Northern Ontario Plant Database

Plant Description

Heracleum mantegazzianum Sommier & Levier

En: giant hogweed, giant cow parsnip, cartwheel flower
Fr: berce du Caucase, berce géante du Caucase

Apiaceae (Parsley Family)

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General: A stout biennial or short-lived (monocarpic) perennial herb that dies after flowering, plants are 1.5—4 m tall, occasionally reaching 5.5 m. Warning: The sap of giant hogweed contains phototoxic compounds (furocoumarin) that can produce a severe rash called phytophotodermatitis (PPD). Full protective clothing, gloves, and safety glasses should be worn when working around giant hogweed. Do not use brushcutters or whipper-snippers on giant hogweed, as this will spray the irritating sap around anyone in the area. Plants should be treated with repeated herbicide applications or removed by a professional weed specialist.

Stems: Non-woody (herbaceous), erect, hollow, smooth to ribbed, and 4—10+ cm in diameter. Stems may be solid reddish-purple, or more commonly, green and spotted with large to diffuse purple blotches that are often bumpy or raised. A short, brittle, and hollow bristle-like white hair emerges from each blotch. These hairs have an enlarged base filled with clear sap; touching (breaking) the hairs will release small quantities of irritating sap.

Leaves: Non-flowering plants have a large basal rosette of leaves, while flowering plants have stems with alternate leaves. All leaves are very large and compound, divided into 3—5 short-stalked leaflets. Each leaf can be as large as 3 m long by 1.7 m wide; leaflets are irregularly and deeply cut (incised) into 3 or more sharply pointed lobes with coarsely-toothed margins. Upper leaf surfaces are glabrous; lower surfaces are hairy when young. Giant hogweed leaves are larger and more deeply divided than those of cow parsnip. Petioles are broad and greatly expanded at the base, enclosing the developing leaves or flowers. At each node a dense ring of stiff white hairs surrounds the stem below the expanded petiole.

Flowers: White, very numerous, and borne in large domed inflorescences (compound umbels), to 80+ cm across. Inflorescences are terminal and often multi-branched, with 4—13 lateral compound umbels surrounding the main inflorescence. Compound umbels have 50—150 branches (rays), 15—40 cm long, which each support a smaller umbel of 20—30 flowers. Individual flowers have 5 small sepals, 5 white petals (to 10 mm long), 5 stamens, and an inferior ovary of 2 carpels and 2 styles. Petals of flowers at the outer edge of each umbel are enlarged and deeply divided into 2 lobes; other petals are unlobed. Flowers bloom in late spring to early summer.

Fruit: Each light brown dry fruit (schizocarp) is flat, oval or elliptic, 6—19 mm long by 4—10 mm wide, and marked on the outer surface with 4 dark brown oil tubes that extend down at least 3/4 the length of the fruit.

Habitat and Range: Planted in large gardens and golf courses; usually escaping from cultivation along streambanks. Giant hogweed is native to Asia and was introduced as an ornamental to Great Britain in the 19th century. It was brought to North America as an ornamental and soon became invasive. It is quite invasive in portions of southern Ontario, but giant hogweed is not found in the Algoma District. However, it has been reported from 2 locations on Manitoulin Island, where eradication measure are being taken to remove it.

Similar Species: In the Algoma District, cow parsnip (Heracleum maximum W.Bartram), a native perennial, is very often misidentified as giant hogweed. Cow parsnip usually grows to 2 m tall, but can exceed 3 m in rich sites; its stems are green to purplish, ridged, and covered with soft white hairs, but lack the purple blotches of giant hogweed stems. Cow parsnip flower clusters are flat-topped, with up to 30 small branches (rays), each of which supports a smaller umbel; fruits are indented at the top and taper at the base, forming a heart-shaped outline.

Toxicity: A severe burning blistering rash will develop when bare moist skin contacts sap from giant hogweed leaves or stems and is exposed to sunlight (UV rays). Exposure to both sunlight and the sap are required to cause this rash, which will develop within 1—4 days. Exposed skin should be washed immediately with soapy water, then bandaged to avoid further exposure to sunlight. If a rash develops, SEE YOUR DOCTOR for treatment as soon as possible. After healing, skin will be more darkly coloured and extremely sensitive to UV light for months to years. This reaction is like a chemical burn rather than an allergic reaction, so no one is immune. If sap from this plant gets in your eyes, temporary or permanent blindness may occur. Avoid contact, report sightings to the local OMNR office, and have the identity of these plants confirmed by a professional before treating with herbicides or removal.