In our back garden on the north side of a hill and in the shade of a house, frost painted the ground white last night. A thermometer registered 31 degrees. In front, there was ice on the cars, but the front garden showed no frost. I suspect the difference is that the back yard is protected by houses and fences from light winds, while the front is open to the street. Where air can circulate a bit, even circulation caused by the passage of cars on a still night, this will often be enough to mix warmer air with the coldest air near the ground. And with the temperature just below freezing, maybe that was enough.
I looked out the front window, while I was fixing the blinds, and there was a young woman looking at the garden, probably as surprised as I am to see the Erysimium 'Apricot Twist', the Convolvulus tricolor, the yellow and red violas, and the Gaillardias still in bloom. She looked up and smiled; I smiled back. Flowers on a frosty morning are special.