Growers in northern climates should be aware of the considerations of rootstock hardiness.
Although most folks make good decisions regarding the variety, one needs to pay attention to the rootstock as well. The aerial portion of your apple tree for instance, will suffer more disease, dieback, and the like. If the rootstock fails due to being out of its zone for hardiness, the tree will just drop dead.
Even the most cold hardy of rootstocks are really only cold hardy down to around 19 degrees F. Since soil will insulate somewhat, even bare soil in winter will afford it some protection. Geothermal heat additionally will leak to the area for further aid. The real helping hand when temps get into subzero zones is through snow cover. This is why areas like the northern plains of the US and the Canadian prairies are the real test areas in most years. When wind strips bare the soil and temperatures plummet to -40F and beyond you had better hope you were smart enough to choose the toughest roots of the lot. Now, damage doesn’t always mean death, and plants can sometimes recover from assaults like this, but they often don’t. Furthermore, damage can be hard to assess since it is, well, underground. Often the only indication is that our trees aren’t doing so well, which can sometimes be even more frustrating than having them get on with it and die. So, choose wisely. we are in the process of giving as much info on this site so you can make a good choice for yourself, and when there is choice we always graft to the hardiest we can so there is less you have to think about.