If you trim these spirea varieties at other times of the year, such as during the winter, the limbs may not be able to recover from the pruning stress; branches will slowly wither, showing stunted. Lightly trimming you spirea after blooming will extend this shrub's season of interest.
The second pruning should occur in late fall or early spring. This pruning will usually be a more drastic trim and you will remove more plant material from the plant than during. If there is a prolonged"January thaw", then the spirea may put out new growth, and it could be seriously set back from that freezing. As well, the longer branches provide a measure of freeze protection to the base of the shrub, so waiting until bud-break or just before to start cutting might be a better idea.
Examine the spirea shrub in late winter after the buds begin swelling but before they leaf out, for dead or damaged branches. Remove all dead and damaged wood, along with any branches that cross or rub together. Japanese spirea varieties require tip pruning at this time, or.