Frost cloth can make a significant difference in how well your plants survive extreme winter cold. It’s lightweight (so it doesn’t weight down your plants unless it’s covered in snow) and porous (so there’s good exchange of air and water).
Put down before temperatures plummet, frost cloth can make 6 to 8 degrees worth of difference in protecting cold-vulnerable plants. Of course, that assumes that you’ve watered the plants to keep them perfectly hydrated, and it also assumes that you’ve chosen plants that at least have a good chance of making it through your local winters. Frost cloth can’t save a tropical plant when it’s 15 degrees outside.
Buy frost cloth ahead of the time of real need. Wider 8-foot rolls make the job a lot easier. Cut the fabric to fit over all the plants you might want to be covering. Carefully fold it up and label each piece, then store them in a black plastic trash bag to protect them and keep them reasonably clean.
When a cold spell is forecast, get some help to pull the pieces over your plants. Hold them firmly to the ground with bricks or large river rocks. Have them stashed out of sight until you need them.
You will find frost cloth at most good independent retail nurseries and hardware stores, also online. Buy it before the time of need, though. Once it turns cold, you may not be able to find it at all.
For the record, old bed sheets work well, too. But don’t cover your plants with polyethylene plastic. Wherever it makes contact with leaves, the foliage will turn brown.