If you’ve wondered why your shrubs don’t live up to their flowering potential, you may not need to look any further than your pruning shears. Sometimes busy moms prune all their shrubs at once to save time, but this may cut off the developing flower buds on some plants and reduce or eliminate their next season’s blooms.
Perennials are the mainstays of your garden, returning reliably every year so you don’t have to plant new things each growing season. Herbaceous perennials die back to the ground each year, leaving little or no evidence of their presence until they spring to life again when warm weather returns. Woody perennials may shed their leaves during winter, but they typically keep their framework of trunks and branches that leaf out again the following spring. This framework supports old and new wood, the significance of which determines when you prune spring- or summer-blooming woody perennials.
Typically, spring-blooming shrubs form their flower buds on the previous season’s one-year-old woody growth, which is called old wood. After a plant finishes flowering in spring, it produces new growth throughout the summer and autumn, which includes flower buds. Some of these flower buds are unseen until they emerge in spring, while others are visible along the stems. If you prune a spring-flowering plant in late winter, it will not bloom in spring because the late pruning removes its flower buds. Forsythia (Forsythia x intermedia, U.S. Department of Agriculture plant hardiness zones 4 through 8) and old-fashioned weigela (Weigela florida, USDA zones 5 through 8) are examples of spring-blooming shrubs that bloom on old wood.
Summer-flowering shrubs typically produce their flower buds on new wood -- stems that grew earlier in the current season. If you prune these plants in spring, they can still produce new growth that is capable of setting flower buds that bloom in summer. Butterfly bush (Buddleia davidii, USDA zones 5 through 9) and Japanese spirea (Spiraea japonica, USDA zones 4 through 8) are summer-blooming shrubs that bloom on new wood. Because summer-blooming plants produce flower buds more quickly than spring-blooming plants, successive prunings during the same season may encourage repeat flowering. (Refs 3 & 4 for USDA zones)
Some shrubs, such as hydrangeas (Hydrangea spp.), may bloom on old wood or new wood, depending on cultivar. For example, bigleaf hydrangea (Hydrangea macrophylla, USDA zones 6 through 9) blooms on old wood, but smooth hydrangea (Hydrangea arborescens, USDA zones 4 through 9) blooms on new wood. If you prune bigleaf hydrangea and smooth hydrangea in spring, the bigleaf hydrangea won’t be able to set flower buds and it won’t bloom. Whenever you prune anything in the garden, keep sharp pruning tools out of the reach of children.