Ontario Weeds: Giant hogweed

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Excerpt from Publication 505, Ontario Weeds, Order this publication

Table of Contents

  1. Name
  2. Other Names
  3. Family
  4. General Description
  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.

Photos and Videos

Transcript | Mobile

Transcript | Mobile

Transcript | Mobile

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 (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.

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.

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.

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 OMAF Publication 505: Ontario Weeds
... on weed control, order OMAF Publication 75: Guide To Weed Control

 

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For more information:
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E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 01 June 2002
Last Reviewed: 01 November 2010