Mrs, I De Ver Warner?
Old House Gardens Heirloom Flower Bulbs has this rare cultivar up for sale and claims it is hardy in the area from which they received it ( most years)- which is down to probably zone 6b. I myself am in a borderline zone 6b to 7a, and have had dahlia tubers winter over in the ground only once, when the winter was really mild. They didn't make it through last winter though.
Anyway does anybody have experience with this dahlia at all to say yay or nay towards it's hardiness?