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Giant Hogweed

Heracleum mantegazzianum

Giant Hogweed is an invasive non-native plant species that grows abundantly along river corridors, steams and beyond. The species can reach a substantial height within in a single growing season, with individuals found on average to be two to five metres in height.

Seeds germinate from March onwards, with the plant quickly establishing a taproot. It takes three to four years to reach maturity, and in the final year produces the main stem and a large flower head. A typical flower head is capable of producing 20 000 seeds, however up to 50 000 have been recorded for a single plant. Giant Hogweed can be found flowering from June to August, with seeds being released from late August each year.

The large number of seeds produced by each plant, combined with rapid and appreciable annual growth, enables them to persist in increasing numbers along the watercourse, using flowing waters for seed dispersal. The massive sizes reached by the mature plants out-shade, outgrow and outcompete smaller native species, which can result in their exclusion from the infested habitat.

Giant Hogweed also poses a serious and significant danger to public health. The toxic sap contained with the plant will, upon human contact, result in the development of painful blisters (particularly in direct sunlight and in damp conditions). Touching the plant or merely brushing against it is enough to induce exposure to the sap, and all persons that come in contact with it seem to be affected to some extent.



Giant Hogweed is one of the main causes of phytophotodermatitis in the United Kingdom and United States, and may also occur via indirect contact with the sap, for example from a cat or dog that has touched the plant. Damage affects the skin’s ability to filter UV light and the phytophotodermatitis may be permanent.


Giant Hogweed is most readily identified by the huge dead stems of previous growth of other, possibly parent plants. It is further distinguished by broad stems (especially when compared to native hogweed) that are a dark reddish-purple, and large, deeply incised compound leaves that grow up to 1-1.7 m in width.

Dead Stem
Giant Hogweed Seeds
Giant Hogweed Stem
Stem close-up
Giant Hogweed Leaves
Emerging Flower Heads
Giant Hogweed Flower
Mature Plants

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