Question: Can I cover tender plants to keep them from freezing? What type of covering is best?
Answer: If you’re talking about tomatoes, marigolds and similar tender annual plants, they can actually be damaged by frost, even when it isn’t actually freezing. Frost can form on exposed surfaces on clear, still nights, even at temperatures as warm as 38 to 40 degrees. You can prevent that kind of damage by covering the plants with old sheets or one of the lightweight frost-cloth fabrics from the nursery. If it’s going below freezing, however, you’ll need to add supplemental heating beneath the cover, so you might want to use plastic instead. You’ll have to remove it quickly in the morning, however, or the plants will warm up much too quickly. By comparison, if you have tender shrubs or plants such as pansies, you can gain several degrees of protection from really cold temperatures by covering the plants securely with the lightweight frost-cloth fabrics. These products can be re-used in future winters, and they’re really great at saving your plants.