Giant Hogweed

Species Description


Scientific name: Heracleum mantegazzianum 


 Identification sheet


Native toAzerbaijan, Georgia and southern Russia


Habitat: Typically occurs by lowland streams and rivers. In Britain it also occurs widely on waste ground and rough pastures. It grows on moist, fertile soils and achieves its greatest size in partial shade.


Aptly named ‘giant’, this umbelifera (member of the cow parsley family) has flowering stems typically 2-3 metres high bearing umbels of flowers up to 80cm in diameter. The basal leaves are often 1m or more in size. It was introduced to gardens in the 19th century and was subsequently planted by rivers and ponds. It is important not to touch this plant with bare skin; the sap is highly irritating and causes blistering of the skin which can recur for many years. In the Ribble catchment, this plant currently occurs in only a few locations, however it may spread if not controlled. The danger to human health posed by this species makes it a high priority for control.


Further information

Dispersal and Reproduction

Giant hogweed reproduces entirely by seeds, 20,000 of which are produced by a single plant. These are dispersed by wind, water and humans. The seeds form a short-term persistent seed-bank. The majority of seeds germinate after the first winter and only about 1% remains viable for more than 3 years. This is a short-lived monocarpic perennial, which grows for 5-7 years, flowers once and then dies.

Known Predators

There is no significant insect or pathogen control of this plant. Grazing by livestock can decrease seed production but also prolong the lifespan before flowering.


Impacts

This species may form dense stands, reducing species diversity. The main impact however is on human health. The sap of this plant contains furanocoumarins which cause photosensitisation of the skin. Contact with this sap burns the skin and causes it to blister when exposed to sunlight. This may recur for several years after the initial exposure. The intensity of the reaction varies with individual sensitivity


Management

Control of this plant is by mechanical or chemical means. No biological control is known. As with other riparian weeds, a ‘top down’ strategy will be most efficient. Monitoring and repeat treatment will be required to prevent re-establishment from the seed bank.


Mechanical

Generally more suited for large infestations, mechanical control can be used in conjunction with grazing for best effect. Prevention of flowering is the primary aim so consecutive cutting of the flower stems is recommended. Plants may be effectively killed by cutting or digging up the plant at 10cm below ground level. 


Chemical


Giant hogweed is sensitive to herbicides based on glyphosate and triclopyr which can be applied as a foliar spray. Chemical treatment is a preferred option for isolated populations and may reduce risk of contact with the plant.


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