you want to cut back Butterfly Bush in late winter to spring in most hardiness zones.
How far should a Butterfly Bush be cutback?
A Butterfly Bush can be cut back to six inches above the ground and it will grow five to eight feet in one year. However, If you have larger pets or other animals which might trample the plant you may what to prune high enough to force them to walk or run around the Buttery Fly Bush.
Pruning for Sunlight
A Butterfly Bush do best in full sunlight, which results in a fuller form, but will tolerate some shade, so, you may also want to consider pruning any nearby plants, which may be shading the Butterfly Bush (when appropriate for the plant type) to provide more sunlight for the butterfly bush.
How frequently to prune?
Butterfly Bushes are cultivated plants and will thrive with annual pruning. I prune my Butterfly Bushes in late wither each year and not only do they do well, but the pruning seems to let the plant’s vegetation spread and flower more.