For the past three nights, lows have been around 0 degrees F (the coldest night was -2 degrees F). The high tunnel bottomed out at +10 degrees, and a low tunnel inside it (covered with AG-50 heavy row cover) dipped only to 29 degrees F. I'm very surprised at how much warmth is being retained so early in the year.
Currently I have claytonia in the low tunnel. The plan is to plant our some kale, Hakurei turnips (thanks Jay for the idea!) and spinach in a few days.