This is an easy one. Simply take cuttings toward the end of the season (before first frost). Put them in a glass of water and watch them sprout roots. Be sure to pinch off any flower heads that might form. And don't leave them near an open window. When temps drop below freezing, they'll succumb.
Once any chance of frost has passed, you can plant your cuttings outside. They'll do very well. Repeat as often as you like.
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