Ottawan,
I didn't mention before that my fig trees currently reside in large tubs (25 gallon maybe) in an unheated hoophouse. They did drop their leaves last winter, so were dormant. At a farm show this week, we attended a seminar on hoophouse growing and it was stated that each layer of plastic adds protection equivalent to one zone. So, since we are zone 5b (-10 to -15 F), we gain to zone 7b (+10 to +5 F) inside the hoophouse because we have a layer of poly over the top of the house and a dropped ceiling that is seven feet high. That is equivalent to Northern Arkansas.
When I get cuttings of each variety growing, the large trees will probably be planted outside since the roots already fill the tubs. I am not sure if I need to attempt to root prune these after they go dormant or not.
All small trees (I got two late this summer and am expecting a few more next week from Jon) will spend the winter in the heated portion of the hoophouse where the thermostat is set at fifty degrees F. Of course the daytime temperature is much higher than that when there is sunshine. And because of the heated side, the cool side may actually gain another zone equivalent since there is no insulation between the sides, just a wall of clear roofing. Last year it still froze in the cool side but it may not do so this year since this is the first year with the dropped ceiling on the cool side.
elizabeth near Kansas City Missouri
zones 5b through 7b or higher :-)