How & when is the best time to trim a butterfly bush?

anytime. I trim it all summer and then cut it to the ground after it dies off in the fall. it always comes back even bigger.

I had wonder that myself a few years ago and found out that the best time to cut it is in the spring. Then cut it to 6 to 8 inches from the ground. So that's what I do every year, it grows really well.

Cib, butterfly bush just keeps on blooming, fading, blooming, drying, then blooming again. When you think it is finally finished, cut it down, but don’t pull it out by the roots. You may want to leave a stake in the ground to show where the butterfly bush is so that you don’t cut it out the next year. It’s a beautiful little bush that bees and butterflies just love. Keep it in your garden. ☺️

I cut mine down to about 12-18 inches at the very end of winter, before trees and plants start to send out new buds. It comes back nicely every year.

Trim them in the late fall and cut back to a few feet above the ground. Don't cut them back more or they might not come back.

Here's a great article that tells you how and when if you're one Zone 5-10. https://www.hunker.com/13428384/how-to-prune-butterfly-bushes

It states later winter or early spring and to watch for signs of new growth at the base of the plant.