Cirsium arvense (Canada Thistle)
|Also known as:||Field Thistle|
|Habitat:||sun; old fields, along roads, waste areas, wetlands, lakeshores|
|Bloom season:||June - October|
|Plant height:||3 to 5 feet|
|Wetland Indicator Status:||GP: FACU MW: FACU NCNE: FACU|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Flower heads are ¾ inch across, lavender to pale pink or sometimes white. The bracts are flat, except for the tip which peels back away from the flower head. The bract tips are pointed but not spined, and typically a purplish color, sometimes the entire bract is tinged purple. One plant typically has numerous flower heads, single or in small clusters at the ends of branching stems.
Leaves and stem:
Leaves are stalkless, up to 6 inches long with yellow spines scattered along the wavy edges. The lower leaves are up to 2½ inches across and deeply lobed, with a spine at the tip of each lobe. The upper leaves are more narrow, and generally more toothed than lobed. Leaves may be somewhat hairy on the underside. The stem is light green, slightly ridged and may have scattered hairs but is not usually spiny.
Canada Thistle is highly aggressive, can form large, dense colonies from its creeping roots, and crowds out native plants pretty much everywhere it grows. It is fairly easy to identify from the numerous small lavender to pinkish flower heads, and bracts that have no spines. While it prefers moist conditions it can sprout up and persist just about anywhere. It is currently designated as a Prohibited-Control noxious weed in Minnesota, but the MN Weed Advisory group is recommending a change to Restricted noxious weed, due to "lack of a viable statewide control method". Unfortunately, the lack of a statewide program applies to pretty much all invasive species in Minnesota at this time and they are in essence giving up the pretense they can do anything to stop its spread. We can only hope that will change someday.
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- budding Canada Thistle plant
- an infestation of Canada Thistle
- Canada Thistle with Crown Vetch
- first year emerging leaves
- spring rosette
- Canada Thistle invading a landscape planting
- unusual white flowers
Photos by K. Chayka and Peter M. Dziuk taken at various locations across Minnesota.
Have you seen this plant in Minnesota, or have any other comments about it?