You have to leave it on the plant for a while. A pod will develop where the flower was. I usually let it fall to the ground as it is big enough to see easily. Then I dry the pod and then crack it open for the seeds. A lot of times the pod falls off the plants before the seed is ready though.

JP

JP...thanks so much for your reply. After the flowers fall off and the part where the flower grew out has fallen off, I've been looking inside that for some seeds or anything! I usually wait til it is dark brown but still no seeds. Hmmm...I've had these two plants for about 4 years now but have never been able to find any seeds. I'll keep looking. Thanks again. At least I know I'm looking in the right place!

Linda

My hardy hibiscus develops pods on the plant that will open up and reveal the black seeds inside. I have already harvested the spring bloom seeds and am waiting for the fall bloom seeds to ripen.

Trudi has this photo of Hardy Hibiscus seed pods in the FAQ:

the only time i have ever found seeds from this plant is when it buds, and the bud does not flower but instead dries up in it's bud form, still unopened and even then i couldn't pull the dried bud off the plant it had to fall off and that's how you know the seeds are mature if you shake the pod and you can hear them rattling around in there they are little dark brown almost black round seeds round like a ball not flat i have grown seven new plants from one dead bud!. That has been my experience hope it helps

jules