I've found with that species that you cannot "fool" them- if anything at all about their environment makes them "want" to go dormant, they will and there's no stopping them. They don't seem to need so much cold as they do reduced photoperiod, and I've had them in trays before, seedlings from cuttings, and they went dormant right next to all my year-round growers.
As to what they're hardy to, I know that I killed some by leaving them outside last year, so a typical Niagara winter (mostly around 0 C but can get down past -20 C) will kill them, but as to what they can "take" for real, I couldn't say, they seem a LOT less tough than the garden variety D.filiformis ssp. filiformis tho