This post is about making some quick garden frost protection by lining a box with radiant barrier.
In my case, I’m using a plastic shoe box, lined with house radiant barrier cloth. But a cardboard box and a cut-up space blanket would work just fine.
Based on prior tests, I’m betting these will provide about 5F of frost protection. Good enough for the 29F predicted here for tonight. (Edit: Confirmed by actual test, see final section.)
A final plus is that I don’t have to store these. That’s nice for an item that I might need once a year. Once we’re clearly past last frost, the plastic shoe-box sized totes can go back to being plastic totes, the radiant barrier pieces can be rolled up and put away.
Any tin-foil hat will do: Radiant barrier, or a space blanket. Or even tin foil.
Post G22-005: Frost planning. Dodging the last breaths of Old Man Winter
The post linked just above summarizes some of the tests I did on frost protection methods, a few years back.
I scienced it. I used a pair of temperature data loggers — little recording-thermometer gizmos that can dump their data into a USB port — to record the difference in temperature between protected and unprotected plants.
Glass (e.g., mason) jars work exceptionally well, as does radiant barrier material (e.g., space blanket). And, both work by stopping the radiant heat (long-wave infrared) that radiates up from the (relatively warm) ground.
By contrast, floating row cover and polyethylene sheet do nothing. The latter seems like it ought to work, but polyethylene is transparent in the far-infrared (long-wave infrared) region. It doesn’t trap the heat radiating up out of the ground.
This year’s problem is the rhubarb
This year, I’m stuck with some early-sprouting rhubarb, and a prediction for 29F tonight. Not an ideal combination, as rhubarb leaves are not frost-tolerant. Mason jars would be the easy solution, but the leaves are already too big.
So I did a little freeze-protection improv, and came up with using plastic shoe-box-sized containers lined with pieces of house radiant barrier (aluminized plastic cloth of some sort.)
As is my habit, I’m going to test these. In this case, I’m testing against a “control” of just a plain plastic container without radiant barrier.
I’m betting that the radiant-barrier-lined plastic shoe boxes are good for 5F of frost protection.
I’ll post the results tomorrow, after the temperature data loggers have run all night.
Tested and proven, good for 4F of frost protection
Nature cooperated with the experiment by providing frost inside the “control” enclosure this AM. This, on a night with a predicted low of 29F. With underlying ground temperature (4″ down) of about 49F.
That’s nice, because Jack Frost left me a benchmark. I’d better see “the data”, out of the little USB gizmo, at 32F or nearly, for the control (clear plastic box). So that the results are all about how much warmer the ground is, under the tin-foil hat (the radiant barrier lined shoe box) and not about how much instrument error there might be.
And, as shown in the graph above, that’s what I got. The ground under the clear-plastic cover froze. It got down to 32F, according to the recording thermometer. All quibbling over the accuracy and resolution of my recorder-gizmos aside, literal frost on the polypropylene tells me it froze last night, inside the clear-plastic enclosure.
Just a clear-plastic cover — in this case, a polypropylene storage container — sealed reasonably tightly over a plant, so as to provide a pocket of “still air” during the night — did essentially squat. For sure, did not do enough to provide frost protection during a predicted 29F overnight.
But it was maybe 5F warmer in the radiant-barrier enclosure. The same box, lined with a piece of radiant barrier. In this case, house radiant barrier. But a cheap space blanket would have worked just fine.
Note that this is about what I expected, based on past use of radiant barrier in my garden. I fudge on the exact amount of warming because, if nothing else, the data loggers only record to the nearest 2F. The resulting temperature difference looks five-ish to me. Plus, it’ll depend on how warm your soil is (mine is currently 49F 4″ down.)
A nice thing about these old-school USB data loggers is the simplicity of use. For an old-school user. This device plugs into the USB port of my laptop, and, using very simple software, spits out a text file which, when copied into Excel, requires but a single chain of clicks to become data in a spreadsheet.
Otherwise, near as I can tell, the devices I’m using are obsolete by at least a couple of decades. You can buy updated, modern (and of course, made-in-China) versions of these data loggers that are superior to what I’m using, in every way.
But 1) they still work, 2) they are good enough for my use, and 3) I already own them.
The trifecta of my material existence.
An addendum on “convection”.
I associate the word “convection” with “heat rises”. Convection, in my mind, is about the natural circulation of heat in (e.g.) the atmosphere, or a cup of coffee, by the physical transfer of the heated gas or fluid.
But that’s NOT only one way in which heat can be transferred by the bulk movement of gas or fluid. Which is what convect (from “convey”) means. Any transfer of heat, by gross movement of a material medium, will do. (To complete the trinity of conduction, convection, and radiation as the ways by which an object may lose heat.)
Outdoors, a breeze results in horizontal convection, for want of an easier term.
When you read about plant frost protection, you need be aware that when air conveys heat away from an object simply by blowing past it, that also counts as convection. Anything other than “still air” provides convective transfer of heat energy. Which is why creating a sealed enclosure to provide “still air” is the first step in many frost-protection methods. It doesn’t much matter how much you an heat up a small area, if all the heat blows away.
More confusingly, preventing convection in the outdoor setting isn’t about “heat rises” much, if at all. It’s about sealing the plant in some sort of breeze-proof enclosure. It’s about providing a pocket of still air around the plant.
That’s because heat-driven convection in air only needs an inch or so of air space to get going. (This, per experts in storm windows and such.) Within these little shoe-box enclosures, for example, I’d expect cold air to be sheeting down the inside of the box walls, resulting in an internal air temperature that fairly uniformly reflects the temperature of those box walls, throughout the enclosure.
Which, in turn, is going to reflect the outside air temperature, less any small effects for insulation.
The upshot is that these plastic containers provide a pocket of still air around a plant. They do not prevent thermal convection within that pocket of air. (But the same uninsulated plastic box, if “powered” by infrared-reflecting radiant barrier, runs about 5F warmer.
But how, exactly, radiant barrier works, is not easy to explain.)
And now that I get well into the weeds, I realize I should have lined the clear plastic box with a piece of polyethylene sheet, to rule out that the insulative properties of the second sheet of plastic as the cause of the warmer temperatures under the radiant-barrier-lined plastic shoe boxes. If I ever redo this, I’ll try to remember to test that. But based on other attempts with radiant barrier, I’m sure it’s the IR-reflecting property of the radiant barrier material.
Further confusing the issue, you see discussions of providing “a pocket of still air” around a plant that focus on the insulating properties of that air. But as this and my prior experiments have shown, the main effect of the “pocket of still air” is the prevention of convection, in the important sense of preventing your warmed air from being swept away by the breeze. (And, noting that thermally-driven convection will continue freely within the enclosure, helping force the inside air temperature down to the outside temperature, less any insulating effect of the thin plastic wall of the container).
In any case, the practical upshot is that if you block both radiation and convection, you can get about 5F of frost protection. Here in Virginia Zone 7A, with typical ground temperature at this time of say 49F or so).
But if all you do is provide “still air” around the plant — the plastic shoe box alone — you get next-to-no frost protection.
A good thing to keep in mind as the spring progresses.
