After the petals fall, the seeds will start to mature and then turn brown. You will want to wait until they are brown b4 harvesting them.

If you scroll down to the very bottom of the link below, you will see the seeds head (not a very good pic) and the seeds after harvest.

Sue

Here is a link that might be useful: Cosmos

thanks very much!

What a useful site The Seed Site (the link provided by Chemocuri) is! I bookmarked it and urge other seed collectors, wintersowers, inside growers, look at it. Thanks.