no responses, so bumping to let it move down

No experience with them here, but if the Canadian source is in Ontario, I would be skeptical. While there are parts that do get to USDA zone 3, be aware that the Canadian zones are a little different (~about a half zone of) if that is the system they use.

All magnolias have a more fleshy root system than most other cold temperate climate woody plants, and so are more susceptible to root damage in winter. As you have probably witnessed, a heavy continuous blanket of snow through the winter keeps the soil warm. The ground here, 40 miles west of Minneapolis, thawed in February under the heavy snow, despite the longest subzero like temps practically in recorded weather history. Most of Ontario is like this every year.

Also, sustained cold air temps like this past winter for us is way different than just dipping down. I have winter burn on on several evergreens that have not ever burned in 20+ years.

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!

Just an aside, most Canadians live south of here! Look it up on a map.

I've resigned myself to not being able to grow M. Grandifolia but hope to order M. Sieboldii Collosus which is rated 4AB. What the heck, that's part of the fun.

I am zone 3 and had one in my yard for two years loved the flowers and it got to about 4 foot high, it was an experimental plant from the nursery that i worked for, our winter was just to harsh for it but in zone 4 should make it - good luck it was a beauty!!!

I think you are pushing your luck. MN Arboretum (SW of Twin Cities) has some bigleaf magnolias and cucumber trees, but grandiflora might be asking too much. For MN, I think cucumber tree might be your best bet for a large magnolia (otherwise there are a number or dwarf type star magnolias that will grow here)