A covering should protect them. And provide a micro environment. Plants and soil activity generate heat. Your trying to contain that and prevent ice forming in your cells. A fan defeats this. Temps are colder due to evaporation. Orchards use them to prevent frost ,ice crystals from forming on plants that can tolerate colder temps. Blow em dry before you cover if wet. If still in the 30's for day? Remove plastic. Blow dry and recover to catch ambient heat and suns IR. Just watch for wilting. Sun is still pretty hot for another month.