Answer: In a word, don't prune it in the fall.
Butterfly bush should be pruned in the spring to shape and encourage new growth since it blooms on new wood of the season. When pruning, first off, there may be some die
back or winter kill after the winter season and that should be removed. In some years this
could mean cutting it nearly to the ground. In other milder years this might be just a few
branch tips. Next, you can prune for shapeliness and to encourage bushiness or
branching; you can also prune to try to limit its overall size. The plant has a naturally
gangly shape and can grow very large in one season, so some gardeners will opt to prune
more than others. Finally, each cut will force the plant to branch and each branch tip will
bloom, so you can also influence the number of blooms. If there are fewer blooms then
each will be larger, with more blooms they will individually be smaller. Some gardeners will deadhead the flowers as they fade to help the bush bloom over a longer period.
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