Frost

[whatatip]

Can someone help me understand frost.
What happens?
Why is it that a bit of fleece over my mother's hydrangeas protects them? Surely the temperature would drop to freezing in the soil regardless of that bit of fleece.
And why, if fleece works, did a person in a nursery tell me that my tender plants (heliotropes) would need to be in a heated greenhouse, and an unheated one wouldn't do?
Do unheated greenhouses benefit only some plants?
And why does frost not necessarily affect all the leaves on a plant?

Thanks!

[cantspel]

It is not the soil that is the main problem. If the we get a frost then the cells of the plant will freeze and expand and this is what will kill off a tender plant.

[Bienchen]

tender plants need a minimum of several degrees C above freezing, some as much as 10 or 15, you would not achieve that with fleece.

Frost damage can also happen from cold wind which is why sometimes only one side of a plant is affected; the side that is sheltered can escape unscathed.

Unheated greenhouses give a little bit of shelter if the temperature is around 0 degrees C, they won't protect if you have hard or prolonged frost. You can increase the insulation of an unheated greenhouse by ysing bubble wrap but for the very tender plants this is not sufficient.