It pays to read. Not long ago I came across this paragraph in a book on tree maintenance, and thought it very succinct in describing what is going on invisibly inside the tree during times we might think it is fully dormant:
Cambial activity shuts down first at the top of the tree, then in the trunk, and lastly in the roots. Therefore a tree may well have an active root system well after visible growth or leaf drop has occurred. Also, the upper roots of trees in the ground are the first ones active in the springtime.
Makes you think, doesn’t it? Another complicating piece of the puzzle is that the top of the tree is much more cold hardy than the root zone. The tags we see on nursery containers that read ‘Hardy to_’ assume you will plant it in the ground, where tree roots are designed to be. Those cold ratings are for the top only; roots are more tender. Quite a cautionary tale for those growing bonsai. Roots are calibrated to be in the ground, not in a pot.
There are a couple of important lessons here. While light frosts in the fall will ease a tree into dormancy, unseasonably deep cold in the fall may kill the tree because the roots are still active. Spring is the usual time to be careful of hard frosts, but fall must be watched too. Furthermore, if we live in a mostly mild climate with infrequent arctic blasts we should be aware that bonsai may never go fully dormant, retaining active roots throughout winter.
Healthy roots of most hardy trees and shrubs are fine with light frosts in the upper to mid 20′s (F), which helps to ease them into winter dormancy, but lower than that they need better protection such as placement on the ground or in an unheated room, greenhouse, or coldframe.
Those bonsai spending their winter holidays out on the benches may need a careful weather watcher (that would be you!)
(Reference: Tree Maintenance, P.P. Pirone, 1988)