Overwintering kale roots: Deep cold winter temps.

As the temps drop for the year, we are wondering about saving kale plants from the garden (roots intact in the root cellar) for re-planting in the spring for seed collection.  Alternatively, we are toying with cutting the plants down to the ground, mulching heavily, and uncovering them in the spring for the same purpose.  Ideally if anyone in northern Minnesota/Wisconsin or Manitoba/Western Ontario of Canada has succeeded in doing either of these, I would be interested in hearing of your successes and failures.   Thanks!....

I live in zone 5. Spring planted kale mostly dies in the winter, but late plantings - young plants - make it through just fine. They all flower in the spring. Plant any time from August on, maybe?

In up state NY  fall planted kale grows most of the winter. When it's covered with snow fall it mostly goes dormant until the snow melts.

Wolf used to grow it in Alberta as a fall planting and harvested up to snow covering, then it would come back as soon as the snow was gone and the sun was out.

Redhawk

Hmmmm....Fall planting as a possible factor here is interesting.  Normally all of our kale is spring planted and only this year did I put in one variety in late August.  It's puttering along with the unusually cold nights we've had and yet maybe has put on enough growth to test this out.  So I will make sure to keep some of the spring-planted roots/plants where they are and compare these to the late summer plantings.....with everybody getting a good layer of mulch for overwintering.  We are right around that 80" frost depth shown below, but it's probably also a factor of soil structure and moisture and duration of deep winter that impacts winter survival.  

Thanks for these responses.....will give it a go!

Off topic, sorry...I normally plant spinach as a fall crop for overwintering. (I have friends in zone 4 that do this too). I usually end up planting too late for fall use, but it has a nice long early spring season.
One plus is, the voles don’t like it much at all. They go for the lettuce first, then the kale, and I have never seen damage on the spinach. One way to avoid them might be to plant late enough so that you are overwintering seedlings rather than plants? Seedlings here, so far, seem pretty tough.