The Best Time to Prune Ornamental Shrubs | Gardening Advice
Early spring, before growth begins, is an ideal time to prune ornamental shrubs, especially those grown for their foliage rather than their flowers. Shearing or pruning evergreen hedges such as arborvitae, yew, and boxwood shapes them and encourages new growth, for a denser appearance. Deciduous plants used for foliage, such as cork bush (Euonymus alatus) and privet, can be treated similarly. As you cut them back, be sure to shape the plants somewhat wider at the bottom so sunlight can reach the lower branches. For flowering shrubs such as azalea, rhododendron, and lilac, pruning before they bloom removes some flowering buds. Unless you don’t care about this year’s flowers, wait until just after they finish blooming to cut them back.
If your plants are really overgrown, you may want to improve their shape by more drastic pruning, perhaps over several years. Removing a third of the large stems and trunks each year will enable the plants to renew young shoots while still maintaining an attractive appearance.
–R. Wayne Mezitt, Chairman, Weston Nurseries, Hopkinton, Massachusetts