I read someone's post that says they just go shake the plant to collect the seeds.
MY question is: When can I start collecting?

I had a few plants which were finished blooming because the blooms had more or less dried. When I cut them off there didn't seem to be any seeds left.
SO when should I collect them or how can I tell if they are ready?
Should I just go shake the plant as suggested in another post?
They are still blooming and our weather is still nice enough I don't want to cut the blooms yet especial if the seeds are not ready.
I want to grow more of them next year and share some from one in particular. (pic attached)

Please advise....thanks so much!
:) Aqua