Errrr......this is the TN gardening forum. We don't know nuthin 'bout Minnesota. ;-)

OTOH, sorry about your freeze! If you are talking about hardy hibiscus, they survive freezing quite well. But if you are talking about tropical hibiscus, they need to come before the freeze hits them. So it all depends!

I would not even let the frost hit them. Wow all this global warming stuff and you are talking about freeze in the middle of Sept.

Heater, Amazindirt is right on target, it depends on what type you have.

Maternut, global warming is causing all kinds of weird weather patterns. Although on average the Earth is warming, the weather patterns are shifting rapidly. Some places are actually cooling because of the pattern changes. A prime example of these weird changes is our own drought. World wide there's an increase in precipitation, but here in Tennessee we're stuck right in the middle of a bad drought. Global warming doesn't change the weather everywhere equally, but it can increase extremes.