Butterfly bush cultivars can grow vigorously enough to require some pruning during the growing season. This in-season size control should be done as needed. Avoid late summer pruning - it can stimulate late succulent growth that is susceptible to cold temperature injury.
Most butterfly bush pruning and cutting back is done in late winter/early spring after the worst of winter is over and before vigorous growth has started. Butterfly bushes are frequently cut to the ground each year to remove winter-killed branches; those known to be hardy can be pruned as needed for size control.
