It's been about a week since the mobile greenhouse was pulled into its place behind the vegetable beds. An earthbox seeded with lettuce and a limestone block was set inside. I have been monitoring the temperature using a wireless temperature monitor (base station located inside my house) and two remote sensors, one inside the greenhouse in a little box to block any direct sunlight from falling on the sensor, and another sensor on the back side of the greenhouse (also in shade) to monitor the outside temperature. The base unit can monitor up to three channels and will record minimum and maximum temperatures since the last reset. You can buy these units at many big box stores for about 20 dollars, although additional sensors will cost more.
Since the greenhouse was set up outdoors the weather has gone from one extreme to another. One day there was a high temperature of 55 degrees F. Last night the low temp was 2 degrees. I haven’t kept detailed temperature records, but have come up with a few general observations. Initially I had one limestone block in the greenhouse for thermal mass, but after a few days of warm weather some blocks which were frozen together came unstuck and were put inside the greenhouse. There’s probably about 200 pounds of stone inside the greenhouse.
A few observations:
The greenhouse will be about 20 degrees warmer than the outside temperature on a cloudy day, and as much as 35 degrees warmer on a sunny day. At noon today it was 15 degrees, sunny and very windy outside.
The temperature inside the greenhouse at noon was 45 degrees, a 30 degree difference. By 1 PM it was 53 degrees (outside temp 18 degrees), for a 35 degree difference. It is 100% sunshine today so that temperature difference is not surprising.
Once the sun goes down the temperature difference slowly diminishes. Last night was the coldest night this winter. By midnight the outside temperature was 12 degrees, while the greenhouse temperature was about 25 degrees. By this morning the outside temperature had dropped to 2 degrees and the greenhouse reached a minimum of 11 degrees. After sunrise the inside temperature rose quickly.
11 degrees is pretty cold for most plants, but there are not many winter nights that get to near zero degrees. Another consideration is the actual soil temperature inside the Earthbox. The temperature of the soil will not reach the temperature extremes of the air around the Earthbox.
Last year I compared the temperatures inside the plastic greenhouse that goes over one of the raised beds each year where spinach is overwintered. As with the mobile greenhouse the temperature inside rose quickly whenever the sun was shining. What surprised me was just how quickly the temperature fell once the sun went down. In less than an hour there was no difference between the inside and outside temperature. I thought that the soil in the bed would retain some heat and keep the space inside a little warmer but that was not the case. A single layer of plastic film just does not insulate well enough to hold in any heat. This is a photo of the plastic greenhouse with the new greenhouse in the background.
So how does the greenhouse perform on a more typical winter day for this area? Actually quite well in my opinion. A few days ago we had an "average" January day – a high of about 35 degrees, partly cloudy with occasional peeks of sun, then a low the next morning of 20. The temperature inside the greenhouse stabilized at just under 60 degrees and held at that until sunset, when it slowly fell until it was 35 degrees at midnight. By the next morning the greenhouse temperature reached a low of 28 degrees (outside temp 20 degrees) then climbed quickly after the sun came up.
The climate inside the greenhouse that day was similar to a March day with a nightime frost. That’s enough to give me hope that lettuce and spinach can be grown in this thing much of the winter. Now if that lettuce would hurry up and germinate.