I am growing many Poncirus seedling, some for pollen or seeds for breeding. Most are for rootstocks for grafting. When they were leafing out this spring, I noticed that many had twigs that had died back some, mostly 15 cm. to 30 cm. That is just the amount of die back, not size of the seedlings. Some of the seedlings have been through 7 winters here, and more have been added every year, in hope that I'll some winter hardy seedling to graft onto them.
Several of the seedling had no die back, but nearly half did.
Its not a big deal if a seedling for grafting has die back before grafting. But when I do graft onto them, 30 cm die back means the scion is dead.
So what is causing this die back and what can I do to prevent it?
I have seldom lost a whole tree to winter kill. It seems to me it is only one summers growth I am loosing, maybe less.