It turns out air layering might just work really well. It is the process of propagating woody plants by promoting root growth on a branch still up in the air and attached to the tree. Bending the branch down to the ground for this is ground layering.
The main steps appear to be:
Pick a two year old healthy branch.
Pick a spot just under a node.
Ring the branch under the node one inch and remove the bark.
Lightly scar the green (xylem?) underneath.
Apply rooting hormone.
Wrap in moist root medium. Peat moss, dirt etc.
Wrap that in clear plastic or a cut up clear cup.
Zip tie both ends or otherwise seal.
Over wrap with light blocker. Dark fabric or aluminum foil are common.
Check every week for drying out and root formation.
Cut off the tree when rooted.
Remove most of the leaves and stems to not overwhelm the tiny roots.
Pot or plant.
Protect like any other new tree.
Seems a lot of steps but there are compensations. The rooting hormone seems to be optional. How you ring or scar the branch affects success but is not critical. Many examples after one month on the tree look root bound in the cup or wrapping. It might be ready in three weeks.
I have about seven going now. We shall see.
Plant hardiness zones and apple trees sounds easy. Not so. There are about 8000 varieties of apple at the moment. It is easy to find hardiness zones. But it is a fuzzy concept. And the US and Canada have different maps. It looks like they have different definitions as well. It is easy to find a few hardiness numbers for trees. Almost never if it is the US or Canadian number. The root stock affects hardiness for any tree on sale, so that muddies the data. I was not able to find any list that includes all my trees in the same place. However, almost all apple trees are hardy to zone 4. Almost all of mine are zone 4 as well. Dan's place appears to be zone 4 or 4a at the moment, and warming up. The Gravenstein stands alone as hardy to zone 2. The Idared is the only one in my garden listed as hardy to zone 3. The Golden Nugget might be hardy to zone 5.
Ploidy refers to the number of chromosomes in the tree. This is a true rabbit hole. Most apples are diploid which means the normal two chromosomes. If you cross an apple with itself a certain weird way you get tetraploid: four chromosomes. These trees are weird. They can be giant or stunted. The fruit is often quite large. Then if you cross the diploid with the same variety of tetraploid you get a triploid tree: three chromosomes. Triploids tend to be over size, with large fruit, high disease resistance, and high vigor. Many heritage apples in the eastern US are old triploid trees. Our garden and the neighbor have these triploid trees: Gravenstein, Bramleys's Seedling, Newtown Pippin, and Mutsu (Crispin). Pollen from triploids is nearly sterile. They do not pollinate other trees. And they require pollen from two diploid trees to set fruit. There is a good side to this: thinning. If there is a lack of local pollination then the tree will set lightly and not require much thinning to get large apples. It makes biennial bearing less likely as well. This is when the tree sets way too much fruit and settles into a habit of cropping every other year with minimal crop in the lean years.
Sports are an interesting highlight. This is where a tree mutates just a branch to an different variety. It can be a minor variation like color. Or a diploid can sport a tetraploid or triploid branch. The branch can even have triploid inside with the bark, leaves, and fruit unchanged from the parent. As usual, sports can be propagated from cuttings or layering to make a new kind of tree. The triploid Gravenstein is very prone to sports. Something to watch for.
Apple trees for sale are normally grafted to tough crabapple roots from a handful of managed varieties. This limits the size of the tree and provides a pest and drought resistant foundation. Air layers or cuttings when planted as is are "own root". The tree will be larger. Some own root varieties are flawed and will not grow or produce well. I have seen a report that Bramley's Seedling is one of the most vigorous own root trees known.
Nearly done but I need to mention deer. It turns out that fruit tree leaves, shoots and even branches are like caviar to deer. These little guys will lunch on your young tree leaving nothing behind. Even mature trees are not safe. Bucks like to rub on trees. They can do serious damage to the bark with this practice. They like apples trees for this. Dang. The deer can leap clear over a 12 foot fence. They have been reported to get over 15 foot fences as well. All is not lost. A tight ring of four foot fence around the tree will discourage deer as long as there is other food available. If they are hungry, the fence will get knocked down.
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