After having constructed two identical cold frames and then planting vegetables in them. I began to think about what might happen if it got really cold this winter. I know that most lettuces and many cole crops (Broccoli, Brussels sprouts, cabbage, cauliflower, kale, collards, and kohlrabi) can survive brief bouts of freezing temperatures. What would happen, I wondered, if it got really cold out there. To that end, I decided to place two wireless 900 MHz temperature sensors into both cold frames just to monitor what was going on.
As luck would have it, last night it did get into the 20’s. I had the opportunity to graph both the ambient outdoor temperature and also the internal frame temps in 30 minute intervals. The graph above is a result of one of those experiments. Note: The Davis reading referrs to the outside ambient temperature.
The results were pretty interesting. For one thing, at 4:30 PM, I switched the sensors just to see if CF2 was really colder than CF1 and yes it is. That may be due t the fact that CF1 has a layer of plastic wrap over the glass which creates a small airspace that may indeed raise the insulating value a little. All other factors being equal, it may have increased the insulation enough to raise the internal temperature about 1 to 2 degrees.
The constant warmth of the ground also creates a warming effect as can be seen by the differing slopes of air temperature versus internal cold frame temperatures. I calculated that it will have to get down around 10 above zero before I would need to go to added measures like turning on an electric light bulb inside the frame. Some night if we do get exceptionally cold I will plan to stay up and record that event to see exactly what would happen.
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