Ministry of Agriculture, Food and Rural Affairs

Ontario Weeds: Poison hemlock

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. 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. 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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