You should actually prune your spirea more than once a year, at least twice. Give it a good trim after it flowers in the spring by cutting back the tips of the stems to the top leaf bud. More drastically cutting back spirea should occur in the fall or in late winter to early spring. Spireas are cut back to the ground so that new, healthy shoots can grow. Wait until late winter or early spring to cut back the spireas. Cut straight through each stem as close to the ground as possible so you don't leave ugly stems protruding from the ground.
Rake up leaves and other debris from around the plant. May 25, The spirea genus (Spiraea spp.) Spireas are cut back to the ground so that new, healthy shoots can grow. The two main trimming periods, in early spring and after blooming, are most important to do each year, but you can also trim your spirea as needed throughout any season. Jul 18, Divide your spirea in the late fall when the plant is dormant. A cloudy overcast day is a good time for dividing plants as the roots dry out too much in sunny hot weather once they are exposed above ground.