A system of plastic covering can be installed to protect raised beds from frost. The plastic must be supported so that it does not touch plants and it must make contact with the ground below. Since the earth is the source of the heat providing protection, a bed raised off of the ground must be covered all the way to ground level. For greater protection during hard or extended freezes, small Christmas tree lights can be added inside the frame to provide extra warmth.
Install wire hoops every 3 to 5 feet along the growing bed. These hoops are available through gardening supply stores and catalogs.
Cover the hoops with plastic sheeting.
Bury the edges of the plastic sheeting at the sides and end of the bed.
Slice a two inch hole in the side of the plastic every 2 to 3 feet along the row to allow ventilation and prevent overheating.
Remove or open the plastic during the day and cover again each evening until the danger of frost is past. On warm nights the plastic can be left open, covering completely on cold nights.
Remove the plastic covering when the weather warms and all danger of frost is past.
Add Christmas tree lights if additional protection is required. Use one string of lights per 5 feet of tunnel. Attach the lights to the wire frames. Turn the lights on at dusk and off at dawn.