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:
Always shear a hedge so that the bottom is wider than the top. Otherwise, lower branches, lacking their full share of sunlight, will thin out and soon die. The difference may be barely noticeable, but it is extremely important.
Unless you have an excellent eye for such things, you should use a template or put up posts and a string as a guide when you’re shearing long, straight formal hedges, so that you won’t end up with a lopsided row.
Don’t shear off the top of a hedge and ignore the sides. Soon it becomes irregular and too wide and sags under the weight of heavy rains, ice, or snow. If anything happens to an individual plant, you can replace it much more easily if your hedge is slender.