Invasive Alien Species in Northern Ireland

Heracleum mantegazzianum, Giant Hogweed

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Heracleum mantegazzianum
© Paul Hackney
Heracleum mantegazzianum
© Paul Hackney
Heracleum mantegazzianum
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Heracleum mantegazzianum Sommier & Levier
A large, biennial or perennial herbaceous plant, looking like an extremely robust cow parsley, with a pale, swollen rootstock which looks very like an inflated parsnip, and, indeed, smells strongly of parsnip, the smell lingering on the hands for several days. Introduced from the Caucasus in the late 19th century as a garden ‘architectural’ ornamental plant, it has escaped and is invasive in suitable habitats, such as river- and stream-sides, railway cuttings and other damp waste places. It has been reported that this species has hybridised with H. sphondylium, the common native cow parsley or hogweed.
It is illegal to introduce this plant into the wild in Northern Ireland. It is also an offence to move ground material polluted with the seed - this material is classed as ‘controlled waste’ and requires Waste Transfer authorisation.
Very large herbaceous plant, growing to 5-6m tall, with a sap which can cause dermatitis when handled in sunlight:
stem - ridged, hollow, up to 10cm diameter, with purple blotches;
leaves -
  • lower up to 250cm, pinnate, irregularly and coarsely toothed, the larger teeth long-pointed, hairless above and covered with minute, soft hairs beneath, leaf stalk stout, enclosing the stem a little at the base;-
  • upper small, the leaf stalk with a greatly inflated base enclosing the stem;
inflorescence - a compound umbel, with 50-150 hairy branches 15-30cm long; the terminal umbel has hermaphrodite flowers, the lateral umbels with mostly male flowers;
flowers - sepals triangular, petals white, styles with enlarged base;
fruit - a flattened, oval, papery disc c.1-1.5cm long, with four longitudinal stripes, containing two seeds.
Country of origin
SW Asia.
Location in Ireland
Along riversides, stream banks, and other damp waste sites. In suitable environments, it can be abundant. It can extend along several miles of river bank.
Life cycle
Heracleum mantegazzianum is normally a biennial, but it can also exist as a perennial, although monocarpic (flowering only once in its life); the seeds, being very light, can be dispersed by the wind over some distance, but they will also be dispersed by the streams and rivers beside which they can often be found. They can also be transported in soil adhering to shoes, machinery etc. The seeds readily germinate.
Wildlife and habitat impacts
Being of rapid growth, and having very large leaves, this plant will shade out less vigorous native plants in its immediate area, wherever it grows, with the resultant loss to dependant insects and other animals. Because of the furanocoumarins present, adverse reactions may occur in birds on contact, and possibly also in insects.
Human impacts
In susceptible individuals, probably especially children, contact with this plant and exposure to light, especially strong sunlight, can cause photodermatitic reaction usually within 24-48 hours - redness, burning sensation, itching, blistering. The lesions are slow to heal, and any consequent pigmentation may take some considerable time to disappear.
Individuals should always assume they are sensitive to the plant; those carrying out eradication programmes should wear suitable protective clothing, especially face masks, gloves and arm covers - Health and Safety regulations may well apply; brush cutting machinery may be covered by sap spray and should be washed after use.
Key vectors
Seed dispersal via water transportation and in soil adhering to shoes and machinery. Seeds can stay viable for several years.
What you can do as an individual
Supply records of sightings to the Botanical Recorder for the area or your Local Records Centre. Contact CEDaR, Ulster Museum.
Management measures
Eradication programmes may vary depending on the degree of infestation. Small numbers can be controlled by digging out the whole individual plant; docking the plant to prevent it flowering will divert reserves to ensuring the plant survives to attempt to flower the following year. It is best to cut the stem at below ground level, to ensure that the rootstock is damaged. Larger numbers can be sprayed, preferably when the plants are actively growing and less than 1m tall, with a glyphosate herbicide (this is the only herbicide which can be used near water). This can be done either as a spot treatment, or using long reach sprays. The monitoring of the treated area for several years is necessary, to find new seedlings. Establishing greensward or reseeding with native plants is also beneficial after initial eradication.
Further information
The Centre for Aquatic Plant Management (Natural Environment Research Council) researches the control of problem plants in the aquatic environment and both the Environment Agency and the Centre for Ecology and Hydrology have information available on this plant.
Centre for Ecology and Hydrology /Centre for Aquatic Plant Management ; Information Sheet 4
Environment Agency; Giant Hogweed information
See the Invasive Species Ireland web site for further information -
Text written by:
Catherine Tyrie, Curator, Botany, Ulster Museum