Answer: That will be a difficult environment for plants--it sounds almost like the desert (very hot days, very cold nights). I think the problem would come when, during sunny winter days, the plants are stimulated into growth, only to be nipped by freezing temperatures at night. Is there any way you can modify the temperatures a little more? Like using fans to direct heat in and out--either into the house or outdoors? Or using a shade cloth to filter out strong summer sun? If you can modify the temperatures somewhat, I think you will be able to find some suitable plants.
If you can't modify the lower temperatures, you could try plants that are adapted to zones 8 or 9 (roughly equivalent to the Gulf Coast area or part of California.) For example, you might try some herbs like lavender, sage (or salvia), various bulbs like lilies, etc. Or you could always stick to annual flowers, and leave the green house bare in the winter.
You may be able to find some appropriate ventilation and shade cloth options from Charley's Greenhouse Supply, 1569 Memorial Hwy, Mount Vernon, WA 98273, ph# 800-322-4707. Hope this helps a little.
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