August to October
It is found throughout the country.
For a map see the National Biodiversity Network Gateway
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.
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
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
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 of 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. This statement needs correcting
as a single plant has grown in the treated area in 2008 and was
flowering in October, sometime after the plants in the untreated areas
had ceased flowering.
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 informatin on this and other
research see The Japanese Knotweed Alliance
Japanese Knotweed rampant
Flower spikes, red zigzag stem and leaves
Close-up of flower spike
You have been warned!
This could happen to you