Answer: By hardy hibiscus I assume here you mean musk mallow or rose mallow. These should germinate quite rapidly at a very warm soil temperature of between 71 and 75 degrees. I can think of several reasons why your seeds might not germinate: too cool a soil temperature, soil too dry or too wet, or if you are collecting them from a hybrid plant it is quite possible that the seeds are not viable to begin with.
If you actually mean the hardy hibiscus shrub, H. syriacus, these should also germinate without pretreatment and in fact the seedlings can be a pest in the garden. Here again, though, there are several modern hybrids bred to reduce the self seeding problem and so produce little if any viable seed.
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