C. Michael Hogan (October 12, 2009). N. Strömberg (ed.). "Marsh Thistle: Cirsium palustre". GlobalTwitcher.com. Archived from the original on December 13, 2012. Retrieved August 3, 2010.
C. Michael Hogan (October 12, 2009). N. Strömberg (ed.). "Marsh Thistle: Cirsium palustre". GlobalTwitcher.com. Archived from the original on December 13, 2012. Retrieved August 3, 2010.