Japanese Knotweed

What is it?

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

Japanese Knotweed was first introduced to Britain by the Victorians as an ornamental plant, and soon after brought to Ireland to the gardens of the big houses.

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 attractive plant Japanese Knotweed is rated among the 100 worst invasive species in the world by the Global Invasive Species Programme (GISP).

Japanese Knotweed in Full Flower

Why is it bad?

Japanese Knotweed is an invasive plant that is slowly but surely invading Ireland. Japanese Knotweed is one of Ireland’s most unwanted species and it poses both huge environmental and economic threats.

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 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.

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.  The loss of biodiversity from the impacts of this invasive plant is a major concern to us all.

Japanese Knotweed on finding any weak spot can grow through all kinds of hard surfaces, including brick, tarmac and concrete and in some cases has been known to undermine the very foundations of a property.

Loss of biodiversity – heavily infested river banks

Young Shoots of Japanese Knotweed coming up through tarmacadam

Japanese Knotweed growing through tarmacadam

What can I do?

It is important to always to seek professional advice or professional services before tackling Japanese Knotweed

When dealing with Japanese Knotweed it is more of a case of what not to do…

• Do not strim, cut, flail or chip the plants as tiny fragment can regenerate new plants and make the infestation harder to control
• Do not attempt to dig out Japanese Knotweed, this can actually encourage the plant  into growing faster, therefore colonising an area more aggressively
• Do not move or dump soil which may contain plant material as this may also add to its spread.
• Do not attempt to pull the plant out of the ground, as this can expose part of the infectious crowns, stimulating growth
• Do not use unlicensed herbicides close to any watercourses, plants or wildlife
• Do not compost any part of the plant as due to the resilient nature of knotweed it could survive and grow on when the compost is ready for use
• Do not dispose of Japanese Knotweed in garden waste allotments as this just transport the plant to new locations
• Do not spread any soil that has been contaminated with Japanese Knotweed rhizome as new plants will sprout

Do not do break the law – Remember it is an offence if you cause the spread of Japanese Knotweed either intentionally or unintentionally.

Lack of awareness about how the plant spreads has meant that hedge cutting contractors, developers and people in general have been spreading it unwittingly for years.