Weather data for 3 years to March 2021.
The aim is for the greenhouse to be frost free but as you might see, this is not quite the case. Temps of -0.5 and -0.6 deg C in Jan/Feb 2019 in fact caused no damage to any of the plants inside. However the minimum of -1.5 deg C in Jan 2021, caused by a combination of an unusually cold spell outside and low greenhouse soil temperatures, did cause one fatality: the mango. (this was a speculative long shot in any case, ah well….)