This guide to hedge-trimming comes from The Pruning Answer Book by Lewis Hill and Penelope O’Sullivan.
Q: What’s the correct way to shear a formal hedge?
A: The amount of shearing depends upon the specific plant and whether the hedge is formal or informal. You’ll need to trim an informal hedge only once or twice a year, although more vigorous growers, such as privet and ninebark, may need additional clippings. Formal hedges need even more attention during their growing season, especially fast growers; clip while new growth is less than 12 inches and remove stray shoots as needed. Shearing hedges is easy if you follow these guidelines: