best time to take cutting mid august.... take a 4 inch cutting with a heal & wound ..... apply 0.8% IBA stick in well drained soil & mist taking 70-100 days to root.
I have found butterfly bush one of the easier plants to clone from cuttings. I cut off a small length of green branch and use rooting hormone and put it in good dirt in a mostly shady spot underneath a large plastic jar that has had the bottom cut out. This keeps it moist. Occasionally, I unscrew the lid and water it. I usually get a viable plant for transplanting the following spring, if I do this in July-August.
I cut off the flowers and stick into potting soil and keep moist. I keep them on the north side of the house in full shade until rooted.
I have just stuck a long branch as deep into the ground as it will go (12-18")and they rooted.