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Giant Hogweed

Hazard Related Topics

Giant HogweedGiant Hogweed is a species of plant that is common in the UK along roads, footpaths and railway lines, rivers, wasteland, demolition sites and agricultural areas. Giant Hogweed is characterised by its size and may grow to between 3 and 5 metres in height. Individual plants can take up to four years to reach this height and flower. Giant Hogweed has a reddish purple stem and spotted leaf stalks, with fine spines that make it appear furry. The leaves may expand to 1.5 metres in width with flower heads commonly 250mm in width. Giant Hogweed produces approximately 1500 seeds per flower head in late summer. These seeds can remain inactive in the soil for several years. The movement of soil polluted with Giant Hogweed seeds must be carefully controlled to prevent the spread of the plant. When Giant Hogweed dies back during the winter months, it leaves bare ground.

Under the Wildlife & Countryside Act 1981/Wildlife (Northern Ireland) Order 1985 it is an offence to "plant or otherwise cause Giant Hogweed to grow" in the wild. This includes spreading the species or transferring polluted ground material from one area to another. Any Giant Hogweed polluted soil or plant material that you discard, intend to discard or are required to discard is classed as controlled waste and should be accompanied by appropriate Waste Transfer documentation.

Health Risks

Giant Hogweed contains a substance within its sap that makes the skin sensitive to ultra violet light. This can result in severe burns to the affected areas, producing swelling and severe, painful blistering. Large, watery blisters usually appear 15 to 20 hours after contact with the sap and sunlight. Damaged skin will heal very slowly leaving residual pigmentation that can develop into Phytophotodermatitis - a type of dermatitis that flares up in sunlight for which there is no straightforward treatment.

Contact between the skin and the sap of this species occurs either through brushing against the bristles on the stem (as with a stinging nettle) or breaking the stem/leaves. In the event of contact with the sap of this plant, the skin should be covered to reduce the exposure to sunlight and washed IMMEDIATELY and thoroughly with soap and water.

Methods of Control

The two methods outlined below are the most effective control measures available in the time scale required by the construction industry.

  1. Spraying with an appropriate herbicide is the most effective treatment option available although it can take several years to eradicate this species if the plants are well established, as there will be thousands of seeds in the soil beneath. Although the eradication of this species before or during construction is unlikely, herbicides can be used to achieve short-term control of the plant, allowing construction works to continue and reducing the risk to the workforce.
    Spraying can only be carried out during the growing season (March to August), when there is green, leafy material present. In order to be effective, spraying must be carried out with the greatest number of green leaves present and before the plant flowers and sets seed, otherwise there will be thousands of additional seeds on the ground ready to grow at some point in the future. As Giant Hogweed seeds can remain in the soil for several years, a long-term strategy involving treatment with herbicide will be required.
    Anyone planning to spray a herbicide must be "competent in their duties and have received adequate instruction and guidance in the safe, efficient and humane use of pesticides." This generally means that the person who will be undertaking the spraying must hold a Certificate of Competence for herbicide use or should work under the direct supervision of a certificate holder. A Certificate of Technical Competence can be obtained by attending a short course at an agricultural college or similar institution. One of the most effective active ingredients for use on Giant Hogweed is called Glyphosate, as found in "Round Up" and other similar herbicides.
  2. Digging and Spraying is a quicker method of removing Giant Hogweed and involves the clearing of above ground leaf/stem material together with the removal of ground material polluted with roots and seeds. Due to the risk of contact with sap from this plant, removal by hand should be restricted and not considered once the plant has grown above 1 metre in height. You will need to prepare a risk assessment for any works to remove, cut or work in the vicinity of Giant Hogweed. It is vital to minimise the potential for workers' skin to come into contact with plants of this species and particularly any sap produced. Measures to reduce the risk of any cutting and removal works should include, but are not limited to:
    • The establishment of an exclusion zone around any Giant Hogweed plants identified on site and around any cutting/removal works. Only personnel involved with the removal of this species should be permitted entry to this exclusion zone.
    • Risks associated with this species should be outlined to employees prior to work commencing.
    • The use of full wet weather clothing with gloves and face visor or similar to undertake any cutting or removal of this species.
    • The provision of washing facilities and eyewash for operatives involved with this operation.
    The majority of seeds that are produced fall to ground within four metres of the parent plant. This area should be considered as polluted and be removed. Even with great care, a certain number of seeds may remain. A watching brief is advised after polluted ground material has been removed for any re-growth.


Polluted material should be removed from the site for disposal, unless otherwise agreed with the Environment Agency and the Client. Any bags/skips containing Giant Hogweed or polluted soil leaving the site should be covered to avoid spread along public highways. Waste Transfer documentation will be required for any polluted material leaving the site.

Check with the disposal site in advance that they can receive material containing incidental contamination with Giant Hogweed. Disposal sites may require notice to allow an area to be prepared for this material away from the landfill liner. As Giant Hogweed is considered to be a pollutant, you can apply to Customs and Excise for a 'Landfill Tax Exemption' for disposing of polluted soil.

Information and Survey

If Giant Hogweed is identified on site, it should be fenced off at a distance of at least four metres from the plants and the workforce notified of its presence and associated risks. Outside of the growing season, there may be little evidence of its existence other than the previous years dried, dead flower stem. If you suspect that there may be Giant Hogweed present on the site, you could check with the Client or the Principal Contractor. The Local Authority or local wildlife trusts may also have historical information. Alternatively, a specialist survey can be carried out.


  1. Ensure that soil from within four metres of the plants is not transferred to other areas of the site as this soil is most likely to contain seeds from previous years' growth.
  2. Never stockpile potentially polluted material within 10 metres of a watercourse.
  3. Use of tracked machinery should be limited until areas polluted with Giant Hogweed have been cleared and/or identified and cordoned off.
  4. On leaving areas of the site known to contain Giant Hogweed, any vehicles and machinery that has been used within the contaminated area should be thoroughly cleaned within a designated cleaning area. This area should be as close as possible to the contaminated area. The cleaning area should be monitored in the spring for Hogweed growth along with the original contaminated area.
  5. Giant Hogweed polluted spoil should only be stored on top of a fabric/membrane in an approved area. Once cleared, these areas should be also be monitored for re-growth.
  6. Everyone working on the site should be made aware of the health, safety and environmental risks associated with this species in order to help limit accidental spread or injury and should evidence of the training should be maintained.
  7. All haulage lorries or dumpers carrying Giant Hogweed polluted material should be covered.
  8. NEVER use a strimmer or chipper on Giant Hogweed material as this can cause sap to become airborne, resulting in injury.

Further information on the control and disposal of Giant Hogweed can be obtained from the DEFRA - Japanese knotweed, giant hogweed and other invasive plants.