Spireas are heavy-blooming, hardy shrubs that can tolerate a heavy pruning to keep them in shape. These shrubs vary in size from 3 to 10 feet high, with a slightly wider spread. Some spireas bloom on old buds while others bloom on new growth. When to prune depends first on which type you have. Luckily, there's a handy trick to tell them apart.
Pink spirea bears its flowers on buds developed in the current season. For this reason, and because it's easier to prune shrub branches before the plant puts out its leaves, prune spirea anytime from late fall to early spring. White spirea develops its flowers from last season's buds, which means pruning needs to be delayed until after the plant flowers -- typically in late spring -- or the flowering display will be diminished.
To tell when to prune, go by the color rule for pink or white flowers (See next step). Common types of white-flowering spirea include van Houttei and Thurberg, while pink-flowering cultivars include Anthony Waterer, Froebel and Goldflame.
To prune the white spireas, remove up to one-third of the old stems right after the plant flowers. Older stems feel thicker, have a more woody appearance and produce fewer flowers. Remove these stems every year to maintain a healthy plant. For pink spirea, cut back all stems to a height of about 4 inches from autumn to early spring. The plant will grow new flowers and bloom again.
Your spirea may not need to be pruned each year. If you are happy with the size and density of the shrub, skip a year. Furthermore, trim off branches outside of the optimal time frame for spirea pruning if doing so benefits the plant's health. Check periodically for broken, damaged, diseased or dead branches; remove these when as they are noticed.