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Carduus

1. Phyllaries (outer) 3โ€“5 (โ€“6) mm wide below the middle; heads ca. 4โ€“9 cm in diameter, nodding at maturity and solitary at the ends of branches.

C. nutans

1. Phyllaries less than 2 mm wide; heads less than 4 cm in diameter, erect, solitary or clustered on branches.

2. Lower surface of leaves glabrate except for long, curled, septate hairs along veins.

C. acanthoides

2. Lower surface of leaves sparsely to densely tomentose with fine, non-septate hairs and often with curled, septate hairs along veins as well.

C. crispus

All species found in Carduus

Carduus acanthoidesPLUMELESS THISTLE 
Carduus crispusWELTED PLUMELESS THISTLE 
Carduus nutansNODDING THISTLE, MUSK THISTLE 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. March 28, 2017. http://michiganflora.net/genus.aspx?id=Carduus.