It's going to get cold.

[Paul in Essex]

The theory behind its efficacy is; since the ground is almost always warmer than the air on cold nights, the warmer temperature can be trapped underneath the cloth. The reason behind the use of stakes is to keep the vegetation away from the cold air.

As I understand it, exactly this. On a small scale this is the same principle as sheltering a tender plant by planting it underneath an evergreen canopy or, on a bigger scale, cloud cover rolling in on a cold night preventing what would otherwise be a nasty frost. If there is a heat source (other than the ground) so much the better but the principle remains the same - to capture heat that would otherwise radiate away into the atmosphere. In the open garden a cover like this can also mitigate the physical effects of frost/snow coming into contact with the plant but that is a different situation.

[habanerocat]

If the heat comes from the ground then bubble-wrap on benches and this type of carry-on can't be of much use surely?
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[Paul in Essex]

There's no 'if' - heat is lost by the ground. Any type of barrier can mitigate this to one extent or another - bubblewrap, newspaper, foil blankets. Even a layer of small children, although I see those more as a heat source.

And 'this type of carry on' is something else entirely - protecting the growth point from the direct action of frost or snow making contact.

[Herts Mike]

Yellowknife in North Western Territories Canada is -41c today warming up to -37c.

[ralphrmartin]

Its cold at night here, but warms up nicely by the time the sun goes down. The temperature in the unheated bit of the greenhouse was down to 1C last night and up to 15C at the warmest today.

[Tina]

unheated bit of the greenhouse was down to 1C last night and up to 15C at the warmest today.

That's a really good variation for the plants

[Patrick]

Its cold at night here, but warms up nicely by the time the sun goes down. The temperature in the unheated bit of the greenhouse was down to 1C last night and up to 15C at the warmest today.

That's a bit like the temperature outside here in Portugal at the moment, although the lowest so far this winter has been 2.5 and it often gets up closer to 20c.

[Aiko]

Up here it is a bit colder. Two nights ago it was around -7C, and last night my plants that I kept in my unheated greenhouse (about half of the cacti) had to endure -9C. Coming week it will be around 0C.

[SimonT]

Much of the UK is predicted to have another period of cold weather (potentially with some snow) next week -7 to -10oC to look forward to in some places.

So following on from habanerocat's question, if you had a plant in a greenhouse, is it safer from cold on the staging or partially sunk into the ground?
Assuming the ground remains dry of course and the greenhouse is unheated and everything else is equal between the two locations.

I've tried both approaches in the past and for me staging wins- but I think this might have been just because of the increased risk of plants gettting wet if sunk into the ground.