Identification
Giant Hogweed
Heracleum mantegazzianum Sommier & Levier.
Apiaceae (the Parsley or Carrot Family)
General appearance: A huge, stout, taprooted biennial or monocarpic perennial. Immature plants form a rosette of arching leaves 1-2 m (3.3-6.6 ft) tall, arising from the root crown. Flowering plants may reach 5 m (16 ft) tall.
Leaves: Rosette and lower stem leaves deeply lobed and toothed (pinnatified), with a strong midvein and a relatively short petiole, becoming shallowly lobed and sessile towards the top of the stem. Leaves very large, reaching up to 3 m (10 ft) long and 1.5 m (5 ft) wide, and covered with scattered short, stiff hairs. Leaves are alternate on the stem.
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Stem: Stout, hollow, and ridged, with short, stiff hairs and purple blotches. At peak flowering (anthesis), plants usually have a single straight, central stem with a few to many, short to fairly long branches near the top, each bearing a flat-topped cluster of flowers. Only mature, bolting or flowering plants have a stalk.
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Flowers: Produced in one or (usually) several flat-topped clusters called umbels. About 50-150 rays (primary branches of the flowerhead) each support a cluster of small, white flowers, with each flower on a short stalk (secondary branch) 1.5-4.0 cm (0.6-1.6 in) long. Umbels can reach as much as 0.5 m (1.6 ft) across. Mature plants bolt in the spring, flowering in late June and July in southeastern Ontario (Morton 1978).
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Seeds: Flattened, elliptical seeds about 3-4 mm (0.12-0.16 in) long, ripening by fall.
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Habitat: Colonizes a variety of moist to wet, sunny to partly shaded habitats. Readily invades open, disturbed sites such as ditches, fields, and right-of-ways, as well as marshes, moist to wet meadows, and woods edges. Particularly aggressive on streambanks and in floodplains, where water spreads the seeds downstream.
Warning: Giant hogweed produces chemicals called linear furanocoumarins. These compounds are found in the clear, watery "sap" of the plant. They cause a condition called photodermatitis, characterized by rashes, discoloration, and blistering of the skin, when exposed to sunlight. These burns can be severe and long-lasting or even permanent. Permanent blindness has resulted from eye contact with the sap. Wear protective clothing around giant hogweed, and avoid any skin or eye contact with the sap.
Sources: Tiley et al. (1996) and Gleason and Cronquist (1991), except as noted.
Udated May 2005.
Visitor since March 17, 2006