QUESTION: What is the best cover to use to protect outdoor plants from the cold air — fabric or plastic?
Threat of frost usually occurs overnight when the temperature drops low enough to freeze the moisture on plant leaves and buds. To protect plants from frost, you will need to cover them to keep the moisture from freezing. While an unexpected frost can leave many gardeners scrambling to find anything to cover their tender plants; it is important to use the right materials.
A fabric covering is best because it will allow moisture to escape while still protecting your plants from frost. Fabric coverings will prevent the freezing air from coming into direct contact with the moisture on the plant while also capturing the heat that is radiating from the ground. Bed sheets or comforters work best for covering large plants and shrubs. Newspaper can be used on low-growing foliage, but it can often be difficult to get it to stay in place. I have used old pillow cases, sheets, towels and even cardboard boxes. Just be sure that whatever you use to cover your plant, the cover reaches the ground and traps the warm air inside the canopy.
Plastic can be used but it is important to note that you should not let the plastic touch your plants. Plastic that touches your plants can often be even worse than no protection since it can hold moisture against plant tissues and cause more serious freeze damage. Many gardeners will often erect tall stakes or forms around their frost-sensitive plants so they can drape their covers over the plants and secure them without worrying about the coverings blowing away in the night or damaging the branches. You can use plastic to cover a structure like this as long as the plastic will not be touching the plant in any way.
And remember…the more layers of cover, the better the insulation provided. So, you can feel free to cover valuable or tender plants under more than one cover, especially during severe freezes. For instance, you could cover the plant with an old comforter and then a sheet of plastic.
No matter what you use, it is important to uncover the plant after the threat of frost has passed so the plant can get light as well as to prevent heat build up under the canopy once the sun comes out.