Tomatoes are a warm-season vegetable. Neither the young spring plants nor the mature autumn plants tolerate frost. A late spring frost quickly kills young tomato transplants, causing them to wilt and die. Mature plants may survive a mild frost, but the fruit will rot on the vine before you can pick it. A hard frost kills the mature plants, putting a stop to further garden production. Protecting your tomatoes from frost is imperative if you want healthy plants and usable fruit.
Place a stake on either side of the tomato plant. Use a stake that is at least 3 inches taller than the plant once it is installed.
Drape a sheet of clear plastic over the stakes during the afternoon while the ground is still warm. Arrange it so none of the plastic is in contact with the plant. If you are covering a row of tomato plants, place a stake between each plant then drape a single sheet of plastic over all the stakes.
Remove the plastic in the morning once temperatures warm and any frost on the ground has melted. Replace the plastic in the afternoon if another overnight frost is expected.
Leave the plastic covering on if temperatures remain close to the freezing point during the day. Open the covering 2 or 3 inches so air flows in and out during the day to prevent the plants from overheating, then close the plastic covering again before nightfall.
String a length of outdoor holiday lights under the plastic between the stakes that are holding up the plastic cover if temperatures are expected to drop more than 6 degrees below freezing. Place the lights under the cover, so the plastic helps trap the heat from the bulbs. Turn the lights on at nightfall and unplug them once the sun rises. Use non-LED, incandescent lights as these provide more heat.