Extreme cold. I cannot put my trees in a garage or shed, and cannot bury the pots in the ground due to lack of space or build any structures around them. Temperatures here can reach minus 25 Celsius(-13 Fahrenheit). When the temperature drops to 5 Celsius, I wrap all pots in bubble wrap, the top soil still exposed. As long as there is no snow the trees are covered in shade cloth for wind protection. When the snows come I throw as much snow over my trees as I can until they are completely buried. Even when the air temp is falling below -20 the temperature in my igloo doesn't fall below -10, and for the most part hovers around -5. There they can remain for 5 months undisturbed. Even when the snow melts everywhere else in the garden, the trees are still covered which is good because it protects them from the wind and early spring freeze-defrost cycle.
In the last 3 years I have lost only 1 tree doing this, although I suspect I might lose some this year as I've bought some trees that may not be so cold tolerant in a pot.