CANADA THISTLE
Scientific Name: 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.
Habitat: Occurs throughout southern Canada and is most often a problem in perennial crops and areas where reduced tillage is practiced.
Seedlings
- Seedling with rosette
- 2 first leaves are opposite, successive are alternate
- Cotyledons:
- Oblong
- Apex rounded
- 8- 15 mm (0.3- 0.6 in.) long
- Fleshy
Stems
- Erect
- 30- 150 cm (1- 5 ft) 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
- Heads numerous
- Comparatively small, 5- 15 mm (1/5- 3/5 in.) wide
- 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
- Stamens and stigmas are sometimes present in the same flower
- All the flowers in 1 head and all the heads on 1 plant being either male or female
- Heads with male flowers somewhat shorter and narrower than heads with female
- Seeds:
- Light brown or straw-coloured
- Smooth
- 2.5- 4 mm (1/10- 1/6 in.) long
- Flowers from June to late autumn
Often Confused With
Biennial Thistles (Distinguished in non-flowering stages by their perennial habit reproducing from spreading underground roots, by the absences of a distinct circular rosette of basal leaves, and by their mostly non-winged stems with generally slender stature)
Bull Thistle (Distinguished by the absences of prickles from the surface of the leaf blades (apart from spines along the margins)
Sow Thistle (Distinguished by the absence of white milky juice from stems and leaves)
Click to enlarge.
