Ontario Weeds: Canada thistle

Table of Contents

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

Name: Canada thistle, Cirsium arvense (L.) Scop.,

Other Names: chardon des champs, Canadian thistle, Creeping thistle, Field thistle, chardon du Canada

Family: Composite or Aster Family (Compositae) 

General Description: Perennial, reproducing by seed and by horizontal roots which produce new shoots, often forming dense patches.

Photos and Pictures

A - nodding thistle; B - bull thistle; C - Canada thistle (purple-flowered male plant); D - Canada thistle (purple-flowered female plant); E - Canada thistle (white-flowered male plant).

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Stems & Roots: Stems erect, 30-150cm (1-5ft) high, usually branched, slender, smooth or occasionally with a few, narrow, spiny-margined leaf-like wings on the lower part; leaves more or less lobed and spiny, alternate (1 per node), elliptic to oblong in outline, stalkless and often clasping the stem; the wide variations in lobing, spininess, hairiness, texture and colour of leaves divide the species into 4 botanical varieties.

Flowers & Fruit: Flower heads numerous, comparatively small, 5-15mm (1/5-3/5in.) wide and about twice as long, the involucral bracts weakly spiny or almost smooth; ray florets absent but disk florets prominent with purplish or sometimes white corollas; plants unisexual; although stamens and stigmas are sometimes present in the same flower, the flowers are functionally unisexual, all the flowers in 1 head and all the heads on 1 plant being either male or female; heads with male (pollen-producing) flowers somewhat shorter and narrower than heads with female (seed-producing) flowers; seeds light brown or straw-coloured, smooth, 2.5-4mm (1/10-1/6in.) long. Flowers from June to late autumn.

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Similar Species: Four botanical varieties of Canada thistle occur in Ontario. They are distinguished by differences in leaf characteristics. Common names have not gained general acceptance for them but descriptive terms are applicable as follows: Spiny Canada thistle; Entire-leaved Canada thistle; Gentle Canada thistle; Woolly Canada thistle

All four varieties of Canada thistle are distinguished from the biennial thistles in non-flowering stages by their perennial habit reproducing from spreading underground roots, by the absence of a distinct circular rosette of basal leaves, and by their mostly non-winged stems with generally slender stature, and from Bull thistle by the absence of prickles from the surface of the leaf blades (apart from spines along the margins); flowering stages are distinguished by their generally smaller flower heads, mostly less than 25mm (1in.) long and 15mm (3/5in.) wide, these unisexual, and the involucral bracts surrounding each head either without spiny tips or with very short weak ones. They are distinguished from the Sow-thistles by the absence of white milky juice from stems and leaves.

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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