Most trees and shrubs produce roots near the surface, where the air is. Mulching with wood chips could improve results, a sprinkle of dead leaves over a narrow area near the bottom of the tree is not much protection. A sheltered position could also improve results, magnolias are woodland trees. A similar small 'Edith Bogue' here had the branches peel off under slushy snow. Plants sold as 'Victoria' that do not have good indumentum may not be the true item. What I consider to be 'Victoria' produces somewhat ovate leaves with conspicuous pinkish sheaths over the newest ones at the ends of the shoots - reminiscent of those displayed by Ficus elastica.

Probably not yet as M. grandiflora flowers are very fast in spring or summer but really slow in winter. A frost might throw down the flowers or buds but the buds return in mild weather no matter at what time of winter but lesser in winter... Here is a group of winter flowering (hardy) rhodos in Victoria. Hardy rhodos may flower at any time of winter but December and Jan are usually the mosty quiet months and Feb is spring to early rhodos. April is the max. Summer is for tender specimens that are hardy subtropicals. Most like water & shade so the hardy ones like winter even if it means chilly periods or periods with morning frost that are near 0c or even minus 2C. Cheers Here is a link that might be useful: Pink Winter Rhodos

It depends what you mean by "grown successfully in MN". Do you mean a "charlie brown x-mas tree" type thing that barely survives with lots of protection or a 50 foot glorious tree with luxurious green leaves blowing in the arctic January breezes in MN? I used to live in Mpls so I am familiar with winter there. I now live in a marginal zone 7a in coastal SE Mass where I do grow southern magnolias. In my experience, there are two varieties that grow reasonably well here.....Edith Bogue and Bracken Brown Beauty. By "reasonably well" I mean they grow into small or medium sized trees 20-30 ft tall. And they bloom in early summer. The Edith Bogue is hardier and experienced no leaf damage here this winter. It is considered the hardiest southern magnolia. The Bracken Brown Beauty is less hardy and experienced about 50% burn this year here--growing in full winter sun I might add--but the branches and buds are all alive. Just for a point of reference, my location had only ONE below zero morning this winter (-7) and we average one such morning every 3 years--hence the marginal zone 7a designation. So I think if you wanted to try growing one you could try an Edith Bogue. But....it must be small enough so that you can protect it--a lot. Zone designations are misleading because they don't include duration of cold or wind. Even a "mild" Mpls winter features many below zero mornings--and many more windy mornings with just above zero readings. And then there is that occasional day where the temp might stay below zero all day. That kind of constant assault of cold and wind probably kill even the hardiest southern Mag without protection. So build an enclosure for it. Fill it with leaves and pine needles. Then wrap it in burlap a few layers deep and hope for the best! There used to be a forum member in Mpls who I communicated with a few times many years ago. He planted a variety called "24 below" but I lost touch and I have not heard anything about that variety since. Good luck!

I have only Little Gem and the non-cultivar standard M.grandiflora. Neither had damage of any type this winter.