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Identifying 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.

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What Does Japanese Knotweed Look Like?

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FlowersFlowers appear on panicles, which are formed of dense clusters of small flowers on thin spikes around 10cm long. Individual flowers on each spike are around 0.5cm wide. They are creamy white in colour and appear very late in the summer and early autumn. The panicles are often crowded by bees as they provide a precious source of late season pollen.
LeavesLeaves 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 a number of similar looking plants that have leaves arranged opposite each other.
StemsStems on mature plants are very tall, up to around 3m. They emerge as clumps of apparently discrete stems from ‘crowns’ where the 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 give them the appearance of bamboo, but unlike bamboo, knotweed stems are relatively easy to snap, whereupon it can be seen that they are complexly hollow. In winter, the leafless stems die back to leave brittle, red-brown or straw-coloured canes.
RootsA bit like fungi – which live in the soil as a network of threads (hyphae) only revealing their presence to produce spores from the fruiting bodies that appear as mushrooms and toadstools – 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 direction. 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 meter below the ground.
HeightThe dense canopies of mature stands of knotweed can reach over 3m in height and cover very large 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, there is no pollen from male plants to produce viable seed. However, Russian vine pollen can fertilise the female knotweed plants, resulting in hybrid seed. Thankfully, this almost never 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, or very much like tiny buckwheat achenes, which they are closely related to.
OriginJapanese knotweed originates from Japan and nearby parts of Southeast Asia, including Korea and China, where there are a number of 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 Spot 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 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 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.

The rhizomes can spread several metres outwards from the visible, aboveground 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 plant’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 its native range in Japan, knotweed can appear quite different to how it appears in the UK. There are a number of reasons for this, not least is the fact that the majority of UK knotweed plants are clones of the original few introductions in the 1800s. There is therefore much greater genetic diversity in the knotweed populations in Japan. There are also many environmental and biological factors that alter knotweed’s behaviour there. This includes the formation of rings of knotweed, where stems stop growing in the centre of expanding stems, giving the stands the appearance of giant donuts.

In the right conditions, knotweed is capable of very quickly taking over. 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 rapidly grow. This can happen if knotweed that is growing along a dense hedge is given its freedom when the competing hedge or trees are cut down.

Knotweed is not fussy. It is very happy in a very wide 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 literature sources state it is particularly common 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 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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