Hoping for an answer about the lobelia also.

Chey

I can't help you with the astile, but I have been trying to get seeds off of my lobelia, also. So far what I have been doing is waiting until the flower stem turns brownish and the petals shrivel/fall off. Then pick the stem and if you open up the base of it or sqeeze gently little tiny ball like seeds will fall out. It has been hit and miss with me as I sometimes don't get mature seeds. Here is a picture off the Seedsite of lobelia. Hope this helps a bit!

Here is a link that might be useful: lobelia seed

The seedpods are tiny on the lobelia, as are the seeds. I usually wait until the pod turns brown, pick it off, pinch it in half, and roll both halves between my thumb and forefinger gently, letting the seeds fall into a plastic cup.
As for the astilbe, the seeds are so very tiny that I usually just crumble up the entire dead flower head after it turns dry and sprinkle it out over whatever container I am starting it in. (It doesn't last more than a few months, by the by.) I have had sprouts this way.

My astilbe flowers finished several weeks ago but the seeds are still green. It seems like I'm waiting forever to harvest them. They are the rows up and down the flower stalk ??? Will they turn brown and break off easily when ready ? It would be my luck to have missed the seeds while thinking it was something else.

Kathy