There are some catastrophic weather forecasts that say that we should expect polar temperatures of -20, or even -26 for this weekend and next week...
My sensitive trees are in my closed garden teahouse. I have 4 termometers, two outside in the garden, and two in the teahouse...now winter shelter. The temperature in the tea house does not drop below -2 or -3 C. But that was up to now, when outside temperature was not below -8 or -10. If I turn the heating on, the temperature in winter shelter is between 0 and +3C. But, in more severe weather, I should probably turn the heating on more often. We will see in the next few days.
I use simple and cheap bathroom heater, it is set on a middle temperature and middle speed of hot air..
Some of my termometers showing -1 to -2C
I also drill and do some shaping in my winter shelter during long winter days...so it is dirty
Now, I can keep my sensitive deciduous trees away from freezing temperatures...the outside temperature cannot be so low that I couldn't be able to keep the trees safe around 0C.