Answer: In zone 5 and often in zone 6 butterfly-bush is treated as an herbaceous perennial because it is often killed back to the ground during the winter. In my experience the shrub can be trimmed back if needed for tidiness in the fall and then it is best to wait until late spring to cut back the remainder as far as is necessary to reach live wood -- if there is any. If not, the plant will resprout from the roots and this is fine since it blooms on new wood anyway. You might tip prune it very lightly a few times during the first half of the grwoing season to encourage it to branch and thus look fuller. (If there is a mild winter, you can certainly trim it back as hard as is needed for shaping in mid to late spring.) Once it starts blooming, dead-heading or cutting off the spent flowers will help it to bloom over a longer period.
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