Japanese Knotweed
Latin name
Reynoutria japonica
Other names
Fallopia japonica
Family
Polygonaceae
Flowering season
August to October
English distribution
It is found throughout the country.
See the BSBI distribution map for Japanese Knotweed
Habitat
It is a neophyte which was originally grown as a garden
plant in the 1820s and first recorded in the wild in the
1880s. It has increased markedly since then.
It is found on waste ground, brownfield sites and
rubbish tips, on the edge of woods and beside paths,
roads, railways, canals, streams and rivers.
Description
Japanese Knotweed is a persistent, rhizomatous
perennial herb, which can form dense thickets of up to
2m when growing on its own or much higher and
extensively when growing through and over other
shrubs.
Flowers are in dense spikes.
Each flower has 5 creamy, petaloid tepals, 8 stamens
and 3 divided stigmas.
The stems are green or red and grow as zigzags.
The leaves are large, broad and oval.
The flowers form decorative creamy white spikes.
They are visited by large numbers of insects in the
late summer months.
It is spread by rhizome fragments thrown out in garden
and other rubbish, and by river floods.
An alien invasion
Japanese Knotweed is classed as an alien, invasive and
aggressive weed. It is listed under Schedule 9 to the
Wildlife and Countryside Act 1981 with respect to
England, Wales and Scotland. It is an offence to plant
or otherwise cause it to grow in the wild.
Under the Environment Protection Act 1990, Japanese
Knotweed is classified as controlled waste. For details
of legislation go to www.nonnativespecies.org/legislation.
Defra, in the guise of the NSSS (GB Non-native Species
Secretariat) is responsible for dealing with invasive,
non-native plants: see Japanese Knotweed
It has infected several areas around Lancaster, especially
at the west end quay on the River Lune and around
Freeman’s wood.These areas were due to be developed
and an extensive scheme of Japanese Knotweed
eradication was begun in 2005.
The picture shown of the triffid-like plants was taken
the following year. Since then, it has not reappeared
in the treated areas although it is still rampant in
untreated areas.Initially, a large number of other
wild flowers were also eliminated, but many plants,
especially ruderals such as Evening Primrose,
Great Mullein, docks and a large range of legumes,
recolonised the treated areas.
In July 2009 Defra was considering the release of a
Japanese phloem-feeding insect to control this
invasive plant. In 2010 permission to use a psyllid,
Aphalara itadori, was granted on 2 sites in England.
For up-to-date information on this and other
research see The Japanese Knotweed Alliance
Previous page: Japanese Anemone
Next page: Jersey Lily
Japanese Knotweed rampant
Flower spikes, red zigzag stem and leaves
Close-up of flower spike
Stamens
Stigmas
Feeding insect
You have been warned!
This could happen to you