Canadian distributions above follow R. J. Moore and C. Frankton (1974); I have not seen those specimens. Carduus crispus has been reported also from Arkansas, Connecticut, Iowa, Illinois, Maine, Maryland, Massachusetts, Missouri, New York, North Dakota, Ohio, Rhode Island, Vermont, Virginia, and West Virginia; I have not seen specimens from those states.

Two subspecies of Carduus crispus have been recognized (S. M. A. Kazmi 1964); those are not differentiated here.

Carduus crispus closely resembles the much more common C. acanthoides. Some published records of C. crispus are probably C. acanthoides. Although the degree of spininess and tough versus brittle stems were used as key characters (A. Cronquist 1980; H. A. Gleason and A. Cronquist 1991) to differentiate the two taxa, both characters are subjective, and the second is impractical with dry material.