I was interested in answers to this, too. When there weren't any, I googled it. (I've over wintered annual geraniums for years).

The consensus is....either take cuttings and start them in a pot, or cut down the petunias, put them in a bright window, and treat as a house plant. In the spring, they'll probably be spindly but you can cut that all back, and they should fill out and bloom again.

I believe that is the right advice. I am not aware that our regular petunias are perennials at all, so I don't think they will survive dormancy, no matter how warm it is.

Petunias are all tender perennials...you can winter them over if you can provide the right conditions. There's about a 50/50 chance they will make it through the winter. But, you've already bought them so what do you have to lose?

Dave
Milwaukee