I have been growing 'Cosmic Kaleidoscope' in my zone 4 garden since 2011 - it has not had any problems so far.
I have had 'Snow Crystal' for about one year so it has lived through one winter.
Qualifiers:
I have good deep snow cover for most of the winter with usually one complete thaw in January each year and then snow accumulates and I again have deep snow cover to the end of winter after that (till about the last few weeks of March to the first week of April).
Lowest minimum winter temperature is about -32C or -25/-26F but that only happens once or twice every few years.
I have dug down through the snow once during several winters and each time the soil has not been frozen under the snow. The snow depth was at least 12 to 18 inches where I dug down.
