Protecting young trees during winter

[newby]

Hello all! I live up here on the edge of beautiful Georgian Bay, zone 5A with temps routinely -25C and below during the winter. I have several Pinus stroba, a convalescing Ilex crenata (Dwarf Pagoda) and several first year Ulmus thomasii trees that are all mame/shohin size. What is the best way to protect these small fellows this winter? I am concerned that the roots will be damaged if the soil in the little pots freeze. Ordinarily I would stick everything in the ground, but we are moving this winter and they would be frozen in.
The holly is another matter- it is listed as an outdoor bonsai but it is not hardy in this zone. I almost killed it keeping it inside last winter!
Has anyone tried overwintering trees in a styrofoam box with pot buried in some sort of mulching material eg pine needles or sawdust?
Any and all suggestions appreciated.

 

[Colorado]

Do you have a detached garage? A shed?

I keep most everything outside for most of the year, but for small plants and borderline hardy plants I bring them into the garage for really brutal cold fronts. It’s a lot of shuffling but works well for me, and not that much of a pain in the ass for shohin/mame size trees.

 

[yashu]

This is one of those things where it really depends on what you have access to and then using what you have to best represent what your particular tree might actually experience during (mild?) winter. There’s a ton of decent information on here but it’s best to ask folks that are near you geographically. There’s no one size fits all answer (to this question and most others as well). Personally I’m zone 4/5 so I have several strategies depending on the tree. I purchased some rooted Kishu cuttings in the middle of last winter and they were shipped from a much milder climate so I chose to leave them in a window with an inexpensive grow light in my climate controlled garage at 1° to 2° C (32° to 35° F) but not everyone has that option. Garages, enclosed porches, sheds, window wells, attics, basements etc. can all be tools.