Knotweed, Japanese

Information on Japanese Knotweed

Common Name: Japanese Knotweed
Scientific Name: Reynoutria japonica
Irish Name: Gliúneach bhiorach
Family Group: Polygonaceae
Distribution: View Map (Courtesy of the BSBI)
Flowering Period

Click for list of all flowering by month
Japanese Knotweed could sometimes be confused with:

Knotweed, Lesser, Knotweed, Himalayan,

This tall, vigorous perennial is a well-known sight throughout the country on roadsides, waste ground, edges of woods and along waterways.  Reaching a height of 2 metres, it has large, stalked, triangular leaves on reddish, stout stems which, from August to October, carry loose clusters of small white flowers. These 5-petalled flowers are extremely pretty when viewed through a hand lens. Sadly this plant is not one which is regarded in any other way than as an invasive alien. It was introduced into Europe as an ornamental plant in the early 19th century and it grows most vigorously, its main and extremely successful method of reproduction being by creeping rhizomes. Very quick to colonise all types of soil, even sprouting through cracks in concrete, it is extremely difficult to eradicate.  It only requires a very small part of its root for it to regenerate. Unfortunately its successful colonising of many habitats is at the expense of some of our native wildflowers. Formerly known as Fallopia japonica, it is obviously not a native plant and it belongs to the family Polygonaceae.  

I first saw this plant growing along a stream at Derrynane, Co Kerry in 1977 and thought it the beautiful, graceful plant, which it is.  I photographed it in 2006 in Co Wexford.   

If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre

There are no legal provisions which protect our flora from the spread of Japanese Knotweed.  However the Wildlife (Amendment) Act 2000 states that 'anyone who plants or otherwise causes to grow in the wild – in any place in the State any species of (exotic) flora, or the flowers, roots, seeds or spores of (exotic) flora shall be guilty of an offence'.

Under the Wildlife (Northern Ireland) Order 1985, it is 'an offence to plant or otherwise cause it to grow in the wild'.  

Knotweed, Japanese
Knotweed, Japanese
Knotweed, Japanese
Knotweed, Japanese