Although all cold has the same effect on plants, it can arrive at those plants in different ways. In order to protect your plants from cold, you need to understand how it happens.
Advective Cold
Advective cold occurs when a cold air mass moves into your region. The air mass tends to linger, perhaps for days, causing periods of unremitting cold. It does little good to cover plants to protect them from advective cold because there is no heat source to warm the inside of the cover. During advective cold periods, the sky is often overcast. If the ground is warm, the advective cold generates fog, sometimes in deep banks.
Radiation Cold
Soil absorbs heat from the sun during the day, and radiates it out again at night, with most of it escaping to outer space. Since this heat travels as radiation, rather than as cold air, it obeys different laws than advective cold. Radiation only occurs on clear nights—clouds or overcast are composed of water droplets, and water absorbs the heat efficiently, radiating it back to the ground.
As the radiation cools the soil, the air in contact with the soil cools also, making a puddle of cold air near the ground. This is called an inversion, a reversal of the normal condition, where the air near the ground is warmest, and that higher up is colder. This cold air is denser than the warmer air above it, so it flows downhill, filling valleys and low places. The cool downhill breeze often felt on hillsides in the evening is caused by this flow.
Heat escapes most rapidly from moist, compact, bare soil. Moisture helps the heat to move through the soil to the surface, where it can radiate out. Compaction has the same effect, squeezing out the air pockets that would otherwise act as insulators, slowing the movement of heat. Mulch, groundcovers, and lawns insulate the soil, slowing the release of heat.
Any cover that comes between the soil and the sky catches the radiation and sends it back to the soil. Opaque covers work better than clear plastic, which is transparent to the infrared radiation from the soil. Glass, however is opaque to infrared and catches it efficiently. Plastic film makes an effective cover if water condenses on it—water droplets traps the infrared just as clouds do.