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Japanese knotweed identification is not always easy; if a potential infestation is ignored, there could be destructive and costly legal consequences. Learn how to recognise and identify Japanese knotweed with our in-depth guide.
We provide a free Japanese knotweed identification service to review photographs of your suspected knotweed plants. Please send us clear images of the plant or leaves and/or flowers and one of our experts will review your photo and confirm whether or not it is Japanese Knotweed.
The following provides comprehensive Japanese knotweed identification guidance. It will help you understand how to recognise Japanese knotweed throughout the year, identify Japanese knotweed characteristics and the different hybrids of Japanese knotweed in the UK. You may also be interested in looking at our detailed guide to plants that look like Japanese Knotweed.
|Flowers||Japanese knotweed has creamy white flowers that appear on panicles, formed of dense clusters of small flowers on thin spikes around 10cm long. Individual knotweed flowers on each spike are around 0.5cm wide. The small creamy white flowers appear very late in the summer and early autumn. Bees often crowd the panicles as they provide a precious source of late-season pollen.|
|Leaves||Japanese knotweed leaves are quite large, around 15cm long by 10cm wide, and are light green in colour. They have smooth edges and a flat base, forming a shield shape, although hybrids have lobes at the base that make them appear more heart-shaped. Leaves are arranged alternately along the stems, as opposed to several similar-looking plants with leaves arranged opposite each other.|
|Stems||Knotweed stems on mature plants are very tall, up to around 3m. They emerge as clumps of apparently discrete stems from ‘crowns’ where the Japanese knotweed roots (rhizomes) poke up above the ground. The base of the stems can be quite thick, around 5cm in diameter, and are light green with purple speckles. Rings, or nodes, around the stems resemble bamboo canes, but unlike bamboo, knotweed has hollow stems that are relatively easy to snap. In winter, the leafless knotweed stems die back and the Japanese knotweed canes are brittle, red-brown or straw-coloured.|
|Roots||Japanese knotweed exists as a perennial network of underground shoots called rhizomes. Thin roots grow from these, supplying the rhizomes with water and nutrients, where starchy energy is stored in the fleshy orange and fibrous tissue. The crowns can be large, often around 40cm in diameter, with thick (often around 3cm in diameter) rhizomes growing from them in all directions. It is often quoted that rhizomes can penetrate 3m into the ground, but this is rare and the majority of rhizomes are usually found less than a metre below the ground.|
|Height||The dense canopies of mature knotweed stands can reach over 3m in height and cover vast areas if allowed to spread.|
|Seeds||Practically all the Japanese knotweed in the UK is a clone of the first plants introduced to the UK in the mid-1800s. These were all female (actually male-sterile). As such, male plants have no pollen to produce viable seeds. However, Russian vine pollen can fertilise the female Japanese knotweed plants, resulting in hybrid seeds. Thankfully, this very rarely results in new plants. The hybrid seeds, called achenes, are black and very small, approximately 2mm in diameter. They look a bit like apple pips cut in half, like tiny buckwheat achenes, to which they are closely related.|
|Origin||Japanese knotweed originates from Japan and nearby parts of Southeast Asia, including Korea and China, where there are several other closely related species. As with many other species, Japanese knotweed was first introduced to the UK by Victorian plant hunters who brought back interesting specimens for botanic gardens and commercial sale. However, it was very quickly realised that knotweed could rapidly grow out of control. By the early 1900s it was not commonly planted anywhere in the UK.|
The most common things that people report to us when they are concerned that they might be affected by Japanese knotweed are as follows:
The above are early indicators that you should look out for if you are worried that knotweed might affect you. Any of these concerns should be reported to a professional to give further advice. However, your first stop should be our Japanese knotweed identification page, which has detailed information on identifying Japanese knotweed to help you determine whether or not the invasive plant affecting your property might be knotweed.
The above are early indicators that you should look out for if you are worried that you might be affected by knotweed. Any of these concerns should be reported to a professional so that further advice can be given. However, your first stop should be our Japanese knotweed identification page, which has lots of detailed information to help you determine whether or not the invasive weed affecting your property might be knotweed.
Young Japanese knotweed stems look very different depending on the maturity of the stand from which they emerge each spring and on what control measures might have affected their development.
From healthy and mature crowns, new shoots appear from thumb-sized buds that are dark red to pink in colour and start showing themselves in late winter. The rapidly extending shoots first look a bit like thick asparagus spears, until the leaves unroll, which happens when the stems reach a height of around 30cm.
When young Japanese knotweed shoots emerge, seemingly without notice, from the ground a few metres from the nearest crowns, they appear as dark red/purple shoots with tightly rolled leaves. This growth arises from rhizome buds underground, which are much smaller than the crown buds.
The small shoots are quite distinctive if you are familiar with them. However, many other plants produce similar shoots (the red colour resulting from protective pigments called anthocyanins) and this can cause alarm to the unwary. The unfurling shoots of peonies are one example of a plant often reported to be knotweed in early spring.
New shoots can appear deformed if Japanese knotweed has previously been treated with herbicides. Micro-leaves and micro-stems often develop when glyphosate is used, producing what’s often referred to as ‘bonsai’ knotweed. Other herbicides can cause twisting and curling of stems and leaves. If crowns have been removed by digging, this can sometimes cause smaller than usual stems to develop.
The variability in the appearance of knotweed often means getting professional help in identifying Japanese knotweed is advisable. This particularly applies if there is evidence of previous ineffective treatment, as this can cause complications for the most effective treatments.
New shoots start to appear in early Spring, around March to April. The rate of growth of Japanese knotweed in spring is rapid. The fleshy, asparagus-like shoots from mature crowns can reach a height of a metre or more within just a few weeks.
If you are interested in foraging, now is the time to collect knotweed stems for eating (Japanese knotweed crumble recipe).
The sudden appearance of the small, dark red/purple shoots from rhizome buds can be alarming, particularly as they can seem to arise from nowhere. The tightly rolled leaves and the colour of the shoots make them particularly distinctive at this time of year.
The way the leaves unfold often results in a pair of almost parallel pale green stripes on the leaves that can persist well into the summer.
From May to September is when Japanese knotweed reveals itself in all its glory.
The stems can reach heights of more than 3 metres, forming a dense canopy of lush, green leaves. The purpose of the stems is to provide temporary scaffolding to support the chemical ‘factories’ in the leaves where they can capture as much light as possible to power food production by fixing CO2 from the air into carbohydrates stored in Japanese knotweed’s rhizomes. As such, no energy is wasted in producing the stems, which are hollow, unlike the permanent, solid stems of woody perennials.
The thicker stems on mature knotweed stands can be 5cm in diameter. Rings, or nodes, around the stems resemble bamboo canes, but unlike bamboo, Japanese knotweed stems can be snapped easily by hand. The purple speckling on the stems also makes them easily identifiable from bamboo.
The zig-zag pattern of alternate leaves and side stems, particularly at the tops of the stems, is another feature that helps with the identification of knotweed. So too is the late flowering. Few plants produce dense stands of 3m tall stems, which flower profusely in September.
Japanese knotweed flowers are often still present in early October at the start of autumn. Some of these will have developed into seeds (achenes – from the pollen of Russian vine, as there are no male Japanese knotweed plants in the UK).
The small creamy white flowers and seeds are arranged in conspicuous panicles. The flowers and seeds are shed by the end of October. Japanese knotweed leaves start to turn yellow and then brown about this time. However, if there have been no frosts, leaves can remain on some plants into December, particularly if they are protected from winds.
The stems also start to die and become brittle, turning a red-brown or straw colour.
Once the leaves have fallen, all that remains are the dead stems, which can persist for a few years. However, as they are quite fragile and brittle, they are usually toppled by the weather or are pushed over as new stems emerge the following spring. As most other non-woody vegetation dies back, knotweed stems, particularly where they appear as dense stands, can be easier to spot in the winter. In particularly mild winters, and in areas where plants are shielded from the weather, some knotweed stems have been seen in a relatively healthy, green state as late in the season as February.
The best time to spot Japanese knotweed is during mid-summer and early autumn. During spring, reddish/purple shoots appear from the ground and fat, asparagus-like ‘spears’ rapidly lengthen from bright pink ‘crown’ buds. These can grow up to 2cms a day, thus rapidly forming dense stands of bamboo-like stems that develop dark green heart- or shield-shaped leaves.
By early summer the mature Japanese knotweed stems are hollow with purple speckles and can reach up to 3 metres in height. The leaves alternate along each side of the stem, producing an obvious knotweed zigzag pattern.
The Japanese knotweed flowers that emerge by late summer are creamy-white in colour and appear in lengthy cluster/spike formations. Japanese knotweed spreads mainly from its underground rhizomes/roots which lie dormant, but alive, over the winter months.
Japanese knotweed rhizomes can spread several metres outwards from the visible, above-ground stems, and to depths of more than a metre. It is therefore very easy to accidentally fragment pieces of rhizome and spread them by disturbing the soil several metres from where the stems appear.
As new growth from seeds is very rare, it is testament to the Japanese knotweed’s incredible invasiveness that it has spread to most parts of the UK (and many parts of western Europe and North America, for that matter) simply through the fragmentation and translocation of rhizomes in contaminated soil.
Japanese knotweed is a rhizomatous plant. This means that it relies on a fleshy network of underground shoots to store energy and spread through the ground. This is a survival strategy carefully designed by nature to allow plants to live on year after year, hedging their bets when conditions for growth are poor, such as during abnormally dry seasons, so that the energy stores sustain the plants until conditions improve.
In this way, the rhizomes can be considered analogous to a seed bank in the soil, where it is another survival strategy of other plants to produce seeds that can lie dormant in the ground during seasons that are unfavourable for germination. Both rhizome and seed survival strategies allow some plants to remain dormant in the ground for many years – decades in some cases.
Japanese knotweed produces particularly large rhizome networks in the ground, which often spread a few metres outwards from the crowns. In the right conditions, the spread can be much further.
Japanese knotweed rhizomes are also renowned for the depth in the ground that they can penetrate, which is often a metre and sometimes much deeper (3m is often quoted in guidance). In Japan, knotweed spread by extending rhizomes causing rings of crowns to appear after decades of growth. This happens as above-ground shoots appear on the edge of the extending growth and the older, central, crowns die back. These ‘ramets’ of stems on the edge of the rings eventually separate from the parent growth, forming new ‘daughter’ populations of knotweed plants. This is a common form of reproduction and spread in rhizomatous plants.
In the right conditions, Japanese knotweed grows very quickly. However, where physical boundaries and competition with more established plants are present, knotweed can appear relatively well-behaved and remain ‘camped on the doorstep’ of a property until or unless conditions change, allowing it to proliferate. This can happen if Japanese knotweed that is growing along a dense hedge is given its freedom when the competing hedge or trees are cut down.
Japanese knotweed is not fussy. It is very happy in an extensive range of soil conditions. This is why it is so ubiquitous across the UK. It thrives particularly well in marginal land, where a lack of management fails to keep it in check. Conversely, various written sources state it is widespread along railways and waterways where it has been spread by uncontrolled vegetation mowing and clearance works.
Japanese knotweed can also hybridise with its related species. The most common of these hybrids is that of Japanese knotweed and giant knotweed.
The hybrid knotweed then has the ability to spread by seed, which Japanese knotweed was lacking during its early introduction due to the absence of any male plants in the United Kingdom. This is a very important distinction because Japanese knotweed plants usually only reproduce from cut pieces of stem and rhizome, whereas hybrid knotweed plants can also, potentially, reproduce from seed, which can remain dormant in the soil over successive growing seasons as a ‘seed bank’.
Although Japanese knotweed hybrids more commonly reproduce vegetatively from rhizome fragments, the potential risk of viable seed being present from hybrids can mean that in order to remove hybrid knotweed from a site:
Japanese knotweed also produces hybrid seed from the pollen of the closely related and common garden ornamental plant, Russian vine, which is also known as ‘mile-a-minute’ and Bukhara fleece flower. Luckily, this seed very rarely develops into viable plants, which is probably due to it being particularly sensitive to the relatively mild and wet winters we experience in the UK and possibly because the seedlings appear quite fragile.
The Russian vine hybrid is known as railway-yard knotweed, which was named after Ann Conolly of Leicester University who, along with John Bailey (who registered the name as a birthday surprise for Ann), was a pioneer researcher on UK knotweed.
Giant knotweed is native to the island of Sakhalin which is just north of Japan but is actually part of Russia. As with Japanese knotweed it was brought to Europe to be grown in botanical gardens. It has also escaped into the wild where it is spreading. It is similar to Japanese knotweed in many respects but is larger, growing over 4m high and having leaves around 20-40cm long. Giant knotweed also has flowers that are greener in colour and leaves that are more rounded at the base than Japanese knotweed. The leaves also have scattered hairs (trichomes) on the undersides.
The hybrid plant grows slightly larger than Japanese knotweed and has slightly larger leaves but is smaller than giant knotweed. The leaves also have a pointed tip and a slightly rounded base, which makes the leaves appear more heart-shaped than its Japanese knotweed parent.
The table below details some of the more obvious leaf features that can be used as a rough guide to identify whether a plant is hybrid knotweed or Japanese knotweed.
You can read more about these and plants that look like knotweed on our Plants that are Mistaken for Japanese knotweed page.
If you are still unsure as to whether you might have an infestation of Japanese knotweed on your property, please send us a picture for your free assessment below.
Alternatively, if you have an infestation then we offer a range of Japanese knotweed surveys and Japanese knotweed removal services – you can also learn more about the cost of removing Japanese knotweed. It might also be a good idea to familiarise yourself with the various legal issues associated with knotweed and learn more about our expert witness services.
Feel free to contact us today to speak with one of our expert consultants who can help with any of your Japanese knotweed identification or treatment concerns.