I'm not sure where you're getting your info but Mine(& yours) have survived winters here so far. I don't have any large enough yet to bloom. We'll have to see.
The link below is to Easyliving Wildflowers, my source of seed. They claim it be zone 2 and that we in Mn are in its normal range.

http://www.easywildflowers.com/quality/cph.occid.htm