Actually the hardiest cultivars of Southern Magnolia are hardy to temps between -20°F(-29°C) and -25°F(-31.7°C). But that's for relatively short periods of time (hours, not days). Protection from winter sun and wind are both important in preventing leaf burning. I think winter sun protection is a little more important with this though. Even though a tree might have lots of leaf burning, it often will not have any stem or wood damage. The ideal spot to plant one in a border line area would be against a NW facing wall, which would give plenty of heat during the summer (plus retain some heat longer into the night during winter) and also afford some winter sun protection since the sun sets in the SW during the winter(in the Northern hemisphere). Some people in zone 5 that try to grow Southern Magnolias many times report they act more like deciduous trees in having most all of their leaves burned off during the winter, but releaf out in the spring and bloom, so no wood damage is done. I grow more than 1 dozen cultivars of Southern Magnolia here in zone 6 KS and none have been killed or had much of a problem with winter hardiness. Even the supposedly less winter hardy cultivar 'Little Gem' grows fine for me. I also know of many decent sized trees(30'-50'(9-15m) tall range) in the Kansas City metro area which have been through their all time record low temp of -22°F(-30°C) and the very tough, cold winters of the late 1970's and early 1980's. So, I think in general most Southern Magnolias are zone 6 hardy and a few can make it in protected spots in zone 5. Duration of cold is important as is summer heat. If you live in a colder area with abundant summer heat(temps mostly in the 80's(26°-31°C) or warmer for most of summer), you probably have a better chance at growing Southern Magnolia than an area with little or no summer heat and relatively mild winters.

I had one case of RRD about Sept 2009, on a very tall mini (5 ft tall, so big for a mini). The bush looked odd in June, so I cut the odd cane off at the ground. However, the new growth looked weird and I dug the entire bush out in early Sept, plus three nearby rosebushes (2 HT and 1 miniflora). I dug deep and got their roots also. I wrote a letter and sent the key diseased canes to Dr. Jim Amrine at West Va. Univ. (by Fed Ex). Dr. Amrine confirmed it was RRD. He said it probably blew in from a highway that is about 3 miles from my house, that has woods growing around on either side. To Henry, I am not sure how he checked for RRD - sometimes by looking thru a microscope and seeing a dead mite; otherwise that he is knowledgeable of the symptoms - feathery, light green leaves, many needle-like thorns, thick stems, reddish growth and performed a visual inspection; or perhaps by another method. Fortunately, I have not had RRD since then. I planted petunias and garlic chives in the rosebed. (Farmers alternate crops.) This fall (2 years later), I transplanted micromini roses from a garden that was getting waterlogged just this year. So the micromini roses may make it or not. But they are 6 inches tall, so far. They do not have RRD, at least. They have 18" wire fences around them to protect them from rabbits and a raccoon. Also, to apply Round-Up so it does not affect neighboring plants, cut or saw a branch of the plant to be removed and paint concentrated Round-Up on the open cut with a paintbrush. For a big bush to be removed, make 2 or 3 cuts and paint those. Lastly, the MD Ag Extension Service does not care. I tried to report the RRD, but no one would take my report. Dr. Amrine made a note with his records.

I've actually talked with some seed companies about this for a project I was working on, and they (at least the ones I talked to) don't go to much trouble to verify the purity of the species of the trees that their seeds come from. I got the impression that they left it mostly up to the people that actually collected and sold them the seed to identify and verify the source. The companies probably aren't going to tell you who they got the seed from, so basically (at least with most oaks) it's just one big roll of the dice.

Prostrata-I am almost 100 percent sure Forest farm uses deodara seedlings. Bob Fincham, who all would agree is an expert, has state it is the best understock and is hardy as an understock. If you are looking for other cedars to try look for Eisregen. There is a collector I talked to in Indianapolis who told me it was his hardiest true cedar and he also has Stenacoma. From what you have said about you weather the wind is most likely more of a problem than the cold. You may want to spray the young cedars with Wilt-pruf for several years until they become more established. Livingfossil-I have Karl Fuchs that I got as a 6' BB 3 years ago. It had grown well and is about 10' now. The needles at the top look a little discolored but I think it will be OK. I also planted a 6'BB at my neighbors house this summer and it to looks fine. One of my other neighbors planted a weeping blue cedar that he got from Lowes or HD and it seems to have done fine also.