Answer: Japanese boxwoods can grow upwards of 10' and 8' wide unless you choose the dwarf varieties. Buxus sinica var. insularis 'Wintergreen,' a selection of Korean boxwood, is hardy to USDA Zone 4. This slow growing cultivar's foliage has a dark green color that lasts through the winter. It displays a more open habit than American boxwood and is a heavy seed producer. A 16-year-old plant may be 5 ft tall by 3 1/2 ft wide.
Buxus 'Green Mountain' is hardy to USDA Zone 4b. It also has dark green foliage color that persists through winter. It displays a dense pyramidal habit. A 10-year-old plant may be 3 ft tall by 1 1/2 ft wide. Ultimate size is anticipated as 5 ft tall with a base 3 ft wide.
Buxus 'Green Velvet' is hardy to USDA Zone 4b. It's a 1973 Sheridan Nurseries hybrid introduction with a rounded habit and vigorous growth which holds color well through winter. Ultimate height is anticipated as 4 ft.
Once you have chosen the variety you want to grow, you can plant them about 2' apart to produce a dense hedge.
Boxwoods are typically slow growing. Plant in well draining soils and mulch around the bases of the plants with compost. An annual feeding of an evergreen fertilizer such as an 8-8-8 should keep them green and healthy.
Best wishes with your new hedge.
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