In truth nearly all temperate woody plants are reliably hardy down to -10°C (15F). Below this temperature all that is required is some protection for the root system which is not as frost hardy as the top growth. To become reliably hardy to these temperatures, trees must fully harden up in the Autumn by being grown outside. Trees are able to survive sub-zero temperatures by being dormant.
Deciduous Trees can be kept in a garage or shed should temperatures fall lower for an extended period, remember as they have no leaves little light is required. Temperatures should not rise and the trees need to be checked occasionally as the soil should not be allowed to dry out. But more often trees are fine kept outside in a place with a little shelter away from freezing winds and heavy rain or snow. Should they become waterlogged for days on end place a piece of wood under the bonsai container, this will help to enhance drainage and oxygen flow throughout the root system.