HARDINESS
The main factor that determines whether a plant will live through the winter in your garden (and it is a factor you can’t control) is minimum winter temperature. This is the basis for the newest (1960) Plant Hardiness Zone Map issued by the U. S. Department of Agriculture. Other uncontrollable factors include: How long the low temperature will last; how early and suddenly a freak freeze occurs; whether or not winter provides a beneficial blanket of continuous snow; whether there are punishing gales with driving sleet and encrusting ice.
But even in severe areas, or with plants of questionable hardiness, there are some ways we can increase the odds against winter-kill. Selection of suitable varieties, careful planting in favorable locations (never in low “pockets” where frost settles, or where water collects around roots under eaves or around walls), careful fertilizing, winter protection, prevention of attacks by insects and disease—all of the cultural principles that promote healthy, thriving growth, will help to increase winter hardiness. In other words, “The combined effects of all (climatic and cultural) factors determine the true plant adaptability.”
Actually, true hardiness means more than mere winter survival. A tree or shrub is hardy when it can live through many years to maturity, and when it not only survives but develops normally and completes its annual growth cycle. For example, in some sections, very late or very early frosts will kill flower buds on some varieties. Trees and shrubs that require a period of winter dormancy are not hardy in Florida, where there is no protracted cold period. In arid areas where summers are extremely hot, dry, and windy, hardiness may depend on how much time you have to spare for special care.
If you would avoid disappointment, especially with rather expensive miniature trees and shrubs, buy the strongest, healthiest plants of the varieties most suitable to your climate, and do all you can to satisfy each individual’s cultural needs. Thus, unfavorable weather has two strikes against it from the start.