Answer: In a cold winter climate, an unheated coldframe is not really suited to overwintering non-hardy plants such as geraniums. Since the coldframes is not very well insulated, it is not suitable for using as a heated "greenhouse" or "hothouse" either -- hence the name "cold" frame. Instead, it can be used in some instances to protect overwintering hardy perennials planted in containers from the most extreme of winter weather, and to harden off transplants in the spring.
The uninsulated cold frame will give only moderate protection from cold temperatures (in addition to the frame itself some gardeners will use row covers and additional blankets to insulate the plants on extra cold nights) and is used to buffer the cold and wind and occasional pounding rain while seedlings are becoming acclimated to life outdoors. While hardy perennials will tolerate some cold, it is still a big shock for them to be moved from the indoors to an unheated cold frame so you still can't rush things too much. Annuals and vegetable transplants can also be hardened off using a cold frame to protect them from late frosts.
A good location for a cold frame is on the ground in a protected spot out of the wind and near the house so you can open and close it or cover it as needed. Be sure to open the frame (either a crack or all the way as needed depending on the weather) during sunny days as it heats up quickly just like a closed up car will do.
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