Hi Mercedes.Once they have finished flowering,cut them back quite low,to where you can see new growth,and use some of the tops for cuttings..They root easily,in a shaded cold frame or similar..You can then plant them out to grow on ..The original ones will grow bushier,and not go woody..mine were supposed to be Biennial,but I have had them four years now..hope this helps,good luck..

once they set seed they will die so nothing to lose in following Bloomers suggestion.