As with knowing how to prune a butterfly bush, when to prune a butterfly bush is another aspect of pruning for which there are no absolutes. In fact, butterfly bush pruning can take place just about any time of the year.However, certain pruning techniques will help promote more vigorous growth and healthier blooms.Generally, most butterfly bush pruning should take place during the winter months, in warmer climates, while the plant is dormant.owever, the butterfly bush can also be pruned in the spring with no ill effects. Just make sure you wait until the threat of frost has passed. Keep in mind that butterfly bush pruning may require an additional layer of mulch around the bush for insulation, especially in colder climates.