Marigolds are very easy to re-seed! Just when you see a flower dying, pull it off and stick it in a paper sack. It'll dry over the winter and the seeds are inside the base of the flower! Just split open the base and you'll find tons of seeds! Then you can just seed them the way you did these.
A note...mine never seed true...the yellows never come back yellow, they all come back that yellowy orange color. Which is ok by me...they are pretty!
Thanks. I told Nana we'd better do that with her 4 O'clocks as well since we want to make sure they come back.