Ontario Weeds: Giant hogweed

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems and Roots
6. Leaves
7. Flowers and Fruit
8. Habitat
9. Caution
10. Similar Species
11. Related Links

Name: Giant hogweed, Heracleum mantegazzianum Somm. & Lev.,

Other Names: berce du Caucase

Family: Carrot Or Parsley Family (Umbelliferae)

General Description: Biennial or perennial, flowering only once in its lifetime and reproducing only by seed. Plants forming rosettes to 1m (40in.) high the first year; in the second year, either sending up a flowering stem, or remaining vegetative and producing a very large rosette of huge leaves, these including their petioles, up to 2m (80in.) high, and flowering in the third year.

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Photos and Pictures

Giant hogweed (A - stand of mature 2-yr-old plants; leafy canopy is about 1.5m (5ft) tall and seed-bearing stems about 3m (10ft) tall; B - a small plant beginning to flower; C - lower part of a stem and leaf petiole with a hunting knife for scale).

Giant hogweed. Second-year plant with rosette leaves up to 125cm (50in.) high plus a central stem 2 leaves emerging through the top of the rosette. The central stem will later produce a very large umbel of flowers.

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Stems & Roots: flowering stems up to 5m (16ft) high and up to 10cm (4in.) in diameter, hollow except at the nodes; both stems and petioles having conspicuous reddish-purple flecks throughout and sometimes nearly solid purple near the base; lower stem often very rough with sharp-pointed, irregularly-spaced bumps.

Leaves: Leaf blades of rosette leaves very large, up to 1m (40in.) across, compound with 3 large deeply cut leaflets, each leaflet with deep irregular lobes and coarse, sharp teeth on all margins; their petioles often with sharp-pointed bumps similar to those on the flowering stems; leaves on the flowering stem of similar shape but smaller, the upper ones often not divided but just deeply 3-lobed.

Flowers & Fruit: Inflorescence a compound umbel up to 120cm (4ft) across, consisting of many (approximately 30 to 50) branches, each branch ending in a simple umbel with approximately 30 to 40 flowers; the whole compound umbel with a flat bottom and gently rounded top; petals white or rarely pinkish and up to 12mm (½in.) long; fruit oval in outline, 7- 3mm (¼-½in.) long, 6-10mm (¼-2/5in.) wide and 1-3mm (1/25-1/8in.) thick, smooth to softly hairy, with usually 4 prominent dark-coloured oil tubes. Flowers from June to August.

Habitat: Giant hogweed was introduced from Europe, presumably as an ornamental, and has escaped along roadsides, streambanks and waste areas in scattered localities in southern Ontario.

Similar Species: It is distinguished by its huge size, its very large, compound leaf blades, its tall, thick, hollow, often sharply roughened stems, and its large flat-topped compound inflorescence with white flowers and large, flat fruits with prominent dark-coloured oil tubes. Flowering inflorescences are often heavily infested by aphids.

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Caution: It has been implicated as a cause of severe dermatitis in a few susceptible individuals. 

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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