Japanese Knotweed Identification

By Paolo Martini on 2nd July 2019 (updated: 6th October 2020) in News

What Does Japanese Knotweed Look Like?

Japanese knotweed is a hotly debated topic in Parliament [1], within the property industry and in the courts, however, positive identification of the plant is required before any legal action is started. Although the plant has a few defining features (broad green, shield-shaped leaves and bamboo-like stems), Japanese knotweed takes on different forms throughout the seasons. Therefore, if you know what you’re looking for, it’s possible to discover an infestation at any point in the year.

Correctly identifying Japanese knotweed requires a keen eye and some key pieces of information. The plant does have some unique characteristics, however, it can also be confused with a number of other plants, which can make isolating an infestation difficult. By assessing each component of the suspected plant you can discover if you’ve found Japanese knotweed on your land or not.

This guide is intended to point property owners in the right direction when it comes to identifying this invasive plant and takes into account how knotweed appears throughout the year, other plants that it’s often mistaken for and what risk factors are related to an infestation.

How can I tell if I have Japanese knotweed?

Japanese knotweed infestations can come in all shapes and sizes, so spotting an infestation is not always easy. In overgrown gardens, a patch of Japanese knotweed could easily be lost, especially during the winter when the plant has died back. There are, however, a handful of neighbourhood risk factors which the RICS have suggested are often linked with Japanese knotweed infestations.

These include:

  • The property is close proximity to a water source, such as a canal, lake, culvert or pond.
  • Nearby public or private footpaths, roads, railways, motorways, or other public land that may have been left to grow.
  • Any large open public space, such as a car park, derelict homes, cleared or unused land.
  • Properties close to large industrial buildings, workshops and storage depots are also considered to be risk factors.

Related: Has Your Surveyor Missed Japanese Knotweed?

Flowers

Japanese knotweed flowers are often described as ‘creamy white’ [2] and appear towards the end of summer, from late August to September. Growing in clusters up to 10cm long, they appear alongside the bright green leaves, combining to create a large vegetative mass. Related species of Japanese knotweed can be adorned with similar clusters of flowers in shades of pink and red.

Stems

Japanese knotweed can be recognised by its red stems when it first emerges in spring, however, this colouration does not remain consistent throughout its lifespan. As the plant matures, the stem takes on a less distinctive green colour with mauve spots. It should be noted that these stems are cane-like in their appearance, growing in clearly defined segments with a hollow centre, similar to that of bamboo.

Leaves

When the plant is in foliage, Japanese knotweed leaves are bright green and feature a signature shield or heart-shaped leaf. Each leaf has a pointed end and is arranged at staggered intervals along the stem. Leaves can be up to 14cm long, but don’t expect to see any during the winter, as the plants green foliage doesn’t last through the colder months.

Species

Japanese knotweed (Reynoutria japonica/Fallopia japonica) is the most common species of the plant found in the UK [3], however, it has been known to hybridise with related species. Giant knotweed (Fallopia sachalinensis/Reynoutria sachalinensis) is found widespread throughout the UK but is not as common as Japanese knotweed [4]. Fallopia japonica var. ‘Compacta’ is the dwarf variety of the plant, that is much smaller in stature.

Fallopia x bohemica or Reynoutria x bohemica is the result of a hybridisation between Fallopia japonica and Fallopia sachalinensis, it tends to grow larger than Japanese knotweed [5]. Hybridisation has also occurred between the closely related Russian Vine (Fallopia baldschuanica), its common name is railway-yard knotweed.

Height

Japanese knotweed can typically be identified during early summer by its hollow stems that feature purple speckles and are up to 3 metres in height. The less frequently occurring Giant knotweed can grow up to 5 metres in height, whereas the hybrid Fallopia x bohemica has been known to grow up to 4 metres. Dwarf knotweed does not typically exceed 1 metre in height.

Seeds

Japanese knotweed has heart-shaped seeds that feature small wings. New growth from seeds is very rare [6], as only the female of the species was imported into the UK. For the most part, Japanese knotweed has been spread throughout the country by the transportation or fragmentation of its rhizomes. The rhizomes are effectively the plant’s root systems, with a fragment of the rhizome capable of generating an entirely new plant if given the proper conditions. Unfortunately, Japanese knotweed has been observed to hybridise with other related species [7] which has led to new plants that are able to then spread by seed down the line.

Japanese Knotweed in Spring

Japanese knotweed buds in spring

Japanese knotweed buds in spring

The plant can be difficult to spot until major growth above the surface begins to take place. Early spring is usually when the first Japanese knotweed shoots begin to appear, usually at the base of old canes (in the case of patches that are more than a season old). Knotweed shoots can be spotted by their bright red or pink tips. These pinkish redbuds grow to be around 1-3cm wide but do not keep this distinctive colour for long.

Young Japanese knotweed shoots

Young Japanese knotweed shoots in spring

Once spring is well and truly underway, shoots take on a greener hue and become easier to spot due to their accelerated growth [8]. Now growing as spear-shaped shoots, sometimes described as ‘asparagus-like, the leaves are red and rolled up, but they soon turn green as the plant grows skyward.

Japanese Knotweed in Summer

Japanese knotweed leaves in summer

Japanese knotweed leaves in summer

Japanese knotweed is typically easier to spot during the summer as this is when the plant is most visible above ground. Growth accelerates significantly during summer making the plant much more conspicuous. The hollow stems grow up to 2cm a day and the leaves now assume their most recognisable shield shape with an alternating stem pattern. This presents a good opportunity to treat the problem with a PCA-accredited glyphosate programme [9], professionals are able to assess an infestation and inject each plant with their specialist equipment.

Related: A Guide To Getting Rid Of Japanese Knotweed

Purple speckled Japanese knotweed stems with new foliage

Purple speckled Japanese knotweed stems with new foliage

By mid-summer knotweed plants are close to reaching their full height (between 1.9-3m). The stems are now less green and are rigid with more pronounced dark purple speckles. Clear nodes are visible on these stems giving them the appearance of bamboo, they can also be snapped easily in the hand. It’s not until late summer that Japanese knotweed flowers; multiple bunches of creamy white flowers appear amidst the leaves, but the seeds that are produced from this process rarely lead to new growths.

Japanese Knotweed in Autumn

Yellowing Japanese knotweed leaves in autumn

Yellowing Japanese knotweed leaves in autumn

As Autumn progresses, the characteristically bright green leaves turn a vivid yellow and the plant sheds seed cases. If the weed is left to its own devices the leaves and flowers will eventually begin to fade and fall back. Leaves fall off the stems which start turning brown and then a pale straw colour as winter progresses, also taking on a more rigid, woody form.

Japanese Knotweed in Winter

Large patch of brown Japanese knotweed in winter

Large patch of brown Japanese knotweed in winter

Knotweed appears to ‘die back’ during winter, but it’s unwise to assume that the problem is simply gone. In the middle of winter, all that remains above ground is a collection of pale, dry canes with the rhizomes lying dormant beneath the surface waiting for warm weather to sprout and spread further. Whilst the plant enters this dormant phase it is still very much alive.

Leaves

Japanese knotweed leaf close up

Japanese knotweed leaf close up

Japanese knotweed leaves are heart-shaped with a pointed tip, some also describe them as shovel or spade shaped [10]. Each leaf is staggered on the stem, forming a zig-zag growth pattern which gives the plant maximum opportunity for soaking up sunlight. Leaves are a lush green colour and grow up to 20cm long. One plant can feature dozens of these leaves which are perfectly designed to deliver energy to the plant’s rhizome system and encourage more growth at a later time.

Reynoutria x bohemica Japanese knotweed hyrbid leaf close up

Reynoutria x bohemica Japanese knotweed hyrbid leaf close up

When trying to differentiate Japanese knotweed leaves and Hybrid knotweed leaves, take a close look at the edges. Hybrid plants tend to have crinkled edges, whereas the original plant’s are smooth. Japanese knotweed leaves also tend to be a lighter shade of green and have a broader base (which some describe as more shield-like), whereas hybrid leaves have a more accentuated heart-shaped profile. You may also notice fine trichomes or hairs on the underside of hybrid leaves whilst Japanese knotweed are hairless underneath [11].

Flowers

Japanese knotweed flowers

Japanese knotweed flowers

Japanese knotweed flowers develop late in summer, towards the end of August and early September [12]. These flowers can be identified by their creamy-white colour and cluster formations which can grow up to 10cm long. The flowers grow alongside the leaf foliage creating a dense appearance which often blocks the trademark stems from view. Each individual flower, from within the cluster, is small in size. A flowering Japanese knotweed plant is a sign that it is well established and could therefore be a challenge to remove. Identifying and treating the plant before it reaches the flowering stage in late summer can prevent a longer-term infestation.

Rhizome

Japanese knotweed roots are comprised of a network of rhizomes. These tough, woody roots can collect together in large crowns which can be difficult to remove from the ground without the help of machinery. Rhizomes are dark brown on the outside and orange on the inside, you can usually break them in your hands unless they have grown into a larger clump or ‘crown’ [13].

Removing Japanese knotweed rhizomes lying dormant underground

Removing Japanese knotweed rhizomes lying dormant underground

Rhizome root systems can burrow up to 3 metres deep into the ground with individual rhizomes growing up to 20cm thick, but this kind of growth only occurs if the plant is given the time and space to flourish. The veracity of this plant can make matters particularly difficult in disputes involving neighbours. A Japanese knotweed infestation can easily spread underground during the winter via its root system without even the most attentive of gardeners noticing. Only in the spring, when shoots begin to emerge, would the knotweed problem be visible above ground.

Stems

Young Japanese knotweed shoots growing amongst old canes

Young Japanese knotweed shoots growing amongst old canes

Japanese knotweed stems are often compared to bamboo [14], as they have a similar node based construction and are hollow on the inside. They start life as a pink or red shoot that is not dissimilar to asparagus in appearance, these are edible at this point and have become popular with foragers in the UK. Once the plant grows it takes on a greenish hue, before developing a darker, muddier shade and purple speckles when it reaches maturity. At its mature height of 2-3 metres it is no longer edible, and each stem can be snapped easily in the hand. During autumn these stems start to turn brown, before becoming even more brittle and pale in winter.

Japanese Knotweed Identification Video Guide

Other Types of Knotweed

Giant Knotweed (Fallopia sachalinensis)

Fallopia sachalinensis or Giant Knotweed leaf

Giant Knotweed (Fallopia sachalinensis) leaf

Growing up to 5 metres in height, Giant knotweed is the largest of all types of knotweed [15]. Whilst the plant is found throughout the UK, it’s not as commonly found as its smaller relatives. The plant was imported to the UK around the same time in the 19th century as a curiosity for Victorian horticulturists who were impressed by the speed of which this plant could grow. Similar to Japanese knotweed, it has spread mostly through the dispersion of its rhizomes as opposed to typical reproduction methods.

Dwarf Knotweed (Fallopia japonica var. compacta)

Considered to be much less invasive than Japanese knotweed, the dwarf variant grows to 1 metre tall at most and retains much of its larger relatives’ physical traits. The plants’ stems grow in nodes and stems are produced in a zig-zag pattern. The leaves are noticeably different, however, having a darker green hue and leathery appearance [16]. The edges of the leaves are crinkles and feature reddish veins which can make it easier to distinguish from the lush green leaves of the more common species.

Fallopia x bohemica

Reynoutrica x bohemica Japanese knotweed hyrbid

Reynoutrica x bohemica Japanese knotweed hyrbid

Also known as Reynoutria x bohemica, this plant is a result of the hybridisation between Japanese knotweed and Giant knotweed [17]. The plant grows in a similar fashion to Japanese knotweed however the leaves grow much longer than its relatives. Both sexes of the plant are found within the UK, with hermaphrodite plants outnumbering the sterile-male plants, however the plant still mostly spreads through dispersal of rhizomes. Fallopia x bohemica is often underreported and is invasive throughout mainland Europe and in some states of America.

Railway-yard knotweed (Fallopia x conollyana)

First discovered in Wales in 1983, this hybrid plant has been reported to be a cross between Fallopia japonica and F. baldschuanica [18], having been discovered growing in close proximity to both plants in disused railway-yards in both the Czeck Republic and Germany. This hybridization theory was later confirmed by artificial cross-pollination within a lab. The plant has a similar structure and appearance to Fallopia japonica, with the stems having a propensity to bow over after reaching a height of 2m. It can be difficult to tell this hybrid apart from Japanese knotweed itself, as the plant’s leaf shape and growth can often mimic its parent as it appears post-herbicide treatment.

What Plants Can Be Mistaken for Japanese Knotweed?

Japanese knotweed is technically a herbaceous perennial plant. The plant is often mistaken for a shrub due to the large amounts of foliage it produces. Common shrubs such as lilac, poplar and dogwood also feature similar-shaped leaves, which can lead to misidentifications. A defining trait that separates Japanese knotweed from shrubs is its hollow bamboo-like stems. Most shrubs feature woody stems that can be stripped away, whereas Japanese knotweed stems will snap, rather than split, if bent.

There are a variety of sub-species and cross-breeds that can easily be confused with this plant. The following plants share a number of features with Japanese knotweed but have a few crucial differences which may aid in confirming, or ruling out a positive identification of the plant.

Russian Vine (Fallopia baldschuanica)

Russian Vine (Fallopia baldschuanica)

Russian Vine (Fallopia baldschuanica)

Commonly known by gardeners as ‘mile-a-minute’, Russian vine is a fast-growing relative to Japanese knotweed [19] which also shares some physical traits which often lead to its misidentification. Whilst its flowering structure and masses of green leaves are arguably a red herring, the key difference here is that the vine is a climbing plant which relies on other structures, be it plants or buildings, to grow upwards, as opposed to Japanese knotweed which supports itself.

Hedge Bindweed (Calystegia sepium)

Hedge Bindweed (Calystegia sepium)

Hedge Bindweed (Calystegia sepium)

Another frequent case of mistaken identity occurs with this climbing plant. Bindweed’s leaves share Japanese knotweed’s heart-shaped leaves [20] which can lead to false alarms amongst homeowners. Another key distinguisher is its large pink or white trumpet flowers which start growing in early summer, these are unmistakably different to knotweed’s small, creamy white flowers.

Dogwood (Cornus)

Red-stemmed Dogwood (Cornus)

Red-stemmed Dogwood (Cornus)

This woody shrub is often mistaken for Japanese knotweed because of the similarity in its leaf shape and colour. During spring these bright leaves are accompanied with creamy white flowers which also make it easy to confuse with Fallopia Japonica. The uninitiated may also be drawn to its bright red woody stems that grow throughout winter [21]. Being a shrub, this plant can also grow readily and may appear to be invasive, when in fact it can easily be trimmed back. The key difference between this plant and knotweed is its woody stems which can be stripped down, as opposed to the hollow Japanese knotweed canes that can be snapped.

Heart-leaved houttuynia (Houttuynia cordata)

Heart-leaved houttuynia (Houttuynia cordata)

Heart-leaved houttuynia (Houttuynia cordata)

This herbaceous perennial is rhizomatous, just like Japanese knotweed, and also features heart-shaped leaves which may cause alarm bells to ring. Despite these initial similarities, however, there are some key differentiators which make it easy to distinguish. Heart-leaved houttuynia produces tiny yellow flowers in spikes [22], as opposed to the large spikes of creamy-white flowers of Japanese knotweed. The plant also grows to a maximum height of 30cm, making it much smaller in stature than even the dwarf variety of Japanese knotweed.

Red Bistort (Persicaria amplexicaulis)

Red Bistort or Himalayan Fleece

Red Bistort or Himalayan Fleece

Also known as Himalayan fleece, Red Bistort is native to China, Pakistan and the Himalayas. Another herbaceous perennial, this plant is a close relative of Japanese knotweed and grows to a maximum height of 1.5 metres. The flowers grow in similar spikes, however they are typically white or pink in colour, as opposed to cream [23]. The stems are hollow, just like Japanese knotweed and leaves are arranged alternately along the stems as well. Red bistort’s stems are usually much thinner, typically less than 1cm in diameter.

Himalayan Balsam (Impatiens glandulifera)

Himalayan Balsam (Impatiens glandulifera)

Himalayan Balsam (Impatiens glandulifera)

This invasive non-native plant grows throughout the year and can reach a height of 2.5 metres, reaching a similar height to Japanese knotweed. The stems are also hollow with leaves arranged in a similar alternate fashion. The leaves are a key differentiator, with Himalayan Balsam’s being much long and thinner than Japanese knotweed’s, with each leaf also featuring a pink mid rib. This plant can often be found growing near Japanese knotweed on watercourses [24], but spreads via seed dispersal as opposed to rhizome translocation.

Broadleaved Dock (Rumex obtusifolius)

Broadleaved Dock (Rumex obtusifolius)

Broadleaved Dock (Rumex obtusifolius)

More commonly known as dock leaf, this plant’s most obvious similarity with knotweed is its titular leaf which is broad and green, however the physical similarities they share ends there. Once fully grown, the leaves themselves are much larger than Japanese knotweed and the plant itself is shorter in stature than knotweed, growing up to 1 metre tall. Although leaves are arranged alternately along stems, they will tend to form in tighter knit rosettes closer to the ground [25], as opposed to the larger bushy formations that Japanese knotweed develops. Stems, whilst being similarly hollow, contain a foamy-pith, and often resemble the Japanese knotweed as it dies back in winter.

Bamboo

Bamboo growing in native Japan

Bamboo growing in native Japan

Bamboos is one of the fastest-growing plants in the world [26], so it’s unsurprising that it’s often confused with the similarly speedy Japanese knotweed, especially when you consider the likeness between the way the plants grow. Like Japanese knotweed, bamboo grows in nodes from difficult to eradicate rhizomes. At closer inspection there are some key differences: although shoots look similar, Japanese knotweed will snap when bent, whereas bamboo will not yield easily. The leaves are also very different, growing up to 50cm long, as opposed to Japanese knotweed’s smaller heart-shaped leaves.

Himalayan Honeysuckle (Leycesteria formosa)

Himalayan Honeysuckle (Leycesteria formosa)

Himalayan Honeysuckle (Leycesteria formosa)

This plant is considered an invasive plant in Australia and New Zealand, whilst being native to the Himalayas and China. It has bamboo-like stems that can be easily snapped, which often leads to it being mistaken for Japanese knotweed. Besides the stems, though, there are many differentiators including the formation of leaves opposite to another along the stem (as opposed to alternating) and a lack of purple speckling along the stems. The flowers form in pendulous racemes, as opposed to spikes, and also feature deep purple berries [27] which makes it much easier to distinguish from Japanese knotweed.

Horsetail (Equisetum arvense)

Horsetail (Equisetum arvense)

Horsetail (Equisetum arvense)

Horsetail will only be mistaken for Japanese knotweed when it is in its infancy. Its shoots can grow at a fast pace in spring and may also appear in a large quantity. These strobili die back after they’ve released their spores, however, and are soon replaced by green stems that are segmented into nodes, similar to Japanese knotweed. As soon as Horsetail’s signature brush-like growth forms it becomes apparent that this plant is very different to Japanese knotweed, however it’s still an invasive plant that should be dealt with [28].

Buckwheat (Fagopyrum esculentum)

Buckwheat (Fagopyrum esculentum)

Buckwheat (Fagopyrum esculentum)

Despite its name, Buckwheat is not wheat or grass. The plant is closely related to plants such as knotweed [29], which explains why it is sometimes confused for it. Buckwheat closely resembles its relative when first shooting. Its stems develop marked nodes and its leaves are also arranged alternately along its stems. The leaves are, however, a much different shape to knotweed, often angular and thin, with the base of the leaf clasped around the stems. The flowers are larger and develop as pink or white clusters at the end of stems.

How do I know if I have Japanese knotweed?

Getting a positive identification of Japanese knotweed can be difficult if you’re unaware of the seasonal changes the plant goes through, or the numerous copycats that it can be mistaken for.

If you have any plant matter on your land that resembles these descriptions or images then it’s worth taking photos and sending them to us using the form on the right. We could help you recover the costs of treatment, especially if you’ve bought a property with Japanese knotweed or if it has entered your property from a neighbouring property.

Write us a message, chat with us, or give us a call on 0151 668 0561.

References

[1] Japanese knotweed and the built environment: Government Response to the Committee’s Seventeenth Report

[2] Japanese Knotweed Profile – RHS

[3] Japanese Knotweed Profile – CABI Invasive Species Compendium

[4] Giant Knotweed Profile – Plantlife

[5] Fallopia x bohemica Profile – CABI Invasive Species Compendium

[6] How is it spread? – Cornwall Council

[7] The Japanese knotweed invasion viewed as a vast unintentional hybridisation experiment – Nature

[8] Japanese Knotweed – Life Cycle & Ecology – Department of Genetics and Genome Biology, Leicester University

[9] PCA Guidance Note on Japanese Knotweed Control: Guidance Notes for Herbicide Treatment

[10] Identification – Department of Genetics and Genome Biology, Leicester University

[11] Giant Knotweed and Hybrids – CABI Invasive Species Compendium

[12] What is Japanese knotweed? – CABI Invasive Species Compendium

[13] How to deal with problems associated with Japanese knotweed (Fallopia Japonica) – PCA

[14] Japanese Knotweed Species Description – NNSS

[15] Giant knotweed identification and control – King County Gov

[16] Prize-winners to pariahs – A history of Japanese Knotweed s.l. (Polygonaceae) in the British Isles – Biology Department, University of Leicester

[17] Japanese, giant and Bohemian knotweed (Fallopia japonica, Fallopia sachalinensis and Fallopia ×bohemica) – Department of Natural History, Savaria Museum

[18] Fallopia x conollyana The Railway-yard Knotweed – Biology Department, University of Leicester

[19] How to Grow Fallopia Russian Vine ‘Mile A Minute Plant’ – Sunday Gardener

[20] Bindweed Profile – RHS

[21] Cornus Profile – RHS

[22] Houttuynia cordata Profile – RHS

[23] Persicaria amplexicaulis ‘Firetail’ – RHS

[24] Himalayan Balsam Profile – RHS

[25] Docks Profile – RHS

[26] The Book of Bamboo

[27] Himalayan Honeysuckle Profile – RHS

[28] Horsetail Profile – RHS

[29] Polygonaceae – Wikipedia

Resources

Japanese knotweed and the built environment: Government Response to the Committee’s Seventeenth Report

RHS Japanese Knotweed Profile

University of Leicester’s Japanese Knotweed Research

Government Guidance on Identifying and Preventing Spread of Japanese Knotweed

Paolo Martini

Paolo Martini is the lead solicitor for Knotweed Help and has over 30 years of experience in the field of Civil Litigation and is an expert on the legal issues faced by individuals dealing with Japanese knotweed on their land. Now considered one of the country's leading litigators in Japanese knotweed law he works alongside the country’s top barristers and experts. His in-depth legal experience and connections to the Japanese knotweed removal industry make him uniquely suited for handling your case.