Paul, the problem with plastic greenhouse is that when there is a temperature inversion (calm clear cold weather at night causes cold air to sink) the cold air from the inversion actually gets trapped in the greenhouse and it is colder than outdoors. Same as when you have a "frost pocket" that is a place in the landscape where cold air gets trapped on this kind of night.
The easy solution for a small greenhouse is to cover it with a tarp when you're expecting a clear and cool night. That would allow it to retain its warmth and keep out the cold air that's sinking.
You can also leave it open at night to allow air circulation, but that means your low will be at best the same as outdoors.
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