Heracleum mantegazzianum aka Giant Hogweed

Heracleum mantegazzianum is also known as giant hogweed. It belongs to Apiaceae, the carrot family. Its native range lies in the Caucasus. This herb grows fast. It is a biennial or a short-lived perennial. It may take several years to reach flowering age. It dies once it flowers and sets seed. Growers once planted it for its bold leaves and flowers. It has since spread from gardens and displaced native plants.

Its listed garden height reaches 15 feet [4.57 metres]. NC State notes that some plants reach 20 feet [6.1 metres]. Its listed spread reaches 4 feet [1.22 metres]. The thick green stems stand upright. They are hollow and have sharp angles. Dark purple-red blotches and coarse white hairs cover them. Large green leaves grow in turns along the stems. These compound leaves each have three deeply cut leaflets. Leaves can span 9 feet [2.74 metres]. Upper leaves are smaller, with less deeply cut edges.

White flowers bloom from June to July. They form broad umbels, or flat-topped flower heads. These heads can span 2.5 feet [0.76 metres]. The flowers draw butterflies, notes Missouri Botanical Garden. Dry, oval fruits called schizocarps form once flowers fade. NC State records fruit growth in August. The flat, cream seeds have brown marks shaped like teardrops. A plant can make thousands of seeds before it dies. This large seed crop helps it spread beyond planted sites.

It grows in moist, rich soil that drains well. It accepts clay, loam, and sand. Missouri Botanical Garden reports best growth in full sun. NC State says it prefers moist sites with some shade. Both sources place it in USDA zones 3–9. Plants spread to stream banks, ditches, fields, and unused land. By streams, they can displace native plants and worsen soil loss. Water can carry seeds downstream to new sites.

Its spread and toxic sap outweigh its past use for looks. Missouri Botanical Garden lists no garden use for it. It reports no major insect or disease problems. Giant hogweed is on the United States Federal Noxious Weed list. A permit is needed to import, sell, or move it between states. Its sap holds furanocoumarins. These make skin sensitive to ultraviolet light. Sun on skin after contact can cause severe burns and blisters. Removing it by hand poses a serious risk. This work calls for skin protection. Missouri Botanical Garden also calls for gloves and eye protection. Seeds may sprout 15–20 years after people remove the plants. Sites thus need checks long after removal.