The parentage of Prospero is "The Knight" x "Château de Clos Vougeot", Lots of Tea Rose very close up on the "Château de Clos Vougeot" side, but lots of cold hardiness on "The Knight" side; "Chianti" x "Gypsy Boy". "Chianti" is hardy to at least 5a and "Gypsy Boy" is rated 3b, so it all depends on which cold hardiness genes are expressed. 5b or 6a would be my guess too.

"The Squire" has identical breeding and I grew him in zone 4b in a pot that I wintered in an unheated shed with lowest temperatures to -31C outside. He lived and thrived for many years there. I have heard though, that Prospero is not as cold hardy.

I love your images of this rose Jeri. Still have not been able to get one though.

Cheers, Rick