I'm also zone 5 and have great success with my hardy hibiscus, in winter /early spring I cut the stalk down to ground level, as it should be. They die back to the ground every year, this is normal.thenslowly emerginf from the roots in late spring. Where is yours located? Nice moist soil and lots of sun for it. Mine are 3ftx3ft in second year... Not sure what could be the issue but someone here will have advice.