Japanese knotweed, giant hogweed and other invasive plants

Treating invasive plants with herbicide can be a very effective method of treatment. You will have to respray. It usually takes at least three years to treat Japanese knotweed until it is dormant. Giant hogweed seeds can continue germinating for 15 years after the last seed fall.

If the plant is in or near to water you must have agreement from the Northern Ireland Environment Agency (NIEA) to use the herbicide. The herbicide must be approved for use in or near water.

The herbicide's effectiveness depends on the type used. An advisor certified by BASIS (the registration, standards and certification scheme for pesticides and fertilisers) will be able to advise you on the most suitable type of herbicide for your situation and when best to apply it.

Find information on pesticide and fertiliser registration, standards and certification.

Giant hogweed and Himalayan balsam both drop large quantities of seeds. A control programme will need to continue for several years, with checks carried out throughout the growing season. If you are trying to eradicate these plants from a riverbank it is important to ensure that any plants upstream are also treated to avoid seeds being washed onto the site.

Japanese knotweed has a large underground network of rhizomes (underground root-like stems). To eradicate the plant you must kill the rhizomes. Picking the right herbicide is essential, as it must travel through the plant and into the rhizome system below. Several herbicides can treat Japanese knotweed successfully - you will need to pick the right herbicide for your situation. Glyphosate is effective because it penetrates through the whole plant.

Using herbicides

The person doing the spraying must hold a certificate of technical competence for herbicide use or work under the direct supervision of a certificate holder. If you plan to spray in or near water, the person carrying out or supervising the spraying must have the appropriate aquatic part of the qualification. The sprayer must also comply with the pesticide product label and meet all of its conditions. Before you spray in or near water you must check that the product is approved for use near water. There are formulations of glyphosate and 2.4-D amine that can be used in or near water and are effective against many invasive plants.

You can get a certificate of technical competence by attending a short course at an agricultural college or similar institution.

For herbicide to be effective, make sure you use it at the correct time of year:

• Japanese knotweed is best sprayed in late summer from flowering onwards.
• Giant hogweed should be sprayed in April or May, before the plants flower.
• Himalayan balsam should be sprayed in spring before flowering, but strimming or hand pulling should be your preferred option for control of this plant.

You must follow the guidance in the statutory code of practice for plant protection products. If you follow its advice you should stay within the law.

Download guidance on the safe use of pesticides for non-agricultural purposes (PDF, 137K).

If the invasive plants are near a watercourse, you should not use herbicides as the first option. If you are planning to use herbicide in or near to a watercourse, you must consult the NIEA.

You must also carry out a Control of Substances Hazardous to Health (COSHH) assessment for any activities that involve herbicides.

Dispose of waste herbicides correctly

You must make sure that all your waste is stored, transported and disposed of safely. Waste herbicides are likely to be classed as hazardous waste. You must keep this separate from other waste.

Herbicide containers must either be rinsed or handled as herbicides. Check product labels to see if your waste containers should be rinsed. Water used for rinsing empty containers is classed as dilute pesticides or biocides. You may need a groundwater authorisation, registered waste exemption or trade effluent consent to dispose of this - see pesticides and biocides.