if you want seeds to set, stop now, though it might be too late. The dianthus will more than likely overwinter- they are pretty hardy, even the ones labelled as annuals. But the marigolds will be done as soon as frost hits them.

Yikes! I actually stopped a few weeks ago. For future reference, when should I have stopped? I do hope the dianthus overwinters! Thanks!

If these plants are hybrids they won't come true from saved seed.

I am aware of that, but thanks so much for pointing that out in case I didn't. I know there are a lot that don't know that, so good to not have unwanted surprises. :)

My suggestions would be to contact your county extension office to ascertain correct timing for seed saving. Hopefully, there'd be master gardeners on duty to help you in this effort. Maybe, there's even a state publication of this very subject, or have a leaflet online for viewing. I'm going to check this out myself, here in Buncombe Co., so thanks for posting this worthwhile inquiry!

I'd probably stop deadheading marigolds in mid August so the plant has a rest and recovery time. You can collect the August seedheads in Sept/Oct as the last flush of bloom is coming out.
Dianthus I just shear after spring bloom and pinch off any odd buds developing over the hot months so I get a better second bloom in late September but that's just the variety I have which are the low, mound growing type. I'm not interested in saving seed so much as getting two good flushes of blooms.

Thanks, everyone! Thankfully, I have been able to get a TON of marigold seeds. :):):) I didn't do so well with the dianthus, but I have a bunch of seeds from the Dollar General clearance sell. I appreciate all the tips!