Hibiscus will get a bit cranky under 40 degrees where they show their displeasure by dropping leaves.
I’ve invested in a few “Frost Blankets” (Amazon) which help to a certain extent. They reduce wind and keep temperatures inside about 3-5 degrees or so warmer than the outside temp.
I got very lucky and had 3 of them survive down to about 19 degrees a few years back. There was some dieback of the branches, but they are still alive today.
If yours are too big to handle (for a frost blanket), you could always prune them back a few feet in late fall so that they fit under the frost blanket.
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