Members of the rose (Rosaceae) family, spirea shrubs produce delicate sprays of tiny flowers, which often adorn bridal wreaths. In addition to the traditional bridal white, the flowers also blossom in shades of pink and red. Some types bloom in spring, while others don't flower until summer. This different timing affects when you prune and the seasonal care you provide your spirea plants.
Spring-bloom spirea produce buds the year before they bloom, which means they blossom on old growth. These buds begin to set the autumn before they open. Meanwhile, summer-blooming types blossom on new growth, since they develop buds in the spring just a season before they open. Summer bloomers are often white in color, while spring bloomers come in white, pink or red.
Bud development on old or new growth dictates the safest time to prune your spirea. You can safely prune summer bloomers in winter, before their buds develop in spring. However, this timing would severely limit the flowering of spring-blooming spirea, since you might unintentionally hack away buds that developed in autumn and have yet to open in spring. Trimming at this time won't destroy the health of the bushes, but it's likely to result in a poor display when the spring flowering season hits. A safer time to prune is in late summer, just after flowers fade.
Spireas appreciate full sun to produce the most flowers, though they'll also grow in partial sun. They're not picky about soil, as long as it's not waterlogged, and mulching around their bases can help conserve moisture and provide a slow, subtle release of nutrients for them. Prune them to keep them tidy, removing old, diseased branches and crossing limbs. If you notice an insect infestation or disease, remove the affected parts of the spirea shrub as soon as possible; you shouldn't wait for the appropriate seasonal timing in this case.
Whether blooming on old or new growth, spirea is a rapidly growing deciduous shrub. Though many prefer cool climates, spirea are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8. Spring-blooming spirea has gently arching branches with more delicate sprays of tiny white flowers than summer bloomers, which have upright flower clusters instead. Typically, spring-flowering spirea needs more pruning than summer bloomers, though this depends on your desired shape for your spirea and whether the bridal wreath spring flowers are appropriate in your garden or unruly.