it can shatter the surrounding concrete and cause more damage. If you have an area of concrete and it’s intact with no cracks and fissures, you should expect it to stay clear of Japanese knotweed.

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How deep do Japanese knotweed roots go?

The roots of Japanese knotweed are a huge problem and can grow as deep as 3 metres which makes it a difficult weed to get rid of. In addition to this, the roots can spread up to 7 metres horizontally. Above the ground, the plant is equally fast-growing and is quickly able to reach heights of three or four metres.

A mature, established plant will grow as much as 20cm a day and it can quickly get out of control. While the above-ground infestation is fairly easy to get rid of, it’s the roots underground that cause the most problem.

They can grow too deep for most normal gardening and digging practices which is why it’s important to combine this process with chemical knotweed control. Even one rhizome remaining in the ground means that the plant will start to grow again and soon start to establish itself.

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Not looking quite right? Check our Knotweed Family Identification and Commonly mistaken plants pages to be sure.

If you are still unsure, we offer a free Japanese knotweed identification service. Email your photos to expert@environetuk.com and we'll tell you if Japanese knotweed is present. If it is, then we will help guide you through the removal and treatment options.

This is a free service. However, if you would like to make a small donation to a worthy charity via JustGiving that would be appreciated.