Yes, that would be appropriate. For mature trees, we don't want growth and so we don't need to keep the soil warm during winter.
Our bare ground occassionally freezes, but my citrus plants inground have soil that has high organic matter and dark compost or dark loamy soil on the top, keeping it always above freezing the entire winter. Organic matter provides good insulation and being dark absorbs more sunlight during winter.
I don't want the plants to grow during winter, but then it is the time when majority of my citrus fruits are harvested. That's one of the reasons why my citruses here is never as sweet as those in Southern California. So I am slowly keeping a keen eye on later harvests, towards spring, summer or fall. But if I have an enclosed structure, for sure, I would put an insulation and heating system bordering underground, just in case I need some growth or ripening during the winter, to make them sweeter. But of course I will have to experiment on that as to how warm the soil would be during the winter.
We know that for oranges and grapefruits, it is the air temperature that can make things sweeter, and not sunlight exposure. I was wondering if soil temperature would play a good part, granting that soil moisture is adequate. What's your experience on this. I would speculate that with warmer soil and good water content, the fruits would be juicier, but as to the taste, I have no idea yet.
