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Japanese Knotweed Internal Working Group- Frequently Asked Questions and Facts

Native to Japan and China Japanese Knotweed was introduced to Europe as an ornamental garden plant in the 19th century. Unfortunately it very quickly established itself in the wild and spread rapidly and is now considered to be one of our most problematic invasive species. It is a common sight on waste ground and along waterways and roads, where it takes over and competes with native species.

What does it look like?
It is a green shrub with a hollow stem, reaching about three meters in height and grows in dense thickets. It has a small creamy white flower in late summer and dies back in winter leaving brown cane like stems on the ground.
Identification sheets can be downloaded at
http://www.biodiversityireland.ie/wordpress/wp-content/uploads/Japanese-...

Why does it cause problems?

  • It has a vigorous growth rate, forming tall thickets which shade out the areas below which leads to the loss of other vegetation including our native plants.
  • It can grow through concrete and tarmac if it finds a weak spot and cause dangerous and expensive structural damage to roads and buildings
  • Along rivers it causes erosion of the riverbanks when it dies back during the winter and can affect flood defence structures and create flood risks.
  • It can create difficult driving conditions with the denseness of the thicket on roadside verges

What to do?
The most important thing to do if you discover Japanese Knotweed is NOT to spread it.

  • Do NOT cut, mow or strim and discard any part of the plant on the ground, as this will cause it to grow and spread
  • Do NOT dig it out of the ground and break-up the rhizome system unless it is part of controlled deep excavation works
  • Do NOT compost cut knotweeds as they will grow from this.
  • Do NOT dig, move or dump soil which may contain Japanese Knotweed plant material as this will also cause it to spread.

Japanese Knotweed can be eradicated effectively by the use of an appropriate herbicide applied by a competent person, however the process requires follow up treatment and the disposal of the plant material and treatment of contaminated soil needs also to be considered and requires a license.
Please report sightings to the National Biodiversity Data Centre: www.biodiversityireland.ie/projects/invasive-species/submit-sightings/ Once the sighting has been verified, it will be made available online through Biodiversity Maps for all to access.

The National Biodiversity Data Centre have produced a list of FAQ’s on Japanese knotweed that can be found here.

Tuesday, 21 November, 2017 - 12:45
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