Root cellar question

A listener just asked me, "Once upon a few years ago, @Marjory Wildcraft wrote of her intention to experiment with root cellars and how deep one would need to be in TX to keep things at the correct temperature. Do you know if she ever published anything on this? my home is not so hot, but I have not found good info for the depth of root cellars in my region."

Does anyone know the answer (I know Ms. Wildcraft is very busy right now).