You can either yank them out and toss them on the compost then buy new in the autumn or you can leave them for another year. They will get a bit bushy and maybe a little leggy but will flower earlier. After that pull them out.

I frequently do this unless I need the space.

Thanks Arbuthnot

Cut off all the dead flowers if you plan on leaving them in.

Thanks Mg

You could leave one plant, and save the seed.

You could also take cuttings,which I have been doing for a few years now,from the original plants..keep them in a cold frame or similar,in a shaded area.. I've just done mine this week..,and if the original plants aren't too woody,cut back to where you can see a little new growth,and they will flower again next year..

Thanks V much Bloomer and Dianne

Yuo are welcome,Davros :o)