Spacing of English boxwood for a hedge or parterre
How far apart to plant English boxwood is by far the most common question, rife with confusion and conflicting advice online. The most likely culprit in all of this confusion is people not aware there are many types of boxwood. If you were asked how tall does a tree grow, would not your first question be ‘What kind of tree?’. English boxwood is correctly identified as Buxus sempervirens ‘Suffroticosa’, and English boxwood used in hedges or parterre gardens are planted close together (really close).
For English boxwood, which grow excruciatingly slow, spacing 10″-12″ is not out of line. If you have large English boxwood, then you are fortunate and will space further apart.
Protect the boxwood from sun and wind.
Boxwood are generally easy to grow, the greatest danger being the drying effects of strong wind combined with hot sun. Ideally, plant boxwood where it is protected from both wind and afternoon sun. Boxwood will generally grow fine in full sun, given proper moisture.
Considering that even small English boxwood may represent a significant investment, it is wise to plant properly. The two primary factors that determine whether a plant thrives or dies are:
1) location (does sun/drainage/moisture/pH/fertility match the needs of the plant) and 2) proper planting. Of the two factors, location is by far the most important, but this is no excuse for lazy planting. Proper planting for boxwood translates into this: a reasonably rich soil, and do not plant too deeply. Mix in plenty of compost and/or mulch and set the plant an inch or so above existing grade. SFGate offers a detailed planting guide for English boxwood.