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Herbicide treatment for Japanese knotweed

Fortunately, Japanese knotweed is susceptible to a range of herbicides including glyphosate, the active ingredient in products such as 'Roundup biactive' and 'Glyphos biactive'. Whilst chemicals such as triclopyr and picloram are also effective against knotweed, glyphosate has many properties which make it more suitable for use by householders. Many formulations of glyphosate, are suitable for use in or near water, the product is deactivated by micro-organisms in soil, it doesn't leach and it possesses low toxicity to animals. The biactive formulations of glyphosate are generally regarded as the most suitable.

Glyphosate is a translocated herbicide, which means the plant carries the herbicide down to its rhizome. Contact herbicides may appear to kill the leaves and shoots of knotweed, but unless the herbicide is translocated down to the rhizome the plant will regrow.

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Knotweed survives the winter as an underground rhizome. During autumn the plant draws food stores from the dying leaves and canes, down to the rhizome. Herbicide treatment during late summer and autumn appears to be more effective, as translocation is assisted by this process. If treating mature canes in autumn is likely to cause problems, earlier applications of herbicide during spring and summer can be used to control its growth. However, it is essential that there are plenty of green leaves to absorb the herbicide before application is undertaken.

Glyphosate is usually sprayed onto the foliage of knotweed. Beware of drift on to non-target plants and lawns. If you are concerned about the risk to other plants, use a weed wiper to apply the herbicide instead of a spray.

Great care needs to be exercised when using herbicides near lawns and shrubs. There are no sprays which are specific to knotweed, but products such as 2,4 D amine can be used at concentrations that do not harm grass. You must contact your local Environment Agency office before using a herbicide in or near a stream or river, a pond that discharges to a watercourse or a pond that is fed by groundwater. You can use a herbicide near a contained garden pond that is not supplied by groundwater or a watercourse without notifying the Environment Agency, although care is still needed to avoid damage to wildlife.

Always follow the guidance provided on the label of the product you intend to use. Wear suitable protective clothing and dispose of packaging appropriately.

If you are not confident with the use of herbicides, or lack the equipment and protective clothing, you should use the services of a qualified landscape gardening contractor. When employing contractors for spraying, you should insist on seeing their National Proficiency Tests Council (NPTC) Certificate of Competence for the use of pesticides. Each certificate has an individual number. Use the information on this page to test the knowledge of potential contractors, to ensure that you employ someone with a good knowledge of knotweed control.

It will take at least three years of herbicide treatment before knotweed has been eradicated. By the third year of treatment, knotweed growth may be only a few centimetres tall and easily overlooked amongst grass and herbage. It is essential that treatment continues until no further growth appears. Any disturbance to the rhizome at this stage will stimulate further growth, however, controlled rotavation of the infested area can be used to stimulate the exhausted rhizome to produce more foliage, which can then be sprayed. If a rotavator is unavailable, a series of spade cuts will also stimulate the rhizome. Clean implements thoroughly after use to prevent spread of fragments.

© Cornwall Knotweed Forum.

The information on these pages has been compiled by the Cornwall Knotweed Forum from information provided by its members and partners. It is provided in good faith to raise awareness of the issues and to promote best practice in the control of Japanese knotweed. The information may be used freely for private and educational purposes but may not be copied for commercial use. We update information and advice on a regular basis. However the Cornwall Knotweed Forum, or any of its constituent members cannot be held responsible for the efficacy of the treatments described, or any damage to persons, property or the environment that arises from interpretation of the advice given.

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Always read the product label.