Where I grew up, the freeze line went down a couple-three feet, but we had onions throughout the winter in the cellar.
I don't remember how The Mom kept them, but I now understand that they must be kept dry, in the dark, and not touching each other.
But we're not talking about root cellars, are we? What we are talking about is how produce survived pre-refridgeration. Not being of German extraction, we didn't grow cabbage to allow it to rot in a crock and survive to be called kraut. OK, bad taste. But speaking of sauer kraut, bad taste isn't far afield.
Enough ethnic pokes. I'm sure the ethnics learned a few things from each other back in the days. My experience will be similar and maybe different, but it remains that we're not really talking about root cellars.