In a nutshell: I’m going to try draping radiant barrier material over my tomato plants, to allow them to continue ripening tomatoes despite oncoming low nighttime temperatures. I’ll use a couple of temperature data loggers to track the results, which I will report tomorrow.
Let the garden wear the tin-foil hat this time, instead of the gardener.
Background: Damn those Canadians.
If they aren’t sending smoke our way, they’re sending unseasonably cold air.
Starting today, temperatures in Northern Virginia will dip into the low-to-mid 40’s F at night. Except for a few days toward the end of next week, it looks like those nighttime temperatures will likely continue until first frost.
That’s a problem, because my garden still has dozens of baseball-sized green tomatoes. And those green tomatoes can’t take those low temperatures, based on my last bit of research (Post G23-060).
One option is to strip all the tomatoes off those plants today: ripe, partially-ripe, and green. Anything at “mature green” or later stages will (eventually) ripen, sitting my kitchen. But this would also give me a lot of green tomatoes that would never ripen, and so would have to be eaten, in some form, as green tomatoes.
I would much prefer that those green tomatoes begin to ripen on the vine before I pick them. Once they start to ripen, I can bring them inside where they can finish ripening.
I only need five or ten degrees F of additional nighttime warmth, for another week or two, to achieve that. Ideally, those tomatoes would be in a greenhouse now. Unfortunately, as I neglected to build a greenhouse around them in the spring, it’s a bit late for that. Constructing some sort of temporary enclosed shelter now, around these fully-grown in-situ tomato plants, would take a bit of effort.
So maybe it’s time to drag out the radiant barrier material again. And see if merely draping radiant barrier around those plants it can raise nighttime temperatures enough for the remaining tomatoes to ripen.
It’s the least I can do. Or damn near.
Recap on frost protection.
Recall my much earlier posts on frost protection. The key insight is that your garden bed is like a big open window, looking up into the sky. As such, even on a cold night, it loses more heat energy through radiation than through conduction. (See “The math behind the space blanket, referenced below.)
As a result, keeping your garden bed warm at night is not at all like keeping your house warm at night. For houses, radiative losses are small, and it’s all about insulation. For a garden bed, by contrast, the biggest bang-for-the-buck is in some form of radiant barrier — something that will stop the soil from radiating its warmth away in the form of long-wave infrared (a.k.a. “heat”).
Not convinced? Consider this: A thin-walled glass mason jar provides excellent frost protection. (See relevant posts below). That jar doesn’t weigh much, so it’s not providing “thermal mass” the way a wall-o’-water device does. It captures only a tiny pocket of air, behind a thin wall of glass, so it’s not providing any material amount of insulation. Instead, it works because glass is almost completely opaque to infrared. It traps a watt or two of the long-wave infrared radiation (“heat”) coming out of the soil of your garden bed, all night long. Effectively, it’s like having a little night light giving off heat, under the jar. It’s just that the “night light” is the residual heat in the underlying soil, trying to radiate away. That small energy input is enough to raise the temperature inside the jar, on a freezing night, by about 7 degrees F.
Post #G21-012: Warming a raised bed at night, part 1: Floating row cover does nothing
Post #G21-014: Warming a raised bed at night, part 2: Success!
Post #G21-018: Radiant barrier for keeping a raised bed warm at night, final post.
Post G22-005: Frost planning. Dodging the last breaths of Old Man Winter
Post G22-007: The math of the mason jar cloche (corrected!).
Post G22-008: Plastic cloche surprise, not all plastics are created equal.
What am I testing, exactly?
I already know that radiant barrier works to keep garden beds warm at night. In early spring, with winter-cold soil and freezing air temperatures, a sheet of radiant barrier tightly fitted to a raised bed (to stop air flow), raised the bed temperature by about 5F. Adding some jugs of sun-warmed 75F water increased that to 10F.
Source: See posts above.
And I know that’s the effect of the radiant barrier, not merely the effect of covering the bed with plastic. I know this because a plain polyethylene sheet — transparent to infrared, and so no radiant barrier at all — did very close to nothing at all to keep the bed warm, in the same circumstances. Less than 1F warming, on average.
The upshot is that I’m only testing the unique aspects of my current situation. First, the soil in the bed is a lot warmer than it was in the spring, this providing more heat input. Second, the radiant barrier will fit loosely around the plants, allowing for air infiltration. So it’s really a straight-up empirical question. Will draping sheets of radiant barrier over the plants, with summer-warmed soil underneath, keep my green tomatoes above the critical temperature of 50F, on a fall night when air temperatures will be in the low 40sF?
That’s what I’ll be testing tonight.
A tip if the tin-foil hat
As noted above, I didn’t think this up on my own. I stumbled across a publication from the University of Colorado Extension Service, where they achieved more than 30F of warming with both an air-tight plastic enclosure and a radiant-barrier “cap”.
Like so:
Source: Colorado State University extension service (.pdf).
Weirdly enough, I’m pretty sure this works because the underlying polyethylene sheet does nothing to trap infrared. Effectively, the clear sheeting doesn’t exist, as far as infrared is concerned, and so the radiant barrier has the clear space it needs in order to reflect heat back to the ground.
So the plan is to do what U. Colo did, minus the air-tight plastic enclosure. It’s not that I couldn’t jury-rig something around my plants. It’s that I’m too lazy to. (Mostly because I’d have to remember to open that enclosure every morning, or the sun would likely fry the plants inside.) So I’m going to test whether or not I can get enough heating, absent the enclosure to prevent air infiltration.
All I have to lose is a bunch of green tomatoes. So it’s not as if this is a big gamble.
Results tomorrow.
