Hi Fore (Chris)
I live within 40 miles of you. Near the IL-WI border. Your should be able to winter your new air layer with the rest of your hardy trees.
I have wintered 2 cultivars of Pyracantha outside, one a small leaf, the other a larger leaf cultivar. (names lost long ago) Both wintered in pots. Set on the ground under the bench, leaves piled to bury the pots, tarped all 4 sides of bench to keep out the wind. This spot is in total shade for the winter. Last winter (2012-2013) I have one night of a few hours at -15F (-26 C) though most of the winter nights were above 0 F (-18C). It was less than 5 nights that dipped below 0 F, and only the one night went below -5 F.
For temperate trees, by this I mean trees that are listed for zone 6, and colder, my experience says they all survive the roots be frozen. Actual temperatures are not a big deal. Glance at Chicago building code. Code requires sewer and water pipes to be buried below the frost line, the depth at which the ground freezes most winters. I believe building code is either 36 or 48 inches. Most trees growing in the ground have the majority of their roots in the top 36 inches of soil, feeder roots are literally in the top 10 inches, obviously their roots freeze and the trees in our landscapes survive. Freezing is not the issue. No particular temperature is the issue.
What is different about wintering a tree in a pot? Freeze - Thaw Cycling & Twig and Branch Dehydration.
The ground tends to freeze and stay frozen until spring. In a pot, it is dehydration of thawed out parts of the tree while other parts of a tree are still frozen that causes most winter kill.
The trick to winter hardiness is to minimize the freeze-thaw cycling, and dehydration during the episodes of freeze-thaw cycling that can not be avoided. Key for trees in pots is that they be in the shade during the winter. Sun can heat branches above freezing, allowing the branch to dehydrate, and if the roots are frozen, no sap will flow. So site trees in pots in the shade, I cover the bench sides with opaque painter's tarps, weighed down with cinder blocks at the bottom. This way the trees and pots freeze, and stay frozen. I will shovel a foot or two of snow over the trees when (if) we get snow. Then close the tarp again. This keeps the winter shelter cold like a beer cooler would for that week or so we get in late January or early February that warms up. The tarps keep the wind out, which also helps with preventing deydration.
Once I stopped pampering winter hardy trees, I had less trouble with trees sprouting before last frost, then having to do the in-and-out dance. I don't have a greenhouse, so if something sprouts early there is no place I can give it enough light for good growth and still protect tender new growth from freezing. It is best for everything to stay dormant until the maples and other trees in my landscape start to sprout in spring. Then eveyone moves back to the top of the bench.
For this discussion I am not talking about zone 7 or warmer growing trees. Those are more a case by case issue. For gardenia, satsuki azalea, serissa, certain bamboos, etc, I have spots for them in a cool temp light garden that stays just above freezing. But space there is at a premium, so I can only have half a dozen plants in this category.
