Answer: It is not necessary to cut back healthy wood on a butterfly bush, however in most winters there is some dieback and that should be removed in late spring.
Butterfly bushes bloom on the current year's growth, so a light trim will enhance the number of blossoms. Without trimming, there will be fewer blooms but they will be larger. The plant will also be more ungainly. Without pruning and without winter kill, the plants can become quite large.
Most gardeners find that the bushes look better with a little shaping. In severe winters, the plants may also kill back to the ground and regrow from the roots; this then requires that the plant be cut way back to remove the deadwood. Any pruning should be done in late spring.
Q&A Library Searching Tips