I've just cut pieces of stem or tip and put right into the ground and kept it moist. Approximately 7 out of 10 cuttings will root for me. I looked on Daves Garden and they had this:
Propagate :By dividing the rootball
From herbaceous stem cuttings
From seed; sow indoors before last frost
Self-sows freely; deadhead if you do not want volunteer seedlings next season
