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Flowering Japanese Knotweed

Japanese Knotweed Identification

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.

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What Does Japanese Knotweed Look Like?

Japanese Knotweed with Flowers
FlowersJapanese 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.
LeavesJapanese 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.
StemsKnotweed 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.
RootsJapanese 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.
HeightThe dense canopies of mature knotweed stands can reach over 3m in height and cover vast areas if allowed to spread.
SeedsPractically 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.
OriginJapanese 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.

When is the Best Time to Identify Japanese Knotweed?

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.

Where does Japanese Knotweed Grow?

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.

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.

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