Where the ground is thawed, even in the bottom of a hole you dig, the roots of plants will still grow, slowly.
Mulch very heavily, if you can, but only up to the trunk. Don't bury that, but several inches of mulch will help keep the soil even in temperature and warmer, most of the time.
Limited oxidation and microbial activity, as the mulch decays, generate heat. Not much, I am sure.And the darker the mulch, the better. I know commercial growers spray evergreens with an evaporation retardant, to keep them from losing water while the roots cannot bring any up. I cannot say you can find such a product for your use.
I cannot imagine they would not do better in the ground than in a pot. exposed on all sides but the bottom to the freezing temperature.