Above, my tomatoes stayed at 50F, despite an overnight low of 43F.
Short answer: Draping sheets of radiant barrier over an air-tight plastic enclosure keeps the plants underneath warm. In fact, about 7F warmer than the ambient air. (That’s the same magnitude I found when I used radiant barrier as frost protection, last spring.). This protection against low temperatures should be sufficient to allow my remaining green tomatoes to ripen, despite cold nights.
Recap from recent posts: I have a lot of lovely, baseball-sized green tomatoes on my tomato plants. I’d love to have them start ripening. But the weather just turned much colder, starting last Saturday. Significant exposure to temperatures below 50F will result in those rotting, rather than ripening, due to the destruction of ripening enzymes at low temperatures.
I first tried a minimum-effort approach of simply draping radiant barrier material over the plants, to try to keep them warm at night. That did nothing, presumably due to the free flow of air under the radiant barrier. (And in hindsight, that was an exercise in wishful thinking. How warm does your house stay in winter if you leave all the windows open?)
On this second try, I first covered the plants tightly in polyethylene sheet (clear plastic from Home Depot), trying my best to keep air from infiltrating the enclosure. Then I then draped radiant barrier material over those enclosures. That combination — air-tight and infrared-reflective — kept the plants 7F warmer than the surrounding air.
(FWIW, prior experiments demonstrated that poly sheet alone provides almost no temperature protection. That’s because it provides almost no insulation, and is completely transparent to infrared radiation. Of the three sources of heat loss — convection, conduction, and radiation — it only prevents convection. Which clearly isn’t enough to do much to keep the plants underneath warm.)
As a bonus, when I remove the opaque radiant barrier material during the day, the clear poly sheet acts like a greenhouse. That extra daytime heat should speed tomato ripening. Assuming I don’t fry those plants with too much heat. Which is one reason that I hesitate to cover them with clear poly in the first place.
The upshot of all this is that I still have some hope of ripening the few dozen green tomatoes still on the vines, over the coming week or so, despite the change to colder fall weather.
