Cirsium clavatum var. osterhoutii
The densely pubescent involucre of var. osterhoutii is a feature that links those plants with Cirsium eatonii var. eriocephalum. Some specimens tentatively assigned to C. clavatum var. osterhoutiii may be derivatives of past introgression with C. eatonii var. eriocephalum. Congested heads and strongly undulate leaves with numerous closely spaced lobes are features that suggest such a relationship. Further studies at both population and regional levels are needed to establish the nature of the variation patterns. Where I have observed var. osterhoutii in the field, populations appeared to be morpologically stable, and there was no evidence of current introgression with C. eatonii.