If you still need to know how to collect seeds from porterweed?
I can try to attach some pictures. I haven't done the purple or red yet but on the coral or salmon, wait until the flower stalk is dry and brown and remove from plant. along the stem, there are small brown scale looking things. These are the pods. You can use your fingers but I prefer a stick pin. I put the stick behind the pod and pull it out. Sometimes the seed will fall out at this point and sometimes you have to remove it from the hull.
You can get a better idea if the pictures come through.
Jan...