Continuous relatively deep (10", 25 cm) snow cover is excellent insulation against the cold. January is apparently the coldest month on average for us, but we have not had continuous deep snow cover during January for many years and even before then it was rare. I have not noticed any losses in my daylilies that could be related to the presence or absence of continuus snow cover during January here.
In the past, there was often a suggestion made that daylilies that might be suspected to be somewhat cold tender should be planted as early as possible in the growing season. I think that is very important, but it may imply that areas with shorter growing seasons might have problems. I suspect that daylily losses of cultivars that are hybridized in locations with mild winters (and long growing seasons) may frequently happen the first year that they are planted in locations with more severe winter conditions (but if such cultivars survive their first winter after being purchased they usually survive their future winters). I think that there are cultivars that take a long time to acclimate to the weather conditions in their new locations with more severe winter conditions. I suspect that many of those cultivars need a longer growing period to properly prepare for winter after they have been dug and divided. I think that they often do not get sufficient growing time in their new locations and then die during their first winter or are badly damaged during their first winter. If they do survive, then their second growing season can be a repetition of their first because of the damage they may have sustained during their first winter mimicking the damage they sustained being dug and divided.
That is what I think happens here to 'Crystal Blue Persuasion'. I bought it several times over the years and it died each time, never surviving its first winter here. Then I bought it, potted it, kept it growing inside over what would have been its first winter. I then planted it outside in the spring, pot and all and left it over winter. It survived. I think this is also what has happened in the past to 'Enchanter's Spell' (ES). I now have ES growing successfully in a location outside here. One year I ran a simple test on it. I used a spade to cut through its roots, lifting it slightly while in the ground to make sure the roots were broken. I then firmed it back into the ground. The next spring it suffered considerable winterkill which it had not shown for years.
I remember reading many years ago that some particular plant species (e.g. Clematis?) did not "like" to have their roots disturbed. The only research I have found of similar effects is that newly planted ramets of Miscanthus differ in winter mortality from established, recently undisturbed ramets of the same cultivar in the same location. The researchers suggest that the disturbed plants do not prepare for over-wintering in time because of the effects of being divided and replanted (disturbed). Any plants that do manage to survive their first winter apparently have then produced/recovered an effective timetable for preparing for winter in their new location.
