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What Does Japanese Knotweed Look Like?

Japanese knotweed has several strong characteristics:

  1. Small, white flowers
  2. Heart/ shovel shaped leaves
  3. Tall stems that grow up to three metres tall
  4. Shoots that are purple and reddish-brown to begin, but soon turn green with red flecks
  5. Dense network of underground root system
  6. Groups of dead canes in winter
Flowers
Japanese knotweed flowers appear in late summer, and are small, white and very delicate. You may see bees flocking to them, as they often bloom after most other flowers have died. The flowers grow on long thin spikes around 10cm in length.
Leaves
Japanese knotweed leaves are up to 15cm in length and fairly flat. Arranged in an alternating pattern, the leaves begin a reddish colour in the spring before turning fully green in summer. They are shovel shaped, although hybrid versions may appear more heart shaped.
Stems
The stem of Japanese knotweed is a hollow culm, similar in appearance to bamboo externally. In spring, the shoots start out reddish-purple, before growing into green with red flecks and rings. In winter, they die back to being brown brittle canes which can be easily snapped.
Shoots
In early spring, knotweed can be identified through often random but very hardy purple, reddish-brown shoots that appear through dead foliage.   The sporadic growth can happen anywhere, including through pavement cracks and walls.
Roots (Rhizome)
The root system of Japanese knotweed is very invasive, and can grow through house foundations, boundaries and more. The roots are thick, and while often found around 1m below the ground, they can be found up to 3m below ground.

How Japanese Knotweed is Identified

RICS Approved Survey

RICS (Royal Institute of Chartered Surveyors) surveys are carried out by one of our accredited surveyors here at Japanese Knotweed Specialists. We most commonly carry these surveys out in the warmer months, especially as people begin to prepare their gardens for summer, or as they winter-ready them.  

Our expert identification looks for signs of knotweed and uses the latest RICS and PCA (Property Care Association) Guidance to provide you with an accurate and actionable report.

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Dog Detection Surveys

In winter months, we use sniffer dogs to help identify knotweed in areas of thick overgrowth or large expanses of land.  

These highly trained sniffer dogs are taught to identify the smell of Japanese knotweed roots and buds, and can cover large expanses of land at rapid speed, quickly identifying knotweed before it takes over

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When is it Best to Identify Japanese Knotweed?

The best time of the year to identify Japanese knotweed is in the middle of summer.  

During early spring, purple shoots will appear above the ground, just as buds form and bloom outward as “spears”. As these grow – reaching up to 2cm daily – these start to form bamboo-like pillars.

In early summer, however, the stems will have matured and developed a purple pattern. At this stage, they begin to grow in a zig-zag, with leaves beginning to sprout.

Bamboo Roots

Download Our Knotweed Identification Guide

As the identification process can be challenging, depending on the time of the year and the conditions of the environment, we’ve developed a guide to help you understand the timeline of growth and change that occurs in this plant throughout the seasons.

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Green Leaves With Flowers

Identification of Japanese Knotweed Throughout the Seasons

Japanese knotweed is a perennial plant, meaning its appearance changes with the seasons. It starts with small fleshy roots in the spring, potentially growing up to 10cm per day and over 3 metres high in the spring and summer, before dying back to brittle, leafless canes in the winter where it is less identifiable.

Japanese knotweed can spread from very small sections of root at an alarming rate, which can cause damage to foundations and drainage of a property if left untreated.

Japanese Knotweed Graphic

Identifying Japanese Knotweed in Spring

In early spring, the reddish-purple shoots begin to appear. These soon turn to smaller reddish-green leaves and begin to grow at rapid pace as we head into early summer.  

Growth will be sporadic, and shoots could appear anywhere, not necessarily near dead canes from the previous winter. By the end of spring, the knotweed will be a metre or so high and beginning to turn a darker green.

Identifying Japanese Knotweed in Summer

In summer, when this invasive plant is at an average of between 2-3 metres high in full bloom, the spade shaped leaves and spiky stems with creamy white flowers make Japanese knotweed identifiable in peak summer.  

With the ability to grow up to 10cm a day in the height of summer, Japanese knotweed will be noticed by the end of August if it hasn’t already been. The combination of sheer height and density of Japanese knotweed in the summer makes this plant easily identifiable.

Japanese knotweed is a good source of nectar and may attract bees and other pollinating insects as full bloom begins around late August. This is another way to identify it compared with other plants

Green Leaves With Flowers
Yellow Dry Leaves

Identifying Japanese Knotweed in Autumn

The appearance of Japanese knotweed in autumn will have very similar traits as in late August, as the flowers will potentially be in full bloom until early October.  

Towards the end of October and through to November, the shield shaped leaves will start to wilt and turn yellow, and the stems will become dark brown in colour and start to become dormant, appearing brittle.

Identifying Japanese Knotweed in Winter

By wintertime, around November, leaves will have fallen, and the dark brown canes will appear brittle, and may start to decompose. Whilst in winter Japanese knotweed remains dormant above ground, below ground the rhizomes are still present and will continue to spread. This is when our highly trained sniffer dogs are used to detect stubborn roots and buds underground. 

At the end of winter, around February, new season’s shoots may already begin to appear.

Dry Plants
Leaves and sky

See More Japanese Knotweed Pictures

Don’t rely on guesswork, especially when it comes to such a destructive and costly plant. When attempting to identify Japanese knotweed, it’s helpful to look at pictures – many of which highlight just how invasive this plant can be.

But if you’re still unsure – always contact a specialist for a professional survey. Misidentifying Japanese knotweed can lead to costly and damaging consequences.

Plants That Can Be Mistaken For Japanese Knotweed

On your journey to identify Japanese knotweed, you could easily mistake another plant that isn’t invasive for knotweed. Here are three common plants that people often mistake for Japanese knotweed.

White Russian Vine

Russian Vine (Fallopia baldschuanica)

  • As fast growing as Japanese knotweed
  • Same colour leaves and flowers
  • Unlike Japanese knotweed, Russian Vine is very disorderly in appearance
White Himalayan Balsam

Himalayan Knotweed (Persicaria wallichii)

  • Similar stem to knotweed, so they may look alike when not in bloom  
  • Spreads densely like knotweed
  • Their main difference is that their leaves are half the size and flowers are pink

Houttuynia (Houttuynia cordata)

  • Houttuynia is a perennial plant also with heart shaped leaves and white flowers
  • Grows at a similar time and rate to Japanese knotweed
  • However, it is much shorter with a large yellow stigma when compared to Japanese knotweed

Why Do You Need an Expert for Japanese Knotweed Identification?

Why Do You Need an Expert for Japanese Knotweed Identification?  

It is highly important to make sure you have an expert carry out Japanese knotweed identification to determine whether you have an infestation or not. Failure to do so can lead to the plant quickly spreading across your premises and neighbouring properties, leading to damage.  

Experts like our team here at Japanese Knotweed Specialists will be able to conduct a full survey, provide visual identification, or in winter, use a dog detection survey to sniff out knotweed underground.  

Get in touch with the experts at Japanese Knotweed Specialists today to find out how we can create a treatment and removal plan.

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Identify your Japanese Knotweed Problem Today

Japanese Knotweed Specialists are the leading invasive species identification and removal professionals in the UK, capable of treating and disposing of various aggressive plants.
If you suspect growth of Japanese knotweed on your property, you have a legal obligation to prevent it spreading. Contact a specialist today to ensure peace of mind – we’ll help you identify and treat any infestations.

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Bamboo Stems