Hi Tracy,

Thank you for reaching out to us at the UNH Extension Infoline. You technically can prune in late fall/once your butterfly bush goes dormant if absolutely necessary, but it recommended to prune/shape in early spring versus late fall. There is always a chance for winter-related damages! Butterfly bush and many other summer blooming perennials/shrubs grow on the new wood that they develop in springtime. Due to this, it is always recommended to prune in early spring before it breaks dormancy and starts to show signs of new growth

I have one butterfly bush in my yard that I prune down to about a foot or a little bit less than a foot to the ground in early spring every year. Even with such a drastic pruning, my butterfly bush is at least 6 feet tall right now and is loving life! 

Here are a few resources to reference:
"On the other hand, plants that bloom in the summer should be pruned before growth begins in the spring, because these plants develop their flower buds on the current season’s growth."-  UNH Extension resource here: https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/Resource007692_Rep11274.pdf

https://extension.psu.edu/how-and-when-to-prune-flowering-shrubs#:~:text=Summer%20blooming%20shrubs%20such%20as,produced%20during%20the%20current%20season

https://www.johnson.k-state.edu/programs/lawn-garden/agent-articles-faqs-and-resources/agent-articles/trees-shrubs/butterfly-bush.html

I hope this information helps! Have a great week!