Are you truly zone 7 in SLC? Most of your local hardiness maps say 5 or 6.......and Sunset lists you as a 3a with a minimum range of 4-5 for evergreen magnolias.

Extremes of your location might account for the lack of mature specimens, not just the winter lows. EB and BBB should be able to tolerate a zone 6 without too much issue but I wonder about the aridness.........and perhaps soil alkalinity. Too dry and too alkaline a soil could certainly compromise the success of this plant.

'Victoria' is another very hardy choice.

'24 below' might be the hardiest of them all.....its a hard to get cultivar however.....survived -24 F temperatures in the 80's and 90's with little to no damage

Aside from hardiness zone, you have to consider annual rainfall. I planted two EB Magnolias about six years ago. They were about two feet tall at planting--even bloomed the first year. Now they are 12-15 feet and sprawling but we get between 45-50 inches of rain per year on average. Your cold would be okay, it is just not wet enough for them to thrive.

The zone 7 is from the usda. I'm not sure about it's reliability. As I recall, a few years ago we were listed in zone 5. But, judging from the temps they use it is about right. In the Salt Lake valley we will get down to 0 and sometimes 2 or 3 below. Usually we stay in the +1 to +5 range.

Anyway, I can remember two Ediths in my area. One was planted on the west side of the Tabernacle and kept small purposely. I guess they were growing it for it's leaves not so much for it's flowers. The other was a younger tree that was seemingly healthy, but I never saw it flower. It was a lawn planting on a lot that had been given an award for beautification and was declared blighted just a couple years later so the city could turn the subdivision into a high rise hotel.

My problem is that I love the Southern Magnolias. I just love the flower, the tree and that intoxicating aroma. It is fall sale season in the nurseries hear and one has some Ediths on sale. I'll plant one, keep it watered and hope for the best.