I live in an area of the United State where it's never really safe to say that winter is over. Just when you think it's spring, it can be blizzarding and below zero. Anyway, should I wait until it's for sure spring to repot my Juniper bonsai? Will it show signs of life that it is out of dormancy?
I always do my repotting and trimming of roots and heavy pruning during the time the plant is dormant.
Others might argue with me, but I don't think the beginning of spring is the best time to do it because the plant is coming out of dormancy and wants to be growing. The idea of pruning and repotting during dormancy is to do it at a time when the plant is going to experience the least amount of shock.
My father in law raised bonsai for over 50 years and he taught me to do heavy pruning and repotting in winter when the plant was dormant and light pruning during late spring.
There is usually a second growth spurt during mid-autumn and then late autumn the deciduous trees will drop their leaves. I try to do it a couple of weeks after the cold has set in rather than late winter. In late winter you are running the risk of shocking the plant when it is coming out of dormancy.
Every trees (though dwarf) have to be replanted during winter, when sleeping.
When they sleep, the metabolism of the plant is stopped, so do not be afraid to thermal shock. but above all, you need not to fear a biological shock, relating to remove the roots from the earth. This means that the lymph flow is almost stationary, so you need not fear losses of sap from the roots (which the cold makes it harder to heal), or, worse, infected infiltrations into roots: the sap stops ago by cap.