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For those of you worrying about plant freezing and protection. I have done a little research to clarify this issue and hopefully explain in layman's terms and that it is actual freezing in many plants, particularly for an extended period, that causes damage or fatality. When the temperature drops below freezing ice crystals form within the plant causing water from within the cells to seep out, thus with less liquid within the cells the plant wilts. The reduction of water within the cells reduces the freezing point that can then cause dehydration (aka burn, scorch). Hardy plants have a protein that protects the plants against freezing and the cell membranes will have changed in them as they harden off (aka acclimatise) going into winter.
When there is a thaw water re-enters the cells in hardy plants and they normally recover and resume their previous form, possibly with some damage. In tender plants the cell membranes are damaged by the freezing, water cannot reenter the cells and the leaves and or plant stays wilted and may die.
In summary don't panic as for the majority of plants it is the action of actually freezing that is the risk. This is unlikely to happen when there is a light frost, it's a combination of duration and depth of cold that's the threat. This does not apply to all plants that may be tender or semi-hardy so it's best to do your research and know your plants tolerence levels.
Horticultural fleece is thermal, breathable and allows light to penetrate. Bubble wrap is not breathable and there is a risk of plants sweating in warmer weather. Hessian can be used but it can trap water in its fibres and freeze. Nevertheless bubble wrap and hessian are used successfully by some gardeners. Horticultural fleece's thickness is determined by grams per square metre (GSM) and is available up to 70 GSM, but 30 GSM is more readily available. You can buy it in packs, rolls and as fleece bags with draw strings at the open end. Fleece generally comes in white, but you can get green which is less obtrusive in your garden. The important factor is to create layers that deters penetration of the cold to the plant and preserves heat beneath the fleece or other layers you may use.