As long as your ground temp below the surface is above freezing you should be fine don't mix it too wet and put some firm of insulation over the top. I've used concrete bags with grass clippings or even some of the dirt you dug out over tge top.
I put in hundreds of the old big satellite dishes all over idaho and Wyoming 30 years ago and it was usually a 4" steel pipe in a 36" hole. We only worried about it during real cold spells and when the frost level was down deeper than a foot. In 91 we had -40 degree Fahrenheit weather for about a month and that slowed us down but it was far too cold to be mixing mud anyways.
Bb
