thank you for your advice..
my trees are during Winter time on a „closed“ (with a movable window) balcony facing north. So I tried to keep temperatures not to low, most times between 2-5 grad celsius. In two of three nights water froze on the balcony cause ist was really really cold outside (about-18) but Not colder than -2/-4 inside. I also keep the soil moist….
The others, Apple, mume, hainbuche are coming with power
but the Good News: the cambium layer is green:
I try the black bag Trick….