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.