I like the "shake in a bag" idea, even though I sometimes over-pulverize flower heads instead.
"Shake" creates much less fine chaff, so you can clean the seeds easier. I imagine that I may miss some seeds that way, and sometimes pull flowers apart and rub them after they're dry ... then try to find seeds among the dust and chaff.
It has to be smarter to just shake. If you don't have enough mature seeds to harvest them that way, try again next year.
We have a non-rainy daty forcast for Saturday (hurray) so i'll go harvest all brown spikes and pluck a few colored florets to see if they have a white or green BB or speck.
I've been waiting months for my California Poppy seeds to mature. Seemingly overnight, many pods went from green with white seeds to brown split-open pods.
