Answer: By perennial I assume you mean one of the herbaceous types of rose mallow such as Hibiscus moscheutos. The lanky stems can be cut back to ground level now or in the spring. Mark the spot where they emerge because they resprout very late in the spring and it is easy to lose track of them in the spring rush -- or to think they are dead when in fact they are simply very slow to emerge.
If you mean the shrub hibiscus, Hibiscus syriacus, pruning is best done in late winter. The shrub blooms in mid to late summer on the current season's growth, so pruning during the growing season will remove the year's flower buds.
Enjoy your hibiscus -- they are all great plants!
Q&A Library Searching Tips