Some plants can and some do not keep producing flowers while others are maturing and setting seed.. Blue Flax for sure is one that can set seed and continue to bloom non-stop from April thru end of July.
I always dead most of my plants I want to seed from and then let the new flowers bloom and go ahead and set seed..Around here my last deadhead is mid July. I'm not sure of the Buddelia Bushes flowering habit (Butterfly Bush) , but I wouldnt deadhead any more this year if you want seed.

Vera

The dead flowers need to stay on the bush to produce seed pods. The pods are un-mistakable, they look like elongated footballs and they will hundreds on one flower head. I don't usually get ripe seeds until after the first few light frosts here in zone 5. It takes a couple months for the seed pods to go from green to brown (ripe).

BTW, the seeds are just tiny looking hairs and are very hard to see. It is almost impossible to separate from chaff unless you have just the right screens.

BF Bushes will continue to bloom even if you leave a couple of the flower brackets un-cut to produce the seeds you want.

Thanks for the help. I'm watching and see some seed pods developing.

Geez, I feel like a REAL dummy! I didn't realize there are Flower seeds and Plant seeds!!

I have never tried to save seeds before and know absolutely nothing about it, but have many lovely flowering plants here in my new abode that I would like to save and spread around :)

So I am here for some help and patience with my lack of knowledge.

Last year I bought 3 plants - Rudibeckia (splg?) This year they are huge and loaded with big flowers. Now they are fading and have large heads of seeds. What so I do now??