High 20's, no problems. Teens, think about protection for unfolding buds but probably still OK. Single digits, wrap them.

Thank you; sounds like this is a more hardy plant than I'd realized.

While awaiting an answer, and to be on the safe side, especially since they're also predicting some high winds, I constructed a small "teepee" of bamboo poles and a piece of solar pool cover left over from the greenhouse. There's enough room in there for three one-gallon jugs of water. I'll remove the cover in the daytime so the plant doesn't get cooked in the sun.

Di

Tree Peonies are hardy to zone 3 or colder, below -30F for short periods, but the buds which are set in the previous summer are a little less hardy. The biggest danger for most people is a late freeze which can zap the fully-open foliage or the flower buds when they are ready to open.