nexxogen:
Then you have to take into account the warming during the day.
How old was he at the time of planting?
Did you put any mulch in?
On those cold days, I don't think that daily highs exceeded 8°C, sometimes even less. But generally, during January which is the coldest month, daily highs are around 12°C.
The trees were about 3 year old seedlings I believe. They were over 1.7m tall. I did not mulch at all. Maybe it's worth mentioning that winters here are very rainy.
Indeed this is important because just like many tropical trees, if the water is too important in winter I notice that the roots risk rotting.
If the roots die, the tree too.
This is why a good mulch (10 cm) allows (in my orchard) to avoid this problem.
This also prevents the soil from freezing and thus protects the roots when the tree is young.
Often too much moisture (air as soil) can kill the tree. I see it regularly with the grenadiers too.
The same temperature (- 7°C) can kill the tree if the humidity is too high, whereas elsewhere it will not necessarily be the case.
This moisture problem, I also observe with palm trees.
If the humidity is too high a slight protection is may be helpful.