Image by: wintersown
After hostas flower leave the flower stems on your plants, do not deadhead them. Those which are fertile will begin to develop small green oblong pods along the stems where the flowers were attached. The pods will grow and swell as the seeds develop inside...this requires patience as it can take several weeks, even a few months.
Close to the time of seed maturity you will first see the stem and pods begin to yellow, and then turn a brownish color. You will then soon see the ends of the browning seed pods split open. This is a visual signal that the seeds are matured and the plant is ready to disperse the seeds.
Remove the flower stem and leave the pods attached. Place it on a tray lined with waxed paper and stash it someplace safe and dry, warm but not hot, and out of direct sunlight. Allow the stem and pods to dry for at least a couple of weeks more, they will become papery dry, the pods may even open further. When they are completely dry you may remove the pods and split them open. Ripened hosta seeds are blackish-brown and their shape reminds me of fly wings. Any seeds which are light gray, white, tan, or partially colored brown are infertile and should be discarded as they won't germinate.
There are a few points worth noting when saving hosta seeds:
Not all hostas produce seeds. Many modern hybrids will make seeds, observe your plants and you'll be able to visually determine which are fertile and which are not.
Hosta seeds take a long time to mature, it's not a rushed process, expect a need for patience.
Hybrid hostas will pass the genetics of their mixed parentage onto their offspring, you can expect some differences from the parent in the leaf shape, coloration, and size of hosta plants that are grown from hybrid seeds.
If you're trading your hosta seeds label them as open-pollinated and give a good description of the parent plant in your trade post. Open-pollinated hosta seeds are an excellent addition to any trade list.
Good luck with your hosta seeds!