Ontario Weeds: Poison hemlock

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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. Similar Species
  10. Caution
  11. Related Links

Name: Poison hemlock, Conium maculatum L.,

Other Names: ciguë maculée, Deadly hemlock, Poison parsley, ciguë tachetée

Family: Carrot Or Parsley Family (Umbelliferae)

General Description: Biennial, reproducing only by seed.

Photos and Pictures

Poison hemlock (A - plant before flowering; B - inflorescence of several compound umbels).

Poison hemlock (A - plant before flowering; B - inflorescence of several compound umbels).

Poison hemlock. A. Top of flowering and fruiting stme. B. Portion of simple and compound umbels showing 2 bracts on only 1 side of hte small simple umbel but several bracts surrounding the base of the larger compound umbel. C. 1 two-seeded fruit showing prominent dark-coloured vertical ribs and lighter valleys between them. Poison hemlock. A. Top of flowering and fruiting stme. B. Portion of simple and compound umbels showing 2 bracts on only 1 side of hte small simple umbel but several bracts surrounding the base of the larger compound umbel. C. 1 two-seeded fruit showing prominent dark-coloured vertical ribs and lighter valleys between them.

Poison hemlock. A. Top of flowering and fruiting stme. B. Portion of simple and compound umbels showing 2 bracts on only 1 side of hte small simple umbel but several bracts surrounding the base of the larger compound umbel. C. 1 two-seeded fruit showing prominent dark-coloured vertical ribs and lighter valleys between them.


Stems & Roots: Stems freely branched to 3m (10ft) in height and smooth, with reddish-purple spots or blotches.

Leaves: Leaves finely divided, consisting of 3, 5 or 7 main divisions, each of which is divided and redivided, the ultimate divisions toothed or incised; the complete leaf from 3.5-40cm (1½-16in.) long, broadly triangular-ovate, smooth and hairless; leafstalks often marked with reddish-purple spots or blotches like the stems.

Flowers & Fruit: Flowers small, crowded in compound umbels 2-8cm (4/5-3½in.) wide; the terminal umbel blooming first but soon over-topped by others on the ends of branches arising from axils of leaves lower on the stem; the base of each main umbel surrounded by several small, ovate-pointed bracts (involucre) 1-3mm (1/25-1/8in.) wide and up to 10mm (2/5in.) long, and the bases of each of the smaller umbellets (simple umbels) in each compound umbel with still smaller bracts, but these on one side of the umbellet only; individual flowers (1) white, broadly ovoid and 2-4mm (1/12-1/6in.) across; mature fruits almost spherical, 2-3mm (1/12-1/8in.) long and equally wide, smooth except for 5 prominent light-coloured, rounded or wavy, lengthwise ribs on each of the 2 halves or "seeds". Flowers from June to August.

Habitat: It was introduced from Eurasia and is now found in a few locations in southern Ontario along roads and in woodlots and waste places. A recent introduction through contaminated forage seed resulted in its occurrence in a few cultivated fields.

Similar Species: It is distinguished from other members of the Carrot or Parsley Family by the combination of these characteristics: its finely-divided leaves. the reddish-purple spots or blotches on its smooth stems and leaf petioles, and its small (up to about 8 cm or 3½in. across) compound umbels with the tiny bractlets only on one side of each umbellet.

Caution: Poisonous: All parts of this plant are very poisonous.

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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