Heracleum mantegazzianum giant hogweed
Giant hogweed in waste ground near Ballymena Co. Antrim 2005

Family: Umbelliferae

Origin: south western Asia

Similar to our native hogweed Heracleum sphondylium but much taller: up to about 12 ft (3.6 m) or more, and with huge, spreading umbels of white flowers.

Giant hogweed is normally a biennial, in the first summer producing only leaves above ground while a large swollen tuberous root develops underground. This tuber provides the food for rapid growth and flower and fruit formation in the second season, after which the whole plant usually dies. Large numbers of single-seeded mericarps (fruits) are produced after flowering from about late August onwards and these are the normal means of spread.

Flowers of giant hogweed 2005

This plant was introduced originally into parks and demesnes in the Victorian era, but has spread by fruit dispersal beyond the boundaries of these parks into wild vegetation, especially along river banks. The sources of these infestations are often easily identifiable.

Giant hogweed fruiting; near Tillysburn, Co. Down 2005

Many Northern Ireland rivers are now affected such as the River Lagan, Newry River near Newry, River Roe near Limavady and the Ballinderry River in Co. Tyrone. This infestation seems to have started mainly during the 1940s. It also persists in some parks where it was originally planted such as Belvoir Park near Belfast, and can also be found on damp waste ground.

Immature mericarps (fruits) of giant hogweed; near Tillysburn, Co Down 2005

This species has acquired some notoriety for producing skin burns or blisters when the sap contacts the skin on sunny days. However, to a lesser degree, this is also a property of the sap of the common native hogweed Heracleum sphondylium.

It is illegal to introduce or plant this species into the wild in Great Britain or Northern Ireland.

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