Butterfly bush, know botanically as Buddleia davidii, is a deciduous perennial shrub that produces flowers in the late spring, summer and fall. Named for its power to attract bees and butterflies, the shrub has numerous species and cultivars in circulation. It is consistently winter-hardy down to USDA Zone 6, and typically dies back to the ground with the first hard frost. According to Purdue University Extension, butterfly bush should be prepared for winter with pruning in the fall, before the first hard frost occurs in your region.
Remove any broken, diseased, damaged or dead branches on your shrub, cutting back down to a point of healthy tissue or to the crown of the plant at ground level. Pull out and discard or compost any cuttings that are not diseased.
Shear down the shrub to just a few inches above the crown of the plant in colder climates USDA Zone 7 and below.
Cut back the shrub to the desired size in areas in USDA Zone 7 and above where it will not die back entirely with winter conditions. Remove up to half of the shrub if desired, to re-size it for the landscape and encourage new, more compact branching.
Clear up all the cuttings from the soil with a fine rake to prevent the creation of a breeding area for insects and disease over the winter. Discard or compost the cuttings.