I've started bringing the plants from the enclosed porch outside, but I still have to bring them in and out. They stayed outside two nights ago, but the weather was supposed to cool down significantly yesterday. It was sunny and I didn't pay attention. When I checked the weather website, it was -4C, and it was very, very windy. I went outside and found that the plants were solid frozen.
Everything (i.e., soil, plants) froze in a matter of hours. I brought them all in, but they recovered really fast. These are all alpine plants (hellebore, blue campanulas, violas), so, fortunately, there was no damage.