A few months ago, I wrote a blog post detailing how to build a cold frame made from recycled materials. In that post, I mentioned that I would test to see how "warm" my cold frame really was, using two temperature probes. I placed one probe inside the frame, and the other one nearby in the garden. I put radiation shielding on each probe (aka pvc pipe) to try and measure shade air temperature. By comparing the diurnal temperature profiles for each prope, I should be able to tell how effective the cold frame is at raising nighttime temperatures.
Anecdotally, the box is working wonderfully! I just had my first spinach salad from the box, only 24 hours after it snowed 2", and in a neighborhood known for deer and deep morning shade.
After building the frame, sealing up air gaps with soil and leaves, and letting the cold frame thaw the soil, I shallowly cultivated the soil, threw a few handfulls of spinach seed down, and walked away. I left the frame in my father's garden, and I could only get by a few times a week to check on the seedlings. I would occasionally prop the lid open, or close it, but it was mostly left propped about 1/3" open all day, to protect from frost in case my father forgot.
It turns out that the cold frame did an excellent job protecting against frost, as I expected. However, I also learned that despite satisfactory results (greens to eat), I need to do a much better job of opening the box on clear, sunny days.
A summary of my findings: Out of 30 nights where it got below freezing outside the box, it only froze on 9 nights inside. I just extended my season by 21 days out of the 45 that I ran the experiment. The coldest it got in the box was 24.7 degrees, which did not kill my spinach. The maximum frost protection provided was 22 degrees on March 17, when the lid was closed and covered with styrofoam and a rock. It got down to 13 F outside that night, and stayed 35 F inside the box.
Unfortunately, the box also got above 100 F on 13 different days. Luckily, the spinach, and some lettuce transplants managed to survive these hot conditions, including a scorching 123 degree afternoon at the end of March.
Stay tuned for more news about our cold frames at the Missoula Co Fairgrounds! We are in the process of construction preparation right now, and they should be up within the month.
If you are really interested....here is all of the data graphically instead of just highs and lows, or contact me for a spreadsheet.