I've trained a young tree for bonsai recently and left it out in our own very cold/harsh winter. I've set my pots in a larger earth filled container for protection.

Even evergreens such as firs, cedars, boxwoods, hollies, pines have difficulties surviving in a pot if the pot is not insulated. The only way is to make sure the containers are large and insulated. Bottomline the temperatures for the plants should remain steady and so once frozen in winter, you should start mulching the top and stop watering.

I like the idea of setting a container inside a larger container. There should be enough space between the pots to place styrofoam insulators (like the ones used in houses), or mulches or just plain soil. You can hide the inner pot by disguising the top with mulches or moss.

Ianna