It’s always a good idea to know what the soil temperature is. One can measure it, or one can be confident that the average daily soil temperature close to the surface (5 cm, or 2 inch depth) is higher than the daily low temperature, and lower than the daily high temperature.
This turfhack works for times when the average air temperature is above freezing. The soil temperatures usually stay around freezing, even when air temperatures are lower.
This chart shows the area between the daily high and low as a shaded grey zone through the year. That’s where one can find the soil temperature.
And here it is for 2018.
