You can plant them any time but look very carefully at the seeds and see if there are holes in any of them. A moth, I don't remember which one, deposits her eggs in maturing seed pods and the larvae burrow into the developing seeds and eat them from the inside out. As a rule I always put a little bit of diatomaceous earth in with my seeds (all of them, not just hibiscus) and shake well to cover all the seeds .. that will take care of any critters that hatch out.
