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Japanese Knotweed Identification

The first rule of Japanese knotweed elimination is to be able to recognise whether or not you have it present and active in your location. Flagging up the problem correctly is the starting point in any war raged on the notoriously territory-grasping weed, a real villain of modern horticultural times. The danger of Japanese knotweed to Britain’s homes and wildlife is clear and ever present, but the successful identification of Japanese knotweed paves the way to effectively tackling the problem and ultimately defeating the enemy within many of our gardens and countryside.

This is where we come in. Essentially, the identification process involves literally getting to the root of the potential problem, starting by establishing whether the unidentified flora object is the nasty Japanese knotweed or not. The identification process is performed by trained professionals carefully combing the location in search of the plant’s large green oval-shaped leaves and bamboo-type stem. Although the Japanese knotweed is nothing if not a master of disguise, taking on somewhat differing guises and characteristics throughout the calendar year, which certainly keeps us on our toes as we set about making a positive ID. This is why it needs to be done by the professionals like us who know what we’re talking about. Not everyone out there who claims to do this service is legitimate.

How do I identify Japanese knotweed?

Choose one of the below three options.

Leaves

Japanese knotweed leaves are bright green and heart shaped with a smooth, rounded edge, flattened base and pointed tip. The leaves grow from the plant’s stems in an alternating pattern that gives Japanese knotweed branches their distinctive “zig-zag” shape. The leaves generally have pronounced veins and mid-rib, and have a deep red or pink coloured stalk.

The leaves of mature Japanese knotweed plants can grow up to 120mm in length, but other species of Knotweed can grow similar leaves at sizes up to 400mm long.

Early leaf growth in spring can be a deep red colour or light green (sometimes with red spots) but once the leaves have fully opened – often after only a few days of growth – they take on a distinctive bright green shade which they keep until the leaves die back later in the year.

During late autumn or winter, the leaves die back and they don’t re-grow from the same stems the next year, instead forming new stems every spring.

Identify

  • Young Leaves

    Deep red colour or light green (sometimes with red spots) but once the leaves have fully opened – often after only a few days of growth.

  • Mature leaves

    Bright green in colour, the leaves of the mature plant are up to 120mm in length with a flattened base and pointed tip. Leaves and stems have developed a zig-zag pattern.

Flowers

In high summer, Japanese knotweed plants develop many small clusters of creamy-white flowers. Each plant can have dozens of clusters, each of which is made up of many small flowers. The clusters of flowers grow from the point where the leaf joins the stem (the axil). The flowers can look white or pale yellow from a distance, and give Japanese knotweed plants a very distinctive and attractive appearance at this time of year.

Generally, Japanese knotweed plants do not produce viable seeds in the UK – although it is possible for Japanese knotweed to cross-breed with other plants, including other knotweed species. You should still avoid spreading any seeds produced by a Japanese knotweed plant.

Identify

  • The clusters of flowers grow from the point where the leaf joins the stem

  • In summer, each plant can have dozens of clusters of creamy white flowers.

Roots

The underground “roots” of Japanese knotweed are made up of a tree-like structure used to store the plant’s energy reserves. This structure is called a “rhizome”, and grows thick and woody, actually building up in rings, like a tree. Live rhizomes can be snapped like a carrot, and will show a light orange colour inside.

It is the rhizome which causes the greatest problems related to Japanese knotweed. Structures including walls, pavements, hard surfaces and even building foundations can be displaced or damaged by the rhizomes, and the “crowns” – which are large round growths that form at the base of the stems over many years of growth.

Japanese knotweed rhizomes can grow up to three metres deep and extend up to seven metres sideways from the surface growth. Japanese knotweed rhizomes can re-grow into new plants, even from very tiny fragments. It has been documented that a piece as small as 10mm in length, or 0.7g of rhizome can successfully grow into a new plant.

If you are trying to identify Japanese knotweed by samples of root, it is important to make sure that you leave any samples in the areas that you find them, and that you don’t spread any plant material to other areas.

Identify

  • The root system can grow up to three metres deep and extend up to seven metres sideways from the surface growth.

  • Rhizome grows thick and woody. Live rhizomes can be snapped like a carrot, and will show a light orange colour inside.

What Next?

I believe I have a Japanese knotweed infestation.
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