Answer: Boxwoods are very susceptible to "winter burn" due to drying out due to the effects of wind and winter sun, especially if the sun is reflected off a building. They are also shallow rooted and for that reason particularly sensitive to cold soil as they would experience in containers above ground.
To minimize risk, make sure the soil is kept evenly moist (but not sopping wet) until the ground freezes and apply several inches of organic mulch over the root zones. It is important for the plants to go into winter well hydrated because they are always losing moisture through their leaves.
Locate the containers in a sheltered position away from the wind. If possible, it would be best to sink the pots into the ground to insulate the plant roots. If this is not possible, then you might try heaping mulch around the pots or wrapping the pots in some type of insulation.
You may also want to protect the tops of the shrubs with burlap or similar air permeable protective covering to help shield them from wind and sun, especially if it is reflecting off of snow. Another possibility would be to use an antidessicant spray according to the label instructions.
Finally, should there be a heavy snowfall, clean the snow gently off the tops of the shrubs periodically to prevent the snow's weight from breaking branches.
Fertilizing would be done periodically throughhout the growing season, from early spring until late summer. The reason for this is that the nutrients leach through the potting soil and need to be replenished. You could use a long release formulation or a water soluble type, be sure to follow the directions.
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