"Cover plants with cloth or paper (not plastic) to insulate. You can use sheets or blankets for minimal protection or use a frost cloth. A properly applied frost cloth can protect plants at temperatures down to 30F, some down as low as 20F depending on the fabric and the weave. Completely drape the plant from the top all the way to the ground. Do not allow any openings for warmth to escape. This procedure will trap the heat radiating from the soil and maintain a more humid atmosphere around the plant foliage."
This is an excerpt from a University of Arizona Agricultural Extension Publication by Lucy Bradley. For further reading you can find the pdf here.
Garden Gate Nursery's Frost Cloth comes in 15ft. wide rolls and can be cut to length. It's made of woven fiber glass, is light weight and easy to keep stored for many years!