I've had the same thing happen to 3 of my hibiscus. They eventually look stunted and gnarled and don't grow properly. I presented a picture to the forum about 2 years ago and no one seemed to know what it was. In every case the plants eventually died. It looks like some sort of disease to me. I've lifted the plant out of the ground to check the roots and the roots look fine. I'm very sorry to hear that whatever it is has infected your hedge. If I find out what it is I'll let you know.
Hi Andrea, You have Erinose Mite. Treat by pruning plant in Spring, ensuring to remove all leaves. Dispose of all foliage in bin. Feed plant with high nitrogen food and keep feeding it regularly with good NPK food suited for Hibiscus or roses or azalea -(forever will keep mite away). Spray monthly with Lawn Grub killer (Endosulphan)for 3 months, then 6 monthly, then when plant is healthy and fed well, once a year thereafter.
Also, vigilance to remove any leaf with tiny galls on it helps.
Where are you? America? as I don't think Erinose mite is known of there yet, but it will get there, so you might be one of the first.
