When and how do I prune a butterfly bush?
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since this is a summer flowering shrub, the rule of thumb is cut back hard in early spring as it flowers on new growth
Butterfly bushes do not need to be pruned every year. In fact, you only need to prune them when they get too large for the space allotted. But since butterfly bushes only bloom on new growth many gardeners prune them severely each spring to encourage lots of new growth and lots of flowers.
I cut mine back in the fall to about 12" tall and they come back fine in the spring. This year they were almost as tall as me.