Prune back about 1/3 of the leafy part of the stems each at least once a year to maintain nice form and leaf quality and to encourage new growth. This is best done after flowering in the fall or winter either with a pruner, or much more quickly with a manual hedge shear, cutting back into a nice roundish dome-like shape. Plants will rapidly begin to regrow. If desired, spent flower stems can be removed through the flowering season. this may extend the flowering season, but can be quite a lot of work if you have multiple plants. In many inland gardens, cutting back twice per year can be optimal for garden performance: Cut back once in late spring / early summer after spring blooms start to fade. Plants will grow back rapidly and rebloom. Then cut back in fall. It is best to avoid a significant cut back during the peak hot summer months if possible (S). References