Canada thistle, Cirsium arvense
Weed Control Options 
Life Cycle
PerennialPropagation
Reproducing by seed and by horizontal stems.Stems
Stems erect, 30 - 150 cm high, usually branched, slender, smooth or occasionally with a few, narrow, spiny-margined leaf-like wings on the lower part.Leaves
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 and Fruit
Flower heads numerous, comparatively small, 5 - 15 mm wide and about twice as long, the involucral bracts (b) weakly spiny or almost smooth; ray florets absent but disk florets (c) 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 - 4 mm long. Flowers from June to late autumn.Roots and Underground Structures
An extensive rhizome system.Habitat
Found in numerous different habitats throughout Canada.Competitiveness
.Distinguishing Features
Canada thistle is a highly variable species, especially in leaf characteristics that seem to fall into four groups that have been given varietal status: 1) Spiny Canada thistle, Cirsium arvense (L.) Scop. var. horridum Wimm. & Grab. 2) Entire-leaved Canada thistle, Cirsium arvense L. var. integrifolium Wimm. & Grab. 3) Canada thistle, Cirsium arvense L. var. arvense and 4) Woolly Canada thistle, Cirsium arvense L. var. vestitum Wimm. & Grab.Toxicity
Canada thistle is not known to be toxic.Human Health Issues
Canada thistle is not a known allergen.Forage Quality
No information exists at this time.Species Benefits
.Power Ranking Corn
Power ranking↑ 20
Power Ranking Soybeans
Power ranking↑ 17
Biological Control
- The stem gall fly, Urophora cardui
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- The stem weevil, Hadroplontus litura
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- The defoliating beetle, Lema cyanella
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Availability: Not commercially available
- The seed-head weevil, Rhinocyllus conicus
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Availability: Not commercially available
- The root-crown weevil, Cleonis pigr
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Availability: Not commercially available
- The defoliating beetle, Cassida rubiginosus
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- The seed-head weevil, Larinus planus
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- The seed-head fly, Terellia ruficauda
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- The systemic rust fungus, Puccinia punctiformis
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- Small midge with yellow-orange larvae, Dasineura gibsoni
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- Painted lady butterfly, Vanessa cardui
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Biopesticide Control
Currently none exist for this weed.Herbicide Resistance
No documented cases of herbicide resistance to date.For more information on weed resistance:
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Media Items
Figure 1: Canada thistle: A. Horizontal root with 2 aboveground shoots. B. Flowering stem. Top
Figure 2: A young Canada thistle seedling. Top
Figure 3: Canada thistle leaf. Top
Figure 4: Canada thistle stems and alternate leaves. Top
Figure 5: The root system of a Canada thistle seedling. Top
Figure 6: Canada thistles in flower. Top
Figure 7: Weed Control in non-gmo soybeans
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