I believe that a lot of it has to do with the face that the roots remain alive through the dormant season, and can restart the top parts in spring. A lot of cold weather death is because the feeder roots will be killed by excessive moisture while in the dormant state during winter. If the root dies then the top of the plant will certainly do so in spring. This 'root death' is the probable cause of many losses in marginally hardy plants.

George

Thank you "Geoforce"it seems to be a good reason!