Answer: If you are planting a hardy hibiscus shrub (Hibiscus syriacus or Rose of Sharon) or one of the hardy perennial hibiscus such as H. moscheutos, then plant it at the same depth as it grew in the container. The roots will grow down deep into the soil on their own.
If you are planting a tropical hibiscus or Hibiscus rosa-sinensis, the plant will not survive outside in the winter no matter how deep you plant it. You would need to bring it inside each fall for the winter and treat it like a houseplant.
I hope this answers your question.
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