Japanese knotweed

Japanese knotweed facts

Scientific name: Reynoutria japonica

Family: Polygonaceae

Native to: Eastern Asia

Type: Herbaceous perennial

Predators/herbivores: Deer, rabbits, squirrels, birds, rodents and groundhogs. However, it is not a main part of their diet and their grazing will only limit areas of spreading, not eradicate it completely.

Location: Along canal banks and near man-made structures like roads, railways and waste disposal areas.

Japanese knotweed in Britain

Originally native to Japan, Taiwan and China, Japanese knotweed was introduced to Europe as an ornamental plant in the 19th century.

It is now common throughout the UK, but cannot be cut or manually removed because of its ability to grow from tiny fragments smaller than a one pence piece.

This weed is now listed under Schedule 9 of the Wildlife and Countryside Act 1981, meaning it is illegal to cause it to grow in the wild.

Why is Japanese knotweed a problem for canals and native wildlife?

Japanese knotweed is an incredibly persistent plant. It strong roots can grow through tarmac and concrete, breaking the surface open. This has the potential to cause serious damage to our structures like towpaths and bridges, and historical relics such as pillboxes.

Its dense and rapid growth shades out native plans, harming natural habitats for species like coots, moorhens and the endangered water vole.

Insects, including dragonflies and bees, are also left without suitable spaces to forage.

What does Japanese knotweed look like?

Japanese knotweed has a thick, bamboo-like stem, which is mainly green with purple speckles. Dense leaves shaped like spades grow from distinctive zig-zag stems, which are fully purple in colour. The leaves die back in the autumn, but the plant's main stem remains stiff and upright throughout the year, turning brown and brittle over winter.

In summer, the plant produces large masses of creamy-white flowers, which grow to about 10cm in length. These are not believed to produce viable seeds.

If you come across what looks like thick, orange-tinted roots in gravelly areas, these could be Japanese knotweed rhizomes. Rhizomes are underground plant stems that are able to produce roots and shoots from their nodes, which is how this weed is able spread with power and speed. Any rhizome or root belonging to Japanese knotweed has to be removed and disposed of by a licensed professional.

How do you remove Japanese knotweed?

Mowing this weed would cause its fragments to become airborne and settle elsewhere, causing further spread. It is best controlled by using approved herbicides.

How to report invasive species

If you see an invasive, non-native plant species on one of our canals or rivers, please contact us online or call us on 0303 040 4040 and we'll check if it is already included in our treatment schedule.

How you can help prevent the spread of invasive species

Various invasive plant and animal species can be transported on equipment such as paddleboards and oars, and wet clothing including footwear.

Whether you use our canals and rivers for work or pleasure, please remember to Check, Clean, Dry.

These three things should always be done before you leave a waterway, even if you are moving to another spot on the same stretch just a few miles away.

Other invasive species on our canals