Depends on the tree, depends on the pot size, depends on your microclimate (how cold does it get overnight in Jan. in your yard vs someone's a block away because of hedge protection from wind, etc). I'm from Mtl. and if you tell me what your trees are and how big the pots are, I can give you some advice. Do you have a garage, is it attached or not, and how about a shed, or unheated(?) porch?

Hi, I also live in Montreal and have a question about wintering bonsai. I just received a little juniper (procumbens nana I believe)as a gift and from what I have read it seems they are not suitable as indoor bonsai. It is six years old, about 6" high and the pot is roughly 6" x 4".
I live on a 3rd floor with two balconies, one facing N the other SW. Both are unprotected. One option I considered was putting the bonsai in the space between the outer and inner windows, as there is enough room. Would this be appropriate? Does it need better air circulation? I can't tell you how cold it gets between the windows, but it does get down to -30 outside on a good day.
thanks in advance!