Answer: An unheated greenhouse would be used much like a coldframe, providing protection from just a few degrees of frost. You would use it for hardening off the plants prior to transplanting them into the garden. Unheated greenhouses do not provide enough warmth on cold nights to move the seedlings out there until the weather is fairly settled. Many gardeners will also keep some frost blankets handy if an exceptionally frosty night threatens. You will also want to make sure it is vented automatically or manually as needed during the day because heat will build up quickly on sunny days. In the meantime, you may want to use supplemental lighting for your seedlings such as a shop light fitted with one "warm" and one "cool" color bulb with the plants kept within an inch or two of the bulbs.
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