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Identifying Japanese Knotweed

How to Identify Japanese Knotweed

Japanese knotweed identification is not always easy; if a potential infestation is ignored, there could be destructive and costly legal consequences.

We provide a free 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.

You may also be interested in taking a look at our guide to plants that look like Japanese Knotweed.

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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 strands 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.

Below are some images and descriptions of the main features of Japanese knotweed that should help you identify it. You can click on each thumbnail image to make it bigger.

Japanese Knotweed Hybrids

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:

  • Herbicide treatments might be required over longer periods;
  • Pre-emergent herbicides might need to be incorporated into the treatment programme; and/or
  • Plants could be reintroduced back onto the site after treatment from windblown seed from hybrids in the surrounding area.

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.

Japanese knotweed leaf

  • lighter green
  • smooth edges
  • flat base forming shield-shaped leaf
  • mature leaves are smaller than those of hybrid knotweed
  • no trichomes (hairs) on the undersides of the leaves

Hybrid knotweed leaf

  • darker green
  • crinkled edges
  • lobes at base forming heart-shaped leaf
  • mature leaves are larger than those of Japanese knotweed
  • short trichomes (hairs) on the undersides of the leaves

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