The timing depends on whether your shrub blooms on old wood or new wood. Shrubs that bloom in spring (lilac, forsythia, azalea, rhododendron) set their flower buds the previous fall — prune these immediately after they finish blooming in spring, or you’ll cut off next year’s flowers. Shrubs that bloom in summer (hydrangea paniculata, butterfly bush, spirea) bloom on new growth — prune these in late winter or early spring before growth begins.
