Answer: I think it would be easier to choose two different boxwoods for the effect you want to achieve. The taller one could be 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. The shorter one could be English or true dwarf boxwood, B. sempervirens 'Suffruticosa'. It is the most popular and most widely grown cultivar of all boxwoods, predating 1753 in the United States. It is a low (often less than 3 ft), slow-growing, compact shrub which rarely averages more than an inch of growth annually. The plant is rounded with tufts that resemble a cloud. With annual shearing you'll have the formal hedge you want, plus visual interest because the foliage has a slightly different appearance.
Plants that can be used in place of boxwood for a knot garden include lavender, rosemary or lemonbalm.
Fertilizing is most effective in the spring when plants are actively growing. I'd use either a liquid or a granular (time release) 10-10-10 in amounts as recommended on the label.
Best wishes with your landscape!
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