Answer: Unfortunately, the plain truth is that it is very difficult to consistently keep potted evergreens wintered over outside. The plants are very stressed in a number of ways including by exposure to wind, drying of the rootball, crowding of the rootball after a season of growth, eventual deterioration of the potting medium, possible inattention to watering and nutritional needs during the growing season contributing to overall lack of vigor, and of course the exposure to cold.
In many instances the plants are cold hardy when planted in the ground but this does not carry over to holding them in pots when the roots are exposed to colder temperatures than they would be when planted in the ground and insulated by the surrounding soil. The above ground conditions expose the roots to more cold than they can handle.
In my experience, the best way to overwinter these plants at home is to find a location very sheltered from wind and then sink the pots in the ground up to their rims. Alternatively, protect the plants from the wind and then insulate the roots in some way such as by heaping a generous amount of mulch over and around them. It is better to set the pots on the ground than to leave them on an elevated deck where the cold penetrates from below as well as the sides and top. Cold frames (with automatic venting to avoid overheating) or an (unheated, vented) poly house might be another method to consider. I realize that none of these methods are particularly ornamental, however the question of survival is not an easy one.
Wherever the plants are kept, the soil should be kept slightly moist whenever it is not frozen. In addition, care must be taken to water the plants throughout the fall season so that they go into the winter in a well-hydrated condition. Another factor to consider is to avoid late season fertilization so as to avoid encouraging late season growth that will not have time to harden before winter. Next, application of an antidessicant spray per label instructions can also be helpful in some cases.
Since the conditions are so variable -- for instance a mild winter can result in better success than a particularly cold one, or a winter where there are many abrupt swings from hot to cold can be very tough on plants -- in my experience it is better to consider the potted evergreens as an annual planting and be pleasantly surprised when they do in fact survive. Sometimes it is a matter of skill, but at least as often it is a matter of skill plus luck with the weather.
I hope this helps you in addressing the difficulties of keeping these plants alive and healthy.
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