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Giant hogweed flower heads

GIANT HOGWEED  (Heracleum mantegazzianum)

Introduced to Britain in 1893 as an ornamental plant, Giant Hogweed is a large, vigorous biennial, which grows from a seed in year one, and takes between 2-5 years to produce white flowers. A single umbel up to 80cm in diameter can produce between 5000 to 100,000 viable seeds, after which the parent plant dies. The stem of the plant can reach 5m in height with sharp bristles and is blotchy purple in colour, bearing divided, serrated leaves with a bristly underside which can grow up to 3m in diameter.


Giant Hogweed is commonly found growing on waste ground, streams and river banks, road verges and railway embankments. 


Giant Hogweed contains a highly toxic chemical, called furocoumarin, located within the sap found throughout all sections of the plant. The sap photosensitises the skin, and in combination with ultraviolet (UV) radiation in sunlight, it can cause a severe red rash and water blisters. This is not apparent until 20-24 hours hours after initial contact - the initial contact is completely pain free, and the subsequent reaction varies from person to person. The affected skin can remain highly sensitive to UV radiation for  several years.


Effective management of Giant Hogweed can limit exposure of risk to people, prevent spreading and stop any civil actions. It is an offence under the Wildlife and Countryside Act 1981 to cause it to grow in the wild or plant it in the wild.


Features of Giant Hogweed

1. The size of Giant Hogweed makes it easy to identify as it can grow up to 5m in height.


2. Large umbels of white or rarely pink flowers from
June to August. Setting seed and producing
between 5,000 and 100,000 seeds, 1.5cm in
length and lightweight, easily dispersed by wind
and watercourses. Seed bank can remain active
for up to 5 years.

3. Green hollow stems with purple blotches, hairy
 5-10cm in diameter.


4. Serrated and sharply divided leaves with bristles,
growing up to 3m in length.

Giant hogweed features

Giant Hogweed First Aid: If contact is made with the sap of the plant...

  • Immediately wash the affected area thoroughly with soap and water.


  • Cover the skin to reduce exposure to sunlight for at least 48 hours.


  • Seek medical advice as soon as possible. Early treatment with steroids can reduce severity.


  • Use suncream on the affected area for several months after 

Types of Giant Hogweed Treatment

  • In-situ herbicide treatment: This is the application of herbicide to the foliage.


  • In-situ manual treatment: Removal of the seed heads and taproot cutting, killing the plant and reducing further contribution to the seedbank.


  • On-site treatment, ex-situ: Contaminated soil is excavated and moved to a more convenient area of the side for stockpiling and ongoing herbicide treatment.


  • On-site burial: Contaminated soil is excavated and buried at least 1m deep within a convenient area of the site.


  • Remove and dispose: Off-site disposal at a licenced landfill facility can be used as a last resort when other options are not viable.


  • Mowing: Mechanical means to cut the foliage over a number of years to deplete the energy dtores of the taproot and prevent flowering.

  • Combined: excavation to landfill and herbicide treatment

Giant Hogweed in a public park behind a cordon

ERS has extensive experience in identification, management and remediation of Giant Hogweed infested sites. We work with clients to develop a treatment strategy which meets their timescales, budget and future development requirements.


When Giant Hogweed is identified on a site, ERS can develop and implement a Giant Hogweed management plan. This plan is essential, both for the current owner and any future owners, as a record for treatment of the site


To enquire about a survey or management plan for Giant hogweed on your site, contact us today to speak to one of our Invasive Weeds experts.


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