When Should You Prune a Hardy Hibiscus Plant?
Time Frame
Prune hardy hibiscus in the early spring, before new growth appears. Prune established hibiscus by 1/3 every spring to encourage regrowth.
Benefits
Hibiscus plants respond well to pruning, which encourages new foliage growth and blooms. The hibiscus can be pruned to a preferred size without fear of damaging it.
Considerations
Hardy hibiscus will die to the ground if kept outside in areas that experience cold winters. New growth will appear in the spring.
Source...