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Most frost damage is caused by frost physically forming on tender new growth. A mini greenhouse will raise the temperature of the soil higher than that of the surrounding area, hopefully enough to prevent freezing overnight. It will also largely prevent any frost forming directly on foliage. In short, a mini greenhouse will give your plants a better chance.
The likelihood of a plant flourishing is apparently inverse to the care you lavish on it, this is why plants you don't want will self-seed and prosper. The second you remove the mini greenhouse, all your plants will be battered by the frost that wasn't predicted, or dug up by next doors' cat.
ColinTheBald
This might be an obvious question but I can't work it out. What's the warmest temperature that you can still get a plant killing frost at, and how much will a mini greenhouse reduce this to? As in can it be 0 outside but still no danger to plants in a mini greenhouse?
I have just stuck some sweet pea and digitalis seeds in a mini greenhouse in my quite sheltered urban garden. Hoping they will be fine from now on - I read something about sowing them early meaning that they'll be hardier than if you sowed them later or indoors, they get a bit spoilt that way...