Sorry, my long-term memory was off by a zone, it was 4, not 5. I quote from
the "Growing Instructions" I received with Doyle Thornless plants in 1999:
"In zones 3 and 4, plants should be protected, at least, during the first
winter"
To me, that's not hardiness for zones 3 and 4, and therefore doesn't satisfy
the "all 50 states" as had been claimed and as had been in my purchase
decision parameters. Unprotected vines have never survived the winter for me
here.