I've been using the inverted trashcan treatment to trick this Hibiscus grandiflorus plant into flowering earlier. It usually doesn't bloom until very late (close to frosts) when all the other Hibiscus are well over. So every day around 5:00 PM, I cover it with the trashcan and the following morning around 8:00 AM the can comes off. With a few exceptions, I've been doing this for about a month. It seems to be working - see the tiny flower buds that have formed at the growing tip.
Now if I can just hold the other Hibiscus back from setting seeds (by deadheading) and keep them forming new flowers, I hopefully will be able to finally get some hybrids from this really cool species.