Carduus acanthoides (Plumeless Thistle)
|Also known as:||Spiny Plumeless Thistle|
|Habitat:||sun; fields, pastures, roadsides, waste areas, disturbed soil|
|Bloom season:||July - October|
|Plant height:||3 to 6 feet|
|Wetland Indicator Status:||none|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Purple flower heads, occasionally white, and smaller than most comparative thistle species, generally 1 inch wide. Flower heads are solitary or in clusters of 2 to 5. The bracts are narrow and spreading with very short spiny tips.
Leaves and stem:
Leaves are deeply lobed, with lobes further segmented, wavy around the edges, and very spiny. Lower leaves are up to 8 inches long, becoming much shorter in the upper plant. Leaf undersides can be either smooth or have dense, bristly hairs, especially along major veins.
Plumeless Thistle resembles Bull Thistle (Cirsium vulgare), whose flowers are larger and leaves hairier, but is more closely related to Nodding Thistle (Carduus nutans), whose flowers are also larger and have broad showy bracts. The spiniest of all thistles in the state, Plumeless Thistle is on the noxious weed list for Minnesota and a relative new comer here, dispersing rapidly over the last thirty years from the Dakotas to the rest of the state. A common pasture/roadside pest, this species can invade high grade native habitats and is no doubt under-reported in the state. Of note is the MN Weed Advisory Group is recommending this species, currently a Prohibited-Control species, be removed from the noxious weed list because they have determined "infestations [are] primarily caused by human disturbance". The decision seems illogical, since that doesn't make it any less of a pest or threat. According to one source, a mature plant can produce 10,000 seeds. They may disperse within 10 days of flowering and over 90% will germinate. This one is nasty.
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Photos by K. Chayka taken in Hubbard and Wadena counties. Photos courtesy Peter M. Dziuk, taken at west central, Metro and southeastern MN sites.
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