How far back should a Hardy Hibiscus be cut above ground?
I've heard Hearty Hibiscus should be cut back all the way from 6" to 24" above ground.
Hardy hibiscus is tolerant of severe pruning however I would not do it annually. If your shrub is over-grown, you can do "rejuvenation" pruning by cutting all the branches down to 9-12 inches in late winter before leaves appear. On an annual basis, remove 1/3 of the oldest, largest branches to the ground. Every 3 years, you have a new, youthful, fuller blooming shrub. This method also controls overall height.