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Close up of Japanese knotweed stem showing detailed patterning

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HOW TO IDENTIFY JAPANESE KNOTWEED UK

Japanese knotweed is a highly invasive and resilient weed that can cause damage to your property and lower the value of your home. It can be difficult to identify but being able to spot the signs of a Japanese knotweed infestation early is advisable.

If you’re wondering “how can I tell if it’s Japanese knotweed?” this page provides all the information you need, read on to find out what knotweed really looks like.

Why is it important to identify Knotweed early?

Since Japanese knotweed (Reynoutria japonica) is such an invasive and resilient weed, it is important to be able to identify it early. Japanese knotweed can be treated with herbicide, and this will stop the aerial growth that you see above the ground, but the roots – known as rhizomes – are capable of supporting life for up to 20 years.

Japanese knotweed can re-emerge and re-grow of its own accord any time, but especially if the contaminated ground is disturbed by gardening or construction activities. If the Knotweed infestation is left unchecked for several years, it can spread and cause a range of issues including:

  • Impeding amenity land use
  • Increase change of land use costs
  • Material damage to hard standing structures
  • Devaluation of property
  • Creating legal issues

In the UK, we only have the ‘female version’ of the plant. The roots of Japanese knotweed are known as rhizomes, or ‘creeping rootstalk’. These are a horizontal underground plant stems that produce the shoot and root systems of new plants. These networks can stretch for 3 metres or more from the visible ‘areal growth’ of the plant. Even small fragments of rhizome can give rise to a new plant. This is why it is so important to learn how to identify Japanese knotweed.

How to Identify Japanese Knotweed in Spring

Like many species of weed, knotweed is very active during the spring months. This is a good time to watch for signs of knotweed growth in your garden, business premises or land.

New growth appears as rapidly growing soft red/purple shoots reminiscent of asparagus spears, growing into stronger, upward shoots with hollow stems that look like bamboo and can grow up to 10cm per day. This new growth forms dense thickets, known as stands. The emerging leaves grow on alternate sides of the stem producing a zig-zag pattern in the stem, with a leaf shooting from each ‘zig’ and each ‘zag’.

Key Facts about Japanese Knotweed in Spring

  • Knotweed is particularly active during spring months.
  • New growth resembles asparagus spears, soft red/purple in color.
  • Stems grow stronger and resemble bamboo, with growth rates up to 10cm per day.
  • Emerging leaves form a zig-zag pattern on the stems.
  • Dense thickets known as stands are formed from new growth.

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk.

Japanese knotweed shoot with new leaves Japanese knotweed shoot close up

How to Identify Japanese Knotweed During Summer

During the summer months, Japanese knotweed becomes fully formed with large shovel shaped green leaves, forming a dense canopy. Leaves can grow up to 200mm long. Japanese knotweed canes grow to around 3 to 3.5m tall. It will often form as a semi-dense stand of upright stems. As summer continues, knotweed begins to form clusters of creamy-white elongated flowers. The canes grow to around 3 to 3.5m tall in summer, often forming a semi-dense stand of upright stems, with lush green foliage.

Key Facts about Japanese Knotweed in Summer

  • Knotweed has large shovel shaped green leaves forming a dense canopy.
  • Leaves can grow up to 200mm long during summer.
  • Canes grow to 3 to 3.5m tall, forming a semi-dense stand of upright stems.
  • Clusters of creamy-white elongated flowers form as summer progresses.
  • Identifying knotweed can be challenging due to other flowering plants and weeds.

During the summer months, the volume of other flowering plants and weeds can make it tricky to identify knotweed. Our experts are fully trained to identify knotweed (and other invasive species).

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk.

Japanese knotweed flowers   Japanese knotweed leaves showing their distinctive shovel shape

How to Identify Japanese Knotweed During Autumn

In Autumn the dense covering of leaves will remain, however, they start to turn yellow and orange and appear more brittle. The leaves wilt as we move into September and October. At this stage the knotweed plants are still about 2-3 metres tall, and the hollow stems start to turn brown. Once the leaves start to fall the canes become more easily identifiable.

Key Facts about Japanese Knotweed in Autumn

  • The dense covering of leaves starts to turn yellow and orange.
  • Leaves become more brittle and start to wilt in September and October.
  • Knotweed plants remain tall at about 2-3 metres during this season.
  • The hollow stems of the plant begin to turn brown.
  • As leaves fall, the canes or stems become more distinguishable.

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk.

Japanese knotweed leaves in Autumn turn golden in colour before dropping off  Japanese knotweed clump in autumn showing orange coloured leaves

Identifying Japanese Knotweed in Winter

Identifying Japanese knotweed in Winter can be a little tricky as some of the most identifiable signs are its leaves and flowers, and in Winter knotweed will have lost all of its leaves, but the long clustered shoots will remain. These canes will die off and turn brown and brittle however will remain erect through the winter if left undisturbed.

This isn’t the end of the weed, however – its crown and rhizome remain dormant throughout the winter, waiting for spring to return so that they can start the cycle again. Identification of its rhizomes is the perfect opportunity to directly tackle the problem with excavation and removal. Digging out and if required replacing the contaminated ground at a time when the weed is dormant, and when the garden or green areas are generally used less is a great option.

Key Facts about Japanese Knotweed in Winter

  • Most identifiable signs, like leaves and flowers, are lost in Winter.
  • Clustered shoots or canes remain but turn brown and brittle.
  • The canes stay erect throughout the winter if undisturbed.
  • The crown and rhizome of the weed remain dormant in winter.
  • Winter is an ideal time to tackle the problem through excavation and removal.

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk.

Japanese knotweed canes spreading over a wall in winter   Japanese knotweed clump in winter with brown brittle canes

 

Japanese Knotweed Leaf Identification

Knowing what Japanese knotweed looks like can really help you to identify a problem. There are quite a few weeds out there that can easily be mistaken for this foreign invasive species. One of the most identifying features of the Knotweed are the leaves.

Japanese knotweed leaves are shovel-shaped (not to be confused with heart-shaped) with a point at the tip and staggered on the stem (one stem per node), creating a zig-zag stem growth pattern which is quite characteristic. They’re a luscious green colour and grow up to 200mm long.

Key Facts about Japanese knotweed leaves

  • One of the primary identification features of Japanese knotweed is its leaves.
  • The leaves are shovel-shaped, distinct from heart-shaped leaves.
  • Each leaf has a point at the tip.
  • Leaves are staggered on the stem, with one stem per node.
  • This arrangement leads to a characteristic zig-zag stem growth pattern.
  • Leaves have a luscious green colour.
  • They can grow up to 200mm in length.

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk

Japanese knotweed stem showing zig-zag arrangement of leaves   Japanese knotweed close up showing leaf shape

Japanese Knotweed Flower Identification

Another defining feature of Japanese knotweed appears in the summer months towards the end of August and early September when it begins to flower. Knotweed will produce elongated clusters of creamy white flowers that grow to around 0.5cm wide and 10cm long.

The leaves will still be visible along with the flowers and together, can often create a dense foliage. The distinct leaves combined with the flowers can make it easy to identify if the weed is Japanese knotweed.

Key Facts about Japanese knotweed flowers

  • Knotweed begins to flower in the late summer, especially towards the end of August and early September.
  • The plant produces elongated clusters of flowers.
  • These flowers are creamy white in color.
  • Each flower grows to be approximately 0.5cm wide.
  • The flower clusters can reach lengths of up to 10cm.
  • The combination of these distinct leaves and flowers makes it easier to confirm the presence of Japanese knotweed.

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk

Japanese knotweed flowers growing along the branch   Close up of white Japanese knotweed flowers

Identifying Japanese Knotweed Roots

While doing work on your garden, land or commercial site, you may come across the roots of Japanese knotweed. These roots are known as rhizomes and work like underground stems.

If the roots are fresh, they will snap easily like a carrot. The outside is dark brown, and the inside is orange/yellow in colour.

The root system can grow to depths of 2 metres and extend up to 7 metres horizontally from the visible part of the plant. These roots are what allows knotweed to spread and are part of the reason that it is so resilient. It only takes a small amount of healthy root to give rise to an entirely new plant – and cause the infestation to re-emerge.

Key Facts about Japanese knotweed roots

  • The roots of Japanese knotweed are referred to as rhizomes and function like underground stems.
  • Fresh rhizomes can snap easily, similar to a carrot when broken.
  • The external part of the rhizome is dark brown in color.
  • The inside of the rhizome exhibits an orange/yellow hue.
  • The root system of Japanese knotweed can delve to depths of up to 2 metres.
  • Horizontally, these rhizomes can stretch as far as 7 metres from the visible part of the plant.
  • The extensive root system plays a major role in the plant’s rapid spread and resilience.
  • Even a small fragment of a healthy root can spawn an entirely new plant.
  • Due to this, minor remnants of the root can lead to the resurgence of an infestation.

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk

Japanese knotweed rhizome root being held to show orange coloured centre  Japanese knotweed rhizome root in the ground

Japanese Knotweed Stems

Japanese knotweed stems grow to 2-3 metres tall. In the summer months they’re similar to bamboo in shape and size but the stems are distinctive with purple speckles and vibrant green leaves shooting from each node on the stem (as shown by the image below).

At they mature the stems are hollow and when cut have a pale green fibrous flesh inside. During winter the stems dry out, turning brown and brittle.

Key Facts about Japanese knotweed stems

  • The stems of Japanese knotweed can reach heights of 2-3 metres.
  • In the summer, the stems resemble bamboo both in shape and size.
  • These stems are marked by distinct purple speckles.
  • Vibrant green leaves sprout from each node on the stem.
  • As the stems mature, they become hollow.
  • When cut open, the stems reveal a pale green fibrous flesh inside.
  • During the winter season, these stems undergo a transformation: they dry out becoming brown and brittle

Still unsure? Use our free identification service. Use the form at the top of this page to send us a few pictures of the plant that concerns you, or email your pictures to ident@knotweed.co.uk

Close up of Japanese knotweed stem showing distinctive red speckles

Japanese knotweed canes in winter are brown with hollow centre   Japanese knotweed stem in summer showing its fleshy inside

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