Thanks for the additional information.   I built two 4'x20' planting beds insulated and heated with soil heating cables.  Both of these beds are inside my 20x30 hoophouse.  I am able to maintain 80 degree soil temperature throughout the winter with no problem.  The air temperature is another story.  I intend to grow most of my citrus in these conditions.  I intend to experiment outdoors with Yuzu, Saduchi and flying Dragon.  I fantasize about growing something more tasty outside such as an Owari or a another variety with better cold tolerance that I am unaware of.  This winter was exceptionally cold (got down to 7 degrees F. one night) (at least 3 weeks well below freezing) yet I was able to overwinter them with only a modest amount of supplemental heat.  I think I could use less heat by draping some agro-fabric over the trees on the nights below freezing and help them catch more of the heat radiating up from my soil.  Other than the several weeks of below freezing temperatures,  I did not heat the hoop house, just the soil.  I believe we have slightly milder winter extremes west of Portland because the occasional cold arctic blast that blows in from the east through the gorge dissipates up and down the valley without delivering the full force of that extreme this far west.  As you mentioned,  the radiational freeze on clear nights is the one to watch for. The other gotcha as you mentioned is the amount of light the trees receive.   Not sure how well I will be able to ripen fruit.  There is a guy south of here who was selling mandarins he grew in his hoop house.  I don't think he is using supplemental light.  He is heating it though.  When I measured the light inside the hoop house in the winter, it did seem to meet minimum requirements.   I think there is a significant amount of non directional light but the number of hours of light is low being North of the 45th parallel.   Not sure if the long days during the rest of the year will make up for the short days of winter.