D.J.Keil (A.Gray) 2004 - Keil DJ (2004) NEW TAXA AND NEW COMBINATIONS IN NORTH AMERICAN CIRSIUM (ASTERACEAE: CARDUEAE). Sida, Contributions To Botany 21(1): 207-219. Retrieved from http://www.jstor.org/stable/41968995
There is no corrections for Cirsium clavatum var. americanum yet.