Japanese Knotweed

Japanese Knotweed (sometimes spelt Japanese Knot weed) is a non-native, alien invasive plant species, originally from Japan & Northern China and it was first introduced to Europe in the 19th Century.

Japanese Knotweed was first introduced to Britain by the Victorians as an ornamental plant – and was actually awarded a gold medal at a prestigious flower show. In a gardening publication in 1907 it is cited as “easier to plant than to get rid of the garden” (The English Flower Garden)


Japanese Knotweed - fast growing perennial

This fast growing perennial can reach 2 to 3 metres in height during the summer. Its hollow, gnarled stems are similar to those of bamboo, thus the descriptions of Japanese bamboo, or Mexican Bamboo are sometimes attributed to it.

The leaves are heart shaped, with a flat bottomed edge and a lush green colour, in summer it produces an abundance of cream white flowers and at first glance these striking blooms are enough to win over any gardener- but do not be deceived, this beautiful plant Japanese Knotweed is rated among the 100 worst invasive species in the world by the Global Invasive Species Programme (GISP).

Japanese Knotweed is a robust, herbaceous perennial that is extremely invasive. It has deeply penetrating woody rhizomes and bamboo like stems. These rhizomes according to UK Environmental Agency Guidelines can extend laterally for 7 metres and vertically 3 metres away from the parent plant, likewise the bamboo like stems develop into dense stands and can grow to 3 metres tall. Japanese Knotweed can grow to 2-3 metres in just 10 weeks in the growing season. Only female plants are present in Ireland and most seeds produced are unproductive.

The principle means of spread of Japanese Knotweed is via fragmentation of stems and rhizomes and the plants very strong resilient underground rhizome growth.

Japanese Knotweed - rhizomes
Japanese Knotweed - In riparian areas

In riparian areas (the interface between land and a river or stream) high water flows carry fragments of the plant downstream where new colonies quickly form. Japanese Knotweed thrives on disturbance, the tiniest piece can regrow, in the past fly tipping and transportation of soil containing rhizome fragments have been a major cause of spread in both the urban and rural environments

This invasive plant grows in a variety of soils including silt, loam and sand. Japanese Knotweed can tolerate adverse conditions including frost, full shade, high temperatures, drought and flooding.

In its native Japan & Northern China Japanese Knotweed presents nowhere near the problem it now poses across Ireland & Britain. With its natural habitat being on the slopes of volcanoes it is no surprise that our less harsh and more fertile environments has allowed this plant to flourish to extreme proportions.

Additionally outside of its natural habitat of Japan and Northern China the plant has no biological enemies to check its spread, in Japan for example at least 30 species of insect and 6 species of fungi live on the plant.

Japanese Knotweed In its native Japan & Northern China

Japanese Knotweed is now very common and widely distributed across a variety of habitat types in Ireland – It is most prominent on roadsides, hedgerows, railways, waste ground, river banks and wetland habitats due to its vigorous growth rate.

It quickly forms tall stands shading out the areas below it, threatening the survival of native plant species and in turn insects and other animal species

By establishing itself on river banks the Japanes Knotweed plant can affect flood structures and increase flood risks.

Japanese Knotweed can prove a driving hazard as it establishes itself on hedgerows blocking sightlines and damaging road surfaces, it can also grow through concrete and tarmac causing dangerous and expensive structural damage.

Eradication of Japanese Knotweed once established can be difficult and costly. The removal and disposal of the plant from the 10 acre 2012 Olympic site in East London is reported to have cost the British Government £88 million.


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