Posted: 23/03/2015 at 13:26
John, damage occurs when frost actually settles on the plant. This causes the internal structure of the plant to freeze leading to death/damage. Covering the plant stops this, generally the thicker the covering, the more protection a thing gets. Layering affords even more protection, just like clothing, trapping layers of air between each layer. I use fleece down to temperatures of -5, and only removing it when the temperature rises. A cloche or bottle will help, but fleece would afford more protection. Obelixx advice is perfect for the temperature you describe, personally I prefer fleece, but either would be sufficient.