Answer: Although you probably are not aware of it, your question is a trick question! You see, frost forms at 32 degrees. That is, the plant surface where the frost is found is 32 degrees. However, the air temperature on a thermometer nearby may be somewhat warmer, even by a few degrees! Here is how that happens...
During the night, the soil and plant surfaces radiate heat out into the atmosphere. That is, they lose heat quite rapidly, even cooling off below the temperature of the air around them. On a cold, clear night, you may therefore see frost forming by morning on the surface of plants when the air temperature is in the mid to upper 30's.
Any covering that traps escaping heat can be helpful in a frost situation. The heavier weight spunbound polyester rowcovers are especially useful as they only weight about 1.5 ounces per square yard and offer a few degrees of protection. I would avoid plastic covers as they can burn plants where the cover touches the foliage.
I hope this helps. Thanks for the question!
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