Answer: According to Dirr's Manual of Woody Landscape Plants, most boxwood varieties are rated hardy to zone 6. There are a few mentioned as hardier than others, but I would be cautious before investing a lot into a large planting. Microclimate can play a big role in whether a marginally hardy plant survives.
The varieties of boxwood Dirr describes as the hardiest are:
Buxus microphylla var. koreana -- foliage turns yellowish-brown in winter. Grows 2 - 2 1/2 feet tall. Zone 4
Buxus sempervirens 'Northern Beauty' and 'Northern Find' (height up to 20') are described as relatively hardy. The variety 'Northland' is described as hardy in central New York state; it grows to a height of about 5'.
You might contact the American Boxwood Society at ph# 540-939-4646 to get their advice.
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