in my country (Roumania / south -east Europe)from this hibiscus problem we make thus: buy a bottle of beer and washing the hibiscus's leaves with beer. (once per month)
It's not a joke. The properties of the beer are so good from the hibiscus.
Now, I'm not sure if that "recipe" is available for your area!
By the way, what kind (tipe) of Hibiscus you have (Natasja, Ivan Smith, Miss Liberty, Concorde, Eye of the Storm, Fifth Dimensionetc) or Hibiscus Divaricatus, heterophillus, diversifolius, Splendens or Australian Native Hibiscus (all latest are australian species).
If you want I help you, I am looking forward to more details.
I'm sorry for my English, but it's not my native language!
PS: In Romanian the name of hibiscus is trandafir japonez (chinezesc) translate in english word by word japanese (chinese) rose