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Japanese Knotweed

Introduction to Japanese Knotweed

Japanese Knotweed is native to Japan, Taiwan and China; it was introduced into Britain in the mid 1800s as an ornamental plant. Since then it has been establishing itself in this country and is now a threat to our native species. It will grow in any soil type, no matter how poor. The most prominent areas that it is found are places where it is easily transported and has little attention, ie roadsides, railway lines, riverbanks or derelict sites.

Japanese Knotweed is spread by vegetative means, by rhizomes (roots), or by crown or stem segments. The main reason for this spread is due to man's activities such as tipping garden waste, mowing, flailing or even fly-tipping and the erosion of riverbanks where the weed is taken downstream and established elsewhere.

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Description of Japanese Knotweed

Japanese Knotweed is a large invasive perennial weed that appears to have no natural enemies in Britain. It forms thick bamboo like canes during the summer months that die back in the autumn. The canes have a reddy/pinky fleck in them which is prominent in the summer. The shoots appear red/purple in early spring growing into hollow canes that in turn produce large, flat, oval, green leaves, which appear in a zig-zag pattern along the stem. Growth is rapid, up to 2cm a day, and the canes can reach 3 metres in height. Flowering occurs in late summer-early autumn and is recognised by clusters of creamy white flowers. (For pictures of Japanese Knotweed see the external links below).

An extensive underground root (rhizome) network can expand several meters depending on ground conditions, a fragment of root as small as 0.8 grams can grow to form a new plant. Japanese Knotweed is regarded as a troublesome pest in many parts of the country because of its rapid invasion and domination of habitats, resulting in the exclusion of other plants; many insects that are dependent on our native plants are also lost.

Japanese knotweed can also cause damage to property, increase riverbed erosion and create a flood hazard.

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The 'Wildlife and Countryside Act 1981' made it an offence 'to plant or otherwise encourage' the growth of Japanese Knotweed.

The 'Environmental Protection Act 1990' classed discarded weed or soil which contained rhizomes as 'controlled waste' to which the Duty of Care applies.

Soil containing rhizomes can be regarded as contaminated land and as such has to be disposed of at a suitably licensed landfill site. When disposed of, this must be down to a depth of at least 5 metres; it must not be reused in further construction or for landscaping purposes. When disposing of the material the landfill operator needs to be aware of the presence of the Knotweed to ensure it is buried to depth. See also contaminated land.

On external websites:

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This lies with the landowner or the tenant of the land. The local Council and Environment Agency are not obliged to control Knotweed on behalf of other landowners. If the Knotweed is affecting your land the best solution is to co-operate with your neighbours to try and control the problem together, by sharing costs or labour, for instance. Wherever possible it is best to encourage co-operation and support within the community to control and prevent any further spread of the weed.

Japanese Knotweed must not be composted!

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Removal of Japanese Knotweed

Japanese Knotweed is susceptible to certain herbicides including glyphosate, this can be found in the common weed killer 'Roundup biactive'. Other chemicals that are also effective include triclopyr, imazapyr, and picloram. Glyphosate is a translocated herbicide, which means the weed takes the herbicide down into its rhizome (roots). To ensure the most effective treatment the herbicide should be applied in the growing season to the green leaves carefully. The greater number of green leaves treated, the larger the quantity of herbicide that can be absorbed into the plant. It will take at least three years of herbicide treatment before the Knotweed has been fully eradicated. However, treatment should continue until no further growth appears.

A quicker method of removing Japanese Knotweed involves the clearing of above ground leaf/stem material and the removal of ground material polluted with roots. Care should be taken to ensure that all Japanese Knotweed roots are removed. Even with great care, a certain amount of re-growth in the spring would be expected and should be treated with an appropriate herbicide as discussed above.

Cutting, mowing or grazing of the weed gradually weakens it but this method may take up to 10 years to be effective. Cutting should be undertaken at least once a month during the growing season. A strimmer, mower (without a collection bucket) or chipper should never be used on Japanese Knotweed material. Pulling is more effective because it removes the crown and some of the rhizome.

Japanese Knotweed must not be composted!

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Further information and pictures of Japanese Knotweed

Further information and pictures of Japanese Knotweed can be obtained from the following websites and publications:

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