I'm not sure for your area but I know that you will have to do something. I live in zone 9, CA, foothills of the Sierras and my pomagranate bonsai all died two winters ago when it got down to 11 F. The winter before they did OK but it never got below 20 F. Pomagranates in the ground survived both winters however. I think the problem is frozen roots. So my recommendation would be to keep the plants in an environment where the roots will not freeze. They will be leafless and dormant so lack of light whould not be a problem. I have bonsai friends in Montana and they dig a trench about 6-8" deep and put the pot in it and then fill it in with sawdust or straw and mound it up over the tree as well. It works for apples, elms and maples. I'm not sure about pomagranates.

Thanks Jack for the advice...

DaveskiB

In your area, you would need to guard against soil temps dropping to killing lows - something below freezing, but I'm not exactly sure how low that might be. I have one that's going into its 4th winter. I over-winter it in my attached (unheated) garage, but I take a precaution that is very important. I place it directly on the floor under the tables the rest of my more hardy plants are on, but I cover it with an overturned cardboard box. The box traps geothermal heat rising through the floor and guarantees the soil will not fall below around 35*, even if you forget & leave the o/h door open. This works well & adds at least a full zone to the deciduous material you should be able to over-winter.

Al