A question came in recently from Greta over at Washburn school about what temps one might be recording inside the tunnels this time of year. Greta purchased a high-low digital thermometer to record the nightly lows and daily high temps. She has a lowest low of 0 degrees and a highest temp recording of around 70 degrees. The inflation blower wasn't always plugged in either, so max insulation value probably wasn't realized.
To compare, the temps I have been recording here have been relatively consistent with what Greta is seeing in their tunnel. I have three sensors- one outside, one inside the greenhouse, and one inside the greenhouse under the row cover.
Outside 38 -17
Inside 64 -2
Under cover 79 13
Keep in mind that both of my greenhouses are single layer plastic tunnels. Nevertheless, you can see the 15 degrees of added protection inside and the 30 degree difference under the row cover. Even just the 15 degrees in this case can be enough to determine whether an over-wintering crop will survive the winter or not. This depends heavily on the number of days the root base has to sustaine those sub-zero temps. Last winter we had over 40 days where it did not get above zero under the row cover. Everything died except for a small amount of spinach.
The point being here is that the more protection you can provide the plants during the dormancy period, a greater chance of survival and earlier jump on growth can be realized. Some commercial growers will add two if not three layers over the plants this time of year. This might also be a fun experiment for students- monitoring the temperature differences under multiple layers (or different layers) of protection.
Watch the moisture build up however! Wet, humid conditions are a perfect breeding ground for mildew and mold. It is good practice to pull back the covers and even open the peak vents on the greenhouse during warmer and preferably sunny days to let things "breathe". I'll speak to this in greater detail on another post.
Agripreneur Students (2016 posts)
High Tunnel Blog