The caerulea by far is hardest you cant kill it its tuff but be warned its invasive and pops up everywhere but in zone 4 not sure any will make your winter .The grossypiiflora is good but it dies over winter and I get enough seeds to just replant it .And you could do the old plant whole pot in ground then when winter comes just pull it up till spring.
Caerulea
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Grossypiiflora
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My caerulea has survived snow both here & at Mom's..

Thanks for the info! I'll try the Caerulea and take the suggestion of burying the pot and the pulling it up in the Fall to store it in an unheated part of my basement during the winter.

The hardiest Passiflora is probably P. lutea, followed by P. incarnata. These are herbacious and die back each winter. Hardy selections of both can survive to zone 5 or so. Lack of summer heat may limit them more than winter cold.

With P. caerulea, the foliage and wood can survive down to maybe 15 F, maybe even a little lower for really hardy selections, but it can also grow back from the roots like P. incarnata. P. caerulea is probably zone 6 at most.

Incarnata is rated for Zone 5, and is probably tougher than that. It dies to the ground every winter even here in Zone 8, but comes back with a vengence. It also sets a lot of seeds, and runs underground. Probably NOT a problem in your zone, though.