You may leave your potted trees and shrubs outdoors over the winter provided they are hardy in your climate and provided you give them extra protection for the winter. Because the roots are aboveground, they're exposed to colder temperatures than woody ornamentals growing in the ground. Keep the soil temperature in the pots above freezing. With smaller pots, you could dig a hole in the ground and place the pots in the hole, mulching over the top to insulate the area. Because your pots are too large for that, it may be best to group the pots together and cover them with a thick layer of straw, mulch, wood chips, or shredded leaves to protect the roots from freezing. If it looks like they'll freeze, move containers to an unheated garage or shed temporarily for added protection.