Chemical control

Chemical control methods, such as the use of systemic herbicides, may be required for invasive plant species that are encouraged by ground disturbance and those species that have extensive root (rhizome) systems such as Japanese knotweed. Due to the potential negative impacts on some non-target species it is recommend that herbicides are used only after other all other methods have been fully considered. For species such as Himalayan balsam physical control through hand pulling or strimming may be an alternative option to herbicide application yet for species such Japanese knotweed or giant hogweed it is unlikely other insitu control options will be available.

If a herbicide is to be used a systemic herbicide that will be absorbed by the plant’s root system, such as glyphosate based products. Depending on the invasive species being controlled the herbicide active ingredient may vary and subsequently the optimum time of application will vary. Landowners / managers should apply these products responsibly by carefully reading the Pesticide Product label and observing all safety recommendations and restrictions. For example plants such as Himalayan balsam or giant hogweed which spread by seed should be controlled in the early growing season prior to seeds forming, whereas plants such as Japanese knotweed which generally spreads by fragments are not dependent on this.

Avoid using herbicides on foggy days, windy days or if rain is forecast within twelve hours of application.  Care should be taken to apply the herbicide only to the target species and avoid affecting surrounding vegetation by run-off or drift.

  • Foliar application: This type of treatment is usually applied with a sprayer such as a knapsack sprayer. It is an efficient way to treat large monocultures of invasive plants, or to spot-treat individual plants that are difficult to remove mechanically such as Japanese knotweed. For deep rooted species, such as Japanese knotweed, regrowth will occur in subsequent years, all be it much less vigorously, which will require follow up treatment at the appropriate time of year. Spot treatment will be required each year until no regrowth is observed.
  • Cut and inject treatment: This methodology is sometimes used for Japanese knotweed control where for example the knotweed is mixed in with other sensitive vegetation such as a hedgerow. This treatment requires a higher concentration of the active ingredient than is used in foliar applications. In addition many of the recommended herbicides for foliar application are not approved for application by this method.  Treatments for Japanese knotweed, through the application of glyphosate based products, are most effective when applied in the early Autumn (Mid to Late Sept). In the case of Japanese knotweed the stems should be cut approximately 20- 30cms from the base of each cane. Subsequently approximately 10 mls of herbicide mix is injected into each cane at a ratio of 5:1 through the use of a specialist stem injection tool. Delaying the application will reduce the effectiveness of the treatment. Regrowth will occur in subsequent years, all be it much less vigorously, which will require follow up treatment at the appropriate time of year. Spot treatment will be required each year until no regrowth is observed. The cut Japanese knotweed canes must be treated with extra care. They can either be kept insitu, where it is possible to do so, and left ontop of plastic sheeting and subsequently monitored for any sign of regrowth (this is not recommended for a river bank habitat where there is the possibility of flooding occurring). They should not be placed in your green waste recycling bin. Alternatively the canes can be disposed of at a licensed landfill site for deep burial via the use of a licensed waste carrier. The landfill site must be notified that the waste material contains Japanese knotweed. Alternatively, through seeking the necessary exemptions to burn, the canes maybe burnt on site. For more information on the legislation in Republic of Ireland see the following page: http://invasivespeciesireland.com/background/legislation/ireland/section-49/
  • Stump treatment: Woody species such as Rhododendron ponticum or Cherry Laurel can be controlled through stump treatment. This methodology involves the stems being cut as close to the ground as possible, but not so close that you will lose track of them. For Rhododendron and Cherry Laurel apply herbicide, typically a glyphosate based product, directly to the cut surface as soon as possible after cutting. Delaying the application will reduce the effectiveness of the treatment. Alternatively the stumps are drilled and the herbicide is applied to the drilled hole.
  • Stem injection: The stem injection method is sometimes used for Japanese knotweed control. This treatment requires a higher concentration of the active ingredient than is used in foliar applications. In addition many of the recommended herbicides for foliar application are not approved for application by this method. It involves the use of a specialist herbicide injection tool whereby the injection tool injects the herbicide directly into each of the canes approximately 20-30cms from the base of each cane (between the 1st and 2nd nodule). Subsequently approximately 10 mls of herbicide mix is injected into each cane at a ratio of 5:1 through the use of a specialist stem injection tool. The application of glyphosate based products, are most effective when applied in the early Autumn (Mid to Late Sept). Regrowth will occur in subsequent years, all be it much less vigorously, which will require follow up treatment at the appropriate time of year. Spot treatment will be required each year until no regrowth is observed.

Herbicides for control of invasive plants

During the application of any herbicide care must be taken to minimise the effects to any surrounding desirable vegetation. The herbicide selected must be used in compliance with the Pesticide Product Label and any conditions set out in it.  Herbicide users should ensure that they have read and familarised themselves with the Pesticide Code of Practice which advises on measures that can help to prevent against any potential impacts on non-target species such as pollinating insects (e.g. Bees) http://www.dardni.gov.uk/cop-plant-protection-final.pdf

It is recommended that professional herbicides are applied by a trained and suitably qualified operator. In both Ireland and Northern Ireland there are a range of companies which offer this service. Their details can be found via an internet or directory search. It is important to ensure the herbicide selected is of a high concentration of active ingredient to ensure a more effective spray. For example in the case of glyphosate based products it is recommended to use a product which contains at least 360 g/l of active ingredient. It is important to be aware that some herbicides brands, which contain glyphosate, are not approved for use in or near water. The Chemical Regulation Directorate website provides details of which herbicide products are licensed for aquatic use in their database search facility.

Herbicide active ingredient / Invasive Plant Glyphosate ( residual for 24-48 hours). 2,4 – D amine (residual for approximately 1 month) Triclopyr (active for up to 6 weeks) Picloram (residual for up to 2 years).
Japanese knotweed Optimum time of application is late season (Mid to late September). An early season spray maybe required in year 1 to assist with access for the late season spray. Will also kill grasses – only approved for use near water in certain products. The Pesticide Code of Practice (link above) should be consulted as to measures to help to prevent against any potential impacts on pollinating insects during the flowering period. Early season application preferred (Only approved for use near water in certain formulations). Early season application preferred – not approved for use near water. Can be applied anytime during the growing season – not approved for use near water. May also impact on nearby mature vegetation and shrubs.
Giant hogweed Early season prior to shoot with seed head emerging. Ideally spraying should take place as early as possible in the season to assist with access around leafy material before it gets too big (March – June). Repeat checks will need undertaken throughout the growing season for any late germinating seeds. Will also kill grasses – only approved for use near water in certain products. Early season prior to shoot with seed head emerging. Ideally spraying should take place as early as possible in the season to assist with access around leafy material before it gets too big (March – June). Repeat checks will need undertaken throughout the growing season for any late germinating seeds. (Only approved for use near water in certain products). Early season prior to shoot with seed head emerging. Ideally spraying should take place as early as possible in the season to assist with access around leafy material before it gets too big (March – June). Repeat checks will need undertaken throughout the growing season for any late germinating seeds – not approved for use near water. Not recommended – not approved for use near water.
Himalayan balsam Early season prior to seed pods forming (May to Mid June). Follow up checks should be undertaken on a monthly basis. Will also kill grasses – only approved for use near water in certain products. Early season prior to seed pods forming (May to Mid June). (Only approved for use near water in certain products). Not recommended – not approved for use near water. Not recommended – not approved for use near water.