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Pruning times and techniques differ for spiræa species that bloom on old wood and new wood. Prune spring-blooming plants, which flower on old wood, immediately after flowering. Summer-bloomers, including many Spiræa japonica varieties, bloom on new growth and are most easily pruned in winter.
You can also do some pruning while the plants are in bloom, says Lucy Tolmach, director of horticulture at the Filoli Center, in Woodside, California. Regardless of bloom time, always start by pruning out branches that are dead, diseased, weak, crossing over other branches, or split. You'll need a sharp saw with a narrow end in order to get between crowded branches, says Tolmach.
Use a pruning technique called renewal pruning—annually cutting one-third to one-fifth of the oldest canes to the ground to encourage new growth—for spring-blooming spiræas, especially the larger types, such as bridal wreath (S. prunifolia).
In late summer, remove fading flowers from summer-blooming spiræas to encourage a second flush of blooms. In winter, thin crowded and inward-growing branches down to a plump, outward facing bud, and cut a few of the oldest branches down to the plant's base. If you've noticed a decrease in blooms over the past few summers, try cutting all of the branches down to 6 inches in late winter to encourage the plant to put out vigorous new growth in spring.
The Pruner's Bible, by Steve Bradley (Rodale, 2005) has more pruning information.
