Prune shrubs in a formal hedge to resemble a dense, smooth wall. Vigorous, rapidly growing shrubs need more frequent shearing than shrubs that grow more slowly. Dense hedges start with the right pruning techniques at planting time and regular maintenance each year thereafter.
Determine your hedge's habit. When you cut the end off a branch, dormant buds below the pruning cut sprout into new twigs. But not all shrubs have dormant buds, and these shrubs won't grow new twigs if pruned too severely. For this reason, it's best to prune evergreens only into the soft, current year's new growth. Many deciduous shrubs-those that shed their leaves in autumn-can tolerate pruning into old wood from previous growing seasons.
Assemble your tools. Use manual hedge shears for small hedges and electric shears for large jobs. If your shears do not have a pruning notch near the base of the blade, you may need hand pruners to cut twigs larger than 1/4 inch in diameter. Use a tarp to collect trimmings or a rake to clean up afterward.
Prune at planting time. When you plant a new hedge, prune vigorous shrubs, such as privet, by reducing the length of the branches and shrub height by one-fourth to one-third. Do not prune the tops of slower and denser-growing evergreens and conifers, such as arborvitae, until they reach their desirable height.
Establish shape. All branches need adequate sunlight for strong growth. As the hedge grows, prune the sides so the bottom is slightly wider than the top to prevent the upper limbs from shading the lower ones. In snowy climates, give hedges rounded or sloped tops to help them shed snow easily.
Prune to maintain shape and fullness. The timing and frequency of pruning depends on the species of shrub and your climate. Prune the new growth of evergreens in late spring and mid- to late summer and avoiding cutting into old wood. Prune deciduous hedges while dormant in late winter and again in summer.
Yew (Taxus species) can tolerate more severe pruning than most evergreens. You can rejuvenate or reduce the size of yew hedges by shearing twigs back into old wood.
Do not allow vigorous shrubs to grow to their final size before pruning them. To form a dense hedge, prune them as they grow. Although it may take years longer to achieve the desired height, the result is more satisfactory.
Photography by Suzanne DeJohn/National Gardening Association