Japanese knotweed

What is Japanese knotweed?

Japanese knotweed, (scientific name Fallopia japonica), also known as donkey rhubarb, was introduced to Britain from the Far East in 1825 as an ornamental plant.

Why is it a problem?

It is the most invasive exotic plant in the British Isles.  It spreads rapidly, especially on waste ground and next to streams and rivers, forming large stands that shade out and inhibit native plants, and leaves bare ground beneath which damages natural habitats and leaves riverbanks prone to increased erosion.  It can also be seen in gardens and on derelict areas and can pose a problem to property developers due to structural damage to buildings caused by its roots and stems. 

The main problem is that it spreads easily through fragments of the roots (rhizomes), transported in rivers, in contaminated topsoil or by stem cuttings in garden waste.  Knotweed can grow in almost any habitat, and once established, it is very difficult to control.  For this reason, under section 14 of the Wildlife Act 1990 it is an offence to plant Japanese knotweed ‘or otherwise cause it to grow’ in the wild.

What does Japanese Knotweed look like?

In spring, red shoots appear with rolled up reddish purple leaves. The plant grows rapidly, up to 10cm a day, and the leaves unfurl, becoming lime green and later darkening to mid green colour. The stems elongate and look similar to bamboo, as they are hollow with prominent nodes. However knotweed stems tend to zigzag and possess reddish-purple speckles. Each leaf is carried on a short stem at a different level from the next leaf below or above. In summer the mature plant reaches up to 3m in height. Clusters of creamy white flowers are produced in late summer/early autumn.

In winter, the plant dies back leaving woody stems that turn dark brown. These can persist for 3 years and prevent the growth of native plants by covering the ground with dense litter.

The stem (right) has a characteristic red speckled pattern. Shoots (left) appear from nodes on the stem.

If you are unsure whether you have Japanese knotweed send a photograph of the plant to ecopolicy@gov.im for identification.

Note: Unlike giant hogweed (heracleum mantegazzianum), Japanese knotweed is not known to be harmful to humans.

How does Japanese knotweed spread?

Japanese knotweed has extensive, deep roots called rhizomes. These can be up to 3m deep and can extend out to 7m from the parent plant. Think of the plant as an iceberg, with a third of the plant above ground and two thirds underground, with an ability to spread. 

Knotweed does not normally spread by seeds. However it can grow from cut stems, crowns or rhizomes:

• Rhizome fragments of 1cm (0.7g) can sprout a new plant;
• Stem cuttings from mowing, flailing, or strimming can re-grow and establish new plants;
• Crowns can withstand drying and composting to sprout new red buds and create new plants.

This plant easily spreads to other areas if cut stems, shoots, crowns or roots (rhizomes) are moved

DO

• Check your land for the presence of knotweed, especially along watercourses;
• Treat and control knotweed on your land using established control methods (see below);
• Report knotweed to the relevant landowners, if possible.
• If landowners are unknown, report knotweed to the Department of Environment, Food and Agriculture (DEFA) using the contact details found below. Precise location details including grid references and annotated maps are encouraged.

DON’T

• Take to amenity sites with other green waste;
• Tip green waste on verges, riversides, cliffs or derelict ground;
• Cut and spread knotweed cuttings or chippings from affected areas;
• Move or spread topsoil from knotweed contaminated sites;
• Move any parts of the plant (including dead-looking or uprooted stems) any more than is absolutely necessary – it is incredibly easy to spread this plant.  

For further information, contact: