Not too low; that's not a very friendly for the lawn this time of year. Regular mowing kills almost everything but grasses and hibiscus syriacus is no exception. Even bittersweet gives up against regular mowing.
Just a quick note; the National Arboretum selections of Rose of Sharon (easily i.d.'d by their names from Greek mythology, i.e., Minerva, Aphrodite, Diana) are sterile hybrids and won't throw seed.