Giant Hogweed or Giant Cow-parsley, Heracleum mantegazzianum, Apiaceae. Giant Hogweed is a member of the family Apiaceae native to the Caucasus Region and Central Asia. It may reach 2-5 metres, rarely to 7 m, tall. Introduced into Britain, it is now widespread throughout the British Isles, especially along riverbanks, and has become an invasive species. Giant Hogweed is a phototoxic plant. Its sap can cause phytophotodermatitis, severe skin inflammations when the skin is exposed to sunlight or to UV rays. Initially the skin colours red and starts itching, then blisters form, as in burns, within 48 hours.