Several nights ago, we had a heavy frost that, according to Environment Canada's website, was as low as -4C. The greenbelt behind our house was covered in a thick layer of frost, but somehow, our patch of grass had none! The sheltered, south-facing location must have trapped enough heat that we were spared the frost that we could see about five feet away from our fence. Since we ripped out most of our annuals and vegetables, about a third of our yard is bare soil. In some ways, this is better than mulch at this time of year, since the dark colour absorbs heat during the day and radiates it back to our plants at night.
Further protecting our plants is the heat that is lost from the foundation of our house. Although cosmos are supposed to be frost sensitive, ours is still blooming like mad (to make up for not starting until September, I think). I advance this as proof that although we are well into October, I still have not experienced a frost, at least in the part of my garden nearest the house, which shows what a good microclimate can do.