I had planted a few cold-hardy vegetables in my garden weeks prior to last weekend’s deep freeze. I put in some snow peas, potatoes, beets, garlic, onions.
It got down below 20F briefly on one of those nights. I can now say that all of those appear to have survived, with just a bit of TLC. That was in the form of capping the bed with radiant barrier, then adding a piece of plastic for air-tightness. (See Post G21-018, or my just-prior garden posts.)
It’s no surprise that we had a freeze. Our nominal “last frost date” is somewhere around April 22,so these plants were in the ground almost two months ahead of that. Instead, the interesting thing is that I had two weeks’ warning that the freeze would occur. The fourteen-day forecast accurately predicted that there would be a freeze that weekend, although the original forecasts understated the depth of that freeze.
This leads me to ponder the implications of reasonably-accurate long range weather forecasting and our “last-frost” dates. Folklore guidelines (“plant peas on St. Patrick’s Day”) and science-based “last frost date” guidelines predate the era of supercomputers that make long-range forecasting possible. Weather is still chaotic in the mathematical sense, and so not predictable at very long intervals, but we now have two-weeks-ahead temperature forecasts that are reasonably accurate.
I already rang the changes on this once, in post G21-005, Your 70th percentile last frost date is actually your 90th percentile last frost date. What you typically see cited as your “last frost date” is the date on which, historically, frost only occurred after that date around 30 percent of the time. But that’s an unconditional probability, as if you would plant on that date regardless. If, by contrast, you check your 14-day forecast on that date, and refrain from planting if frost is in the forecast, then you’ll convert that to a 90th percentile last-frost date. That conditional probability — chance of frost after that date, conditional on a frost-free 14-day forecast — gives you a much higher chance of avoiding a freeze after that date.
The upshot is that a reasonable prediction of the two weeks following the “last frost date” shifts the odds attached to that date considerably. It’s actually a lot safer to plant frost-sensitive plants on that date, in the modern world, than it was in the era when no forecasts extended more than three days. As long as you make that decision conditional on the extended forecast, and you’re smart enough not to plant if it looks like frost any time in the next two weeks.
At present, we’re creeping up on 14 days prior to our April 22 “last frost date”. And I’m pondering — just as an exercise in probability and statistics — whether that same math works 14 days in advance of the date.
And I’m pretty sure it does. If the 14-day forecast were completely accurate, then the conditional 70th percentile last frost date in this area would be April 9th. No frost in the forecast through April 22 would mean that the conditional odds of frost occurring after April 9 would be the same as the unconditional odds after April 22.
That is, April 9 is our conditional 70th percentile last frost date. If we have a decidedly frost-free 14 day forecast at that point, planting on that date bears the same risk of frost damage as planting blindly on April 22.
The only uncertainty there is in how accurate the 14-day forecast actually is, for daily low temperature.
Weather forecasts seem to be one of the few true ephemera of the digital age. They are published, and then they are replaced with the next day’s forecast. Nobody cares about yesterday’s forecast, other than those who have some deep professional interest in forecast accuracy. Accordingly, where you can look up the actual weather 14 days ago, I haven’t yet located a database that lets me look up the actual weather forecast 14 days ago.
So that’s going to have to remain an unknown, for the time being, unless I want to try to compile the data, for my location, day-by-day, myself. Or if I can find existing research that addresses this exact question of predicting a frost. So I’ll just have to leave that as saying that if the 14-day forecast shows lows that are well above freezing, then you can probably move your traditional (unconditional) 70th percentile last-frost date up by two weeks.
But is this just the second-biggest waste of time in the U.S.?
The second-biggest waste of time in the U.S.A. is doing something really well that doesn’t need to be done at all. (I heard that in a time-use seminar I attended decades ago.)
In the fall, frost protection has some clear advantages. The plants are already grown, the produce is already ripening. Protection from an unexpected early frost is a matter of saving garden produce that would otherwise be lost.
But as I hustle about protecting my plants in the spring, it invites the obvious question: Just how much am I gaining by planting these crops early? And to that, I will add not just planting early, but the whole process of starting seeds indoors, regardless of the planting date.
In reality, is this really just an example of the second-biggest waste of time in the U.S.?
Ultimately, while some plants may grow in the cold, they tend to grow slowly. At some level, that’s just basic chemistry. The rate at which a typical chemical reaction proceeds roughly doubles with every 10 degrees C of temperature increase. Sure, plants will develop enzymes to speed those processes in colder temperatures. But it doesn’t take a genius to notice that while they will grow, they sure won’t grow very fast.
What prompts this is my peas, which are now all of about 2″ high. And it’s getting on close to a month after they went into the ground. Is that head start worth it, compared to simply waiting for the nominal last-frost date and planting them then?
In short, I’m beginning to suspect that my current setup — plant early, provide frost protection, but no greenhouse — might just be the least efficient of all possible worlds. All the hassle of early planting, and (almost) none of the benefit.
Without a greenhouse structure (or poly tunnel, or similar) to warm the daytime air and soil temperatures, it seems like most of what I’ve done is to induce my plants to try to grow under inhospitable conditions. And they are responding accordingly.
Back when I was a low-effort gardener, I seldom mucked around with any type of early planting. I’d start seeds a couple of weeks before I planted them, just to be able to have a tiny visible plant to stick in the ground. (And so, have better chance of survival for (say) tomato plants.) But my opinion then was that the gains from very early planting were minimal. Give it a couple of weeks, and the (e.g.) peas planted later in the year will have effectively caught up with those planted earlier.
As a result, I’m now wondering whether I’ve been taking all this early-planting advice from people who do early planting and have some type of greenhouse arrangement on top of those early plantings. From what I’m observing so far, that would make a lot more sense than just sticking plants in the ground and protecting them from freezing as needed.
When I briefly Google for this topic, all I see is people touting the benefits of early planting. In effect, a series of statements that you’ll get more out of your garden if you do it. I’m not seeing any quantification of just how much more you get, from early planting alone (i.e., with frost protection but not a greenhouse or poly tunnel).
So, before I get any further caught up in this effort to see just how much I can push that last-frost date, and just how well I can protect those tender plants from frost, it seems like I need to assess the cost/benefit tradeoff.
I’ve proven that I can plant well in advance of that last-frost date. I can do that very well, thank you. But should I do that? I don’t think I’ve really answered that question. And, in particular, should I do that without some sort of setup to warm the daytime air and soil temperatures?
Maybe early planting without a greenhouse really is just the gardening equivalent of the second-biggest waste of time in the U.S.A. Clearly, that needs to be the next thing I test. For that, I need some sort of cheap, safe, low-effort greenhouse or poly tunnel. One that minimizes the chances that I’m going to bake my plants to death.
So that’s the next thing on the agenda. Replant what’s in my garden, one month after the original planting date. And work up a greenhouse covering that, as a lazy gardener, I can live with.
In my last experiment, I showed how well a Ball (mason) jar worked as frost protection. In the coldest part of the night, the inside of the jar stayed 10 degrees F warmer than the outside. I thought that was exceptional performance for a lightweight uninsulated glass container. My explanation is that the glass traps long-wave infrared. And so, this works for the same reason that my radiant-barrier frost protection works. It prevents the garden bed from radiating heat energy off into space.
Long-wave infrared absorption would explain why glass worked well but polyethylene sheet was a near-total failure. A sheet of ordinary window glass will absorb about 86% of long-wave infrared, and reflect the rest. Polyethylene, by contrast, was reported to be almost completely transparent to infrared.
Accordingly, where a glass jar works well as a garden cloche, I figured that a plastic jar would not. And that’s what I tested last night.
Never let facts get in the way of a good argument.
There’s just one problem: Different plastics have different infrared absorption spectra. And it took me a while to track that down.
Using Wein’s Law, the spectrum of radiation emitted by my 50 F garden subsoil would peak somewhere around:
10 microns (micrometers) wavelength
10,000 nanometers wavelength
1000 waves per centimeter.
Those are three ways of saying the exact same thing.
So I wanted to find out how different plastics behaved with respect to long-wave radiation somewhere in that vicinity. That’s where most of the power from the upwelling long-wave radiation from the garden bed will be concentrated.
I never did find exactly the data that I wanted. But I came close. And, as it turns out, polyethylene’s absolute transparency in that region of the spectrum is the exception among plastics, not the rule.
The chart below show the absorbance spectra of various common plastics, with the long-wave infrared region highlighted. Note that the line for polyethylene is almost completely flat in that region. It absorbs almost no long-wave infrared. But PETE plastic, just below that, in fact absorbs infrared strongly right at the frequency where infrared from the soil will have its peak — wave number of 1000.
The upshot is that when I condemned all plastic for this use, I was too hasty. Avoid polyethylene, for sure. But, assuming the glass choche works as I have described it, PETE plastic ought to work reasonably well. Not as well as glass, but certainly not as poorly as polyethylene.
As an odd little footnote, Mylar plastic — the kind used to make space blankets — is the same stuff as PET/PETE plastic — polyethylene terephthalate.
Results
Below is a photo of a quart Ball jar (right) and the thick-walled PETE jar that I’m going to test. That was as close as I could get to the same size and shape as the Ball jar. FWIW, the PETE jar originally held salad dressing. You can see that it’s much thicker than (e.g.) a typical disposable water bottle or soda bottle.
When I tested that last night — two temperature loggers on a raised garden bed, one covered with the PETE bottle, one un-covered — sure enough, PETE works pretty well. But not as well as glass.
At the very coldest part of the night, the PETE jar provided between 4 and 6 degrees F of protection, or about half the maximum protection observed for the glass jar.
The lesson here is that when I condemned all plastics for use in frost protection, I was too hasty. Polyethylene sheet is a terrible choice, from the standpoint of trapping long-wave infrared. But PETE’s OK. Not quite as good as glass, but pretty close.
Edit 11/10/2023: On re-reading this, I think it’s wrong. The estimate for the energy radiating into the opening of the mason jar seems right, but the analysis fails to account for the energy radiating out of the opening of the jar. The net radiant energy input is much smaller than what I calculate below.
So now, the excellent performance of the mason-jar cloche is a bit of a mystery.
Sure, it works in practice. But does it work in theory?
My prior gardening post demonstrated that a standard Ball jar (mason jar) provides excellent frost protection, if used in a garden bed with relatively warm soil below the surface.
An inverted mason jar gives six or seven degrees Fahrenheit of frost protection. I never would have guessed that.
I had to do this experiment twice, because I didn’t believe the results the first time. For two nights running, I left a pair of temperature loggers (digital recording thermometers) on a raised garden bed, one beneath an inverted wide-mouth quart Ball (mason) jar, one in the open.
In both cases, the goofy little mason jar provided 6 to 7 degrees of warmth. That wasn’t quite enough to prevent frost inside the jar this morning (the temperature dipped below 20F around dawn). But that is, nevertheless, an impressive amount of frost protection from a device that has, for all intents and purposes, almost zero value as either insulation or thermal mass. Continue reading G22-006: Mason jar as frost protection — a winner!
Bottom line: To protect a raised garden bed from frost, use a space blanket. Or, better yet, use a piece of construction radiant barrier. Same idea, but much tougher material. Alternatively, use glass jars to cover individual plants.
I tested these methods, using temperature data loggers, and you can get anything from 5 to 10F of frost protection from them. And all of them work the same way, by trapping the heat energy from the (relatively) warm underlying soil, that would otherwise be radiated away into the air.
By contrast, a lot of advice you’ll get on the internet — and a lot of frost-protection products offered for sale — don’t do much. In this post, I get around to testing (and flunking) the use of a sheet of polyethylene plastic as frost protection.
Free advice is worth what you pay for it. If you’re lucky.
The internet offers plenty of advice on protecting your vegetable plants from frost.
Some of that is useful, but much of it is wrong. Or, if you wish to be more charitable, much of it is almost, but not quite, completely ineffective. It’s folklore that somebody read somewhere, and passed it along without testing it to see how well it works.
Is anyone surprised?
That includes the commonly-cited advice to cover your tender plants with floating row cover, or to cover them with a frame draped with plastic sheeting (e.g., polyethylene sheet) when there is threat of frost.
Neither of which works worth a damn, by the way. Better than nothing, but not by much. Which I will show, below, using temperature data loggers to test that empirically.
In this post, I’m going to finish what I started last year, regarding the use of radiant barrier material for frost protection. Unlike a house (say), on a cold, clear, still night, a garden bed loses far more energy from radiation that from conduction. Once you’ve established a pocket of still air over your bed (and so, stopped losses via convection), you get far more bang-for-the-buck by preventing heat losses via radiation than you would by focusing solely on preventing losses to the atmosphere via conduction. Hence the use of radiant barrier material.
For those of us whose understanding of heat losses comes from insulating our homes, this may seem counterintuitive. Just keep re-reading the “Unlike a house” line above, until it sinks in. For a garden bed, the relative importance of energy loss via radiation and conduction is flip-flopped, compared to a house.
The practical upshot of that is that if all that stands between your plants and a frosty death is a sheet of plastic, consider tossing a space blanket over that. That thin layer of radiant barrier material will substantially add to the minimal frost protection offered by a plastic sheet alone.
To be clear, I didn’t think up that advice. I got the idea from a document that ultimately comes from the Colorado State University extension service (.pdf). That’s seriously useful advice from people in a seriously cold climate.
Edit: Separately, a mason jar (or any glass jar) also provides excellent frost protection for individual plants. For the same reason — it blocks long-wave infrared radiation. I tested this in Post #G22-006.
This weekend’s problem
Given how quickly the weather can turn hot in this area, I decided to get the earliest start possible on some cool-weather crops. Soil temperatures in my raised beds were at or near 50F a few weeks ago, plenty warm enough for some cold-weather crops. So I planted some peas, beets, turnips, and potatoes, even though we were more than a month away from our “last frost date”.
These have now produced nice green shoots. And, of course, now we’re going to get a frost. And yet, I’m not worried.
It’s not like I didn’t have plenty of warning. Two weeks back, the long-term weather forecast for my area gave a warning of likely frost this weekend. True to the forecast, the National Weather Service is now calling for successive nighttime lows of 27F and 26F in Northern Virginia, before returning to above-freezing temperatures.
I’m not worried, because I have my frost protection already worked out. Tested as to performance (Post G21-018). I should get adequate protection for this freeze event just by covering my raised beds with cheap, sturdy radiant barrier material. (It’s the same concept as a space blanket, but a lot tougher.) But if I’m feeling particularly paranoid, I might throw a few tens of watts worth of electric night lights under that cover, just for insurance, left over from a prior experiment (Post #1412, heated faucet cover). An idea which I also stole from the U. Colorado extension service document cited above.
In this the rest of this post, I’m going to recap the radiant barrier method of frost protection, and then push a bit beyond that. At some point, I’ll come up with a raised bed cover that provides both insulation (against loss of heat energy via conduction) and radiant barrier (against loss of heat energy by radiation). For now, though, I’m mostly going to walk through why radiant barrier is far better than either plastic sheeting or floating row cover, for providing frost protection to your sensitive plants.
Part 1: The tin-foil-hat gardener.
Above, you see one of my raised garden beds with pieces of woven polyethylene radiant barrier clipped to the top.
I went through the some of the science and math behind the use of a radiant barrier for garden beds in Post G21-015. Briefly:
While a traditional tin-foil hat serves to block alien mind control rays and other forms of incoming radiation, garden radiant barrier seeks to block outgoing radiation. In particular, it serves to prevent heat energy in the garden bed from radiating away into space. On a cold, clear night, those radiant heat losses would be large, and preventing them from occurring keeps the bed warmer than it would be, without the tin-foil-hat. So let me start this off by telling you all that you really need to know about radiant barrier.
Radiant barrier greatly reduces heat losses through radiation IF AND ONLY IF at least one clean side of the barrier faces an air gap of at least an inch.
(Or if it faces a vacuum, if you happen to be NASA. Hence the origin of the term “space blanket” for one commonly-available radiant barrier material.)
In particular, it works just as well if either side faces an air gap. Either the one facing the warm object, or the one facing the cold exterior. Which means that your intuition that this works “like a mirror for heat” is only partly right. That’s OK. Just follow the rule and it doesn’t matter if your intuition fails you. This is physics, and nothing changes just because you can’t quite get your mind around it. That’s just the way it is.
For you who need a more practical example, consider Reflectix water heater wrap, sold as “radiant barrier insulation”. The shiny outside serves as a barrier to the emission of infrared energy. Touch it and you’ll feel that it’s warm. And yet, it’s doing its job. It’s warm precisely because it’s difficult for heat energy to radiate away from that shiny, “low-emissivity” surface.
For those of you whose bent is more historical than scientific, look up pre-technology ice making in India and Iran. Under the right circumstances, upward radiative losses from a pan of water can result in the production of (some) ice even when the ambient air temperature is above freezing.
Note that the rule is very specific: This shiny material must face an air gap on at least one side.
Get it dirty, and performance degrades. Why? Because dirt is perfectly capable of radiating heat away. Transfer the heat from the aluminum to the dirt via conduction, and off it goes.
Sandwich it between two other layers and it does nothing. Why? The other materials don’t prevent radiation. They are perfectly capable of radiating heat. Conduct heat through this material, radiate it away using some other material, and the heat is gone.
Use it as a ground sheet for your sleeping bag, ditto. No air gap. Lay it on the ground and cover it with snow, likewise (I think it would be no better than having a simple plastic sheet under the snow.)
You get the drift. One clean side must be open to at least an inch of air, NASA excepted.
Choice of material
Now that I have that out of the way — one clean side must face some sort of air gap — let’s proceed.
What you see in use above is a tough woven polyethelene material, with an aluminized coating on both sides. It’s used in building construction. Here’s an example from Amazon, 62 feet of 4′ wide material, for $40 (reference). At 20 linear feet to cover a 4’x16′ bed, that works out to be about $13 to cover one 4’x16′ bed.
This stuff is very tough and reasonably cheap. Given that I use it just a few days a year, I am sure it will last decades.
There are plenty of easily-obtained alternatives for covering your beds with radiant barrier material. Really, all you need is a thin sheet of something that’s extremely shiny. Think “space blanket”.
But I think they all have significant cost or performance drawbacks relative to woven-polyethelene radiant barrier sold for building construction.
Space blankets work fine (I tested that), but they tear incredibly easily, particularly when clamped in place on a windy day. In the long run, they’ll cost more than woven poly radiant barrier. A typical thickness for space blankets is 12 microns, or about 0.5 mils (thousandths of an inch) thick.
You can buy large sheets of aluminized mylar, anything from “space blankets in bulk” to sturdier, thicker material. Here’s some that’s 2 mils thick, and costs somewhat more than half what the woven radiant barrier costs (reference). The thicker material is tough, but my experience is that it doesn’t like to make the sharp-radius curves required to clamp it to a flat surface. I suspect (but don’t know) that this wouldn’t last as woven radiant barrier.
Reflectix or equivalent should work well, but looks like it’s almost 10 times as expensive as woven poly radiant barrier. Reflectix is a tough bubble-wrap type material, shiny on both sides. Presumably that would give both radiant barrier and some modest insulation against conductive losses through the top of the bed.
Performance
Last year I did a series of experiments to show that this works. I had two cheap temperature loggers — digital recording thermometers. I left half the raised bed uncovered, and logged the overnight temperature of the uncovered and covered portions of the bed.
Here are the results:
A layer of floating row cover does nothing whatsoever. This was thin row cover, but this is also just about exactly what I expected.
A covering of literal space blanket (alone) raised the overnight temperature by about 5 degrees F:
Putting some gallon jugs to warm up during the day, and then covering with space blankets at night, raised the overnight temperatures by 10 degrees F. This had about one gallon per 8 square feet, and the gallons were warmed to about 70F by the end of the day. The results below were completely consistent with the thermal energy stored in the water, relative to the energy storage (and poor conductivity) of the underlying soil bed alone.
Doing the same, but using that heavy-duty woven polyethylene radiant barrier instead of flimsy space blankets raised the beds an estimated 12 degrees F overnight. (Estimated because the battery on the control temperature logger died overnight.)
And now, a new test. You’ll often get advice to protect your plants from freezing with some sort of frame (e.g., “hoop house”) covered by a piece of plastic sheeting. So let’s now test that.
(And, to be honest, maybe the good results above were purely from covering the bed with some type of plastic sheet. Maybe the radiant barrier property of that sheet is just a red herring. My earlier calculations say that can’t plausibly be true, but that still ought to be tested empirically.)
It’s not entirely implausible that a plastic sheet would provide some protection. A single layer of polyethylene sheet should add a roughly R 0.85 (U.S. units, not comparable to S.I. R-values). But that plastic sheet is essentially transparent to infrared, and so provides no radiant barrier whatsoever.
I addressed this question, in theory, in Post #G21-015. At that time, all things considered, I figured I would need something like 60 gallon jugs of 70 F water, to keep the garden bed 10 degrees F warmer than ambient, on a cool night, with plastic sheet alone.. Roughly speaking, a gallon jug every square foot.
Here’s a picture of the setup. I have one data logger under that tightly-tucked plastic sheet. Most of the sheet sits well away from the bed. And a second data logger in the uncovered portion of the bed.
And below you see the results. The side of the bed tightly covered in plastic sheet was less than 1 degree F warmer than the un-covered side.
Summary and further reflection in the performance of unheated plastic greenhouses.
The upshot of this is that at least two common bits of internet advice for cheap frost protection seem to be more-or-less worthless when actually put to controlled trial. Neither floating row cover nor a sealed air space covered with polyethylene plastic was able to achieve even 1 degree F of warming, on average, of the course of a night.
So, despite all the customer testimonials, I remain skeptical of the idea that you’ll be able to save your plants from a hard frost something like this:
Source: Amazon.
Or something like this:
Source: Amazon.
On the latter, real growers understand that. Go on YouTube, and you can find plenty of seemingly reputable gardeners who have measured the impact of their unheated greenhouses and will tell you that the nighttime air temperature in the greenhouse is essentially no different from that of the outside air. (e.g., just over four minutes into this episode of Gardener Scott):
If you have enough thermal mass inside the greenhouse, and the area of the walls is small enough relative to the enclosed volume (i.e., the greenhouse is large), and you have (say) a double-walled greenhouse with two layers of plastic and so much higher insulating value for the walls, then you might get several degrees of nighttime warming.
But a two-foot-tall hoop house covered by a single layer of plastic isn’t going to do much. At least, that’s what my experiment suggests.
There may be some anti-frost advantage to the hoop house, in that the higher daytime temperatures — when the solar energy does drive the interior temperature well above ambient — might heat the soil enough to provide additional energy release from that warmer soil at night. So maybe a tiny hoop house, in place day and night, might serve as enough of a solar energy storehouse that there is some material frost protection.
That said, I doubt it. My soil temperatures right now in that bed are around 50 F, but the air above the bed was essentially no different from the ambient air temperature. I can’t believe that another 10F or so in soil temperature would turn that situation around.
That said, I haven’t tested that, and probably never will. I don’t use cold frames or hoop houses because, in my experience, as an inattentive gardener, I always end up cooking my plants, one way or the other. All it takes is one unseasonably sunny and warm day, and a lack of attention, and the season’s growth can be baked to death in an afternoon.
Finally, one way in which a small single-wall hoop house can provide significant protection is in snow. If it gets covered in snow, the snow layer acts as insulation. This is no different from unprotected plants, who are more likely to survive cold temperatures if buried in snow than if exposed to the air above the snow.
I don’t deny that plastic-covered hoop houses have value. They speed growth by raising daytime temperatures well above those of the ambient air. Within reason, higher temperatures mean faster growth, all other things equal.
But if you’re going to spend $40 on a small plastic hoop house, spend an extra $2 for a space blanket to cover it in case of unexpected frost. The single wall of plastic, by itself, seems to be good for maybe 1 degree F of frost protection. Add a space blanket radiant barrier should boost that to at least 5 degrees F of protection.
Extras for experts: Wall of water and glass cloche
There are at least two other commonly-suggested ways to protect plants from freezing (other than literally heating the space the plants are in).
One of those — the Wall O’ Water (r), has physics that are completely obvious.
But the most time-honored of all – the glass cloche — is a little harder to figure out.
First, it’s no secret how a “wall of water” device can protect plants from freezing. These provide a large thermal mass in the form of 2″ wide plastic tubes filled with water, forming a deep, narrow chamber that protects the plant from excessive heat losses from convection. That mass heats during the day, radiates at night, and keeps the interior of the structure above freezing even in the face of a hard freeze.
Source: Wall-o-water.com. Accept no substitutes
Figuring out why a cloche works is a little harder. Glass choches have been in use for so long — and would have been so incredibly expensive in the past — that I have no doubt that they work to some degree. It’s just not clear why they work.
A traditional cloche is a heavy piece of glass, roughly in the shape of a bell. And yet, I’m pretty sure that unlike the Wall O’ Water (r), these don’t work by thermal mass. Near as I can tell, a mid-sized garden cloche only weighs about 6 pounds. Moreover, the specific heat of common window class is only about 0.2 (reference). As a result, a typical garden cloche would only retain as much heat as roughly 1.2 lbs (or a little over one pint) of water.
N.B. specific heat is the amount of heat energy required to raise a given weight of a substance by a given temperature. In the U.S., God help is, that’s expressed as British Thermal Units required to raise one pound of a substance by one degree F (whilst proceeding at a speed no greater than 10 knots per fortnight). Under that system, water always has specific heat of 1.0, as that is precisely how the BTU is defined.
By contrast, the specification for the genuine Wall O’ Water (r) say that it takes more than 20 pounds of water to fill it (reference).
So, what appears me to be a typical glass cloche has only about 5 percent of the thermal mass of a Wall O’ Water. Moreover, people claim that you can use a mason jar as a cloche — just place it atop your plants if a nighttime frost is predicted. Surely a thin-walled mason jar has next-to-no thermal mass. A thin-walled mason jar surely has too little thermal mass to matter.
Instead, I wonder if the effectiveness of the glass choche is due to the spectral properties of ordinary glass. Glass is transparent to near-infrared — what you’d perceive as the warmth from a glowing filament, say. But glass is opaque to medium and far infrared — the type of heat radiation that would dominate the spectrum of infrared given off by soil.
Among other things, this is why infrared cameras (e.g., a FLIR camera) do not have glass lenses (reference). In the temperature ranges where a FLIR camera might be used — such as thermal imaging of a house to look for heat losses — glass is more-or-less completely opaque. In fact, FLIR cameras cannot see through ordinary window glass for the same reason, and by report, window glass appears both opaque and reflective when viewed by a FLIR camera (reference).
And so, in the end, I suspect that my sheets of radiant barrier and a glass cloche work by the same principal — they both reflect the long-wave IR radiation that is given off by warm ground. By keeping this energy from escaping, and maintaining a still air pocket, they keep everything inside that air pocket materially warmer than the outside air.
I guess, as I put the tin-foil hat on my beds for tonight’s freeze, that’ll be another thing to test. Two temperature data loggers side-by-side, one under a mason jar, one not. I’ll report that out tomorrow.
Can you grow potatoes in your garden by planting grocery-store potatoes? If you search the internet, you’ll find every possible answer to this simple question.
The correct answer is: It depends. But not in some wish-washy, random, some-do, some-don’t, luck-of-the-draw kind of way.
It depends on whether or not those grocery-store potatoes were treated with the sprout inhibitor chlorpropham. That substance keeps potatoes from sprouting by permanently damaging their ability to grow. And it is commonly used in the U.S., for potatoes sold at the grocery store.
Key fact: Potatoes labeled as “organic” cannot legally be treated with chlorpropham.
So, if you want to buy potatoes from the grocery store and plant them, buy potatoes labeled as organic. In five years of doing that, I’ve never had that fail.
By contrast, planting non-organic potatoes from the grocery store may or may not work. It depends on whether or not they were sprayed with chlorpropham before being shipped off to the grocery store. And there’s no way to tell, just by looking at them.
Separately, there’s more to this issue, because, unlike “seed potatoes”, grocery-store potatoes are not certified as “virus free”. But I think that’s more of an issue for commercial growers than it is for the casual home gardener.
Details follow.
The damnable thing about reality is that it keeps changing. Just when you think you have some tiny part of it figured out, somebody upsets the apple cart potato bin.
I’m not talking about the pseudo-culture-wars around the rebranding of the Potatoheads.
And I’m not referring to the consequences of the Federal Web Designer Full Employment Act. This is the 1997 Federal statute requiring that every commercial and governmental website must make significant changes in layout and functionality, any time more than half of its users have figured out how to use it.
Nope, today’s rant is about spuds at Home Depot.Bags of certified seed potatoes, for spring planting, prominently displayed for sale. These were offered with several other items I’d never seen there before, including onion sets and, of all things, pomegranate seedlings. (Apparently, hardy to USDA Zone 7 as figs are — with the occasional die-back in a hard winter.)
I don’t recall ever having seen seed potatoes at my local Home Depot, and I’m not quite sure what that implies.
Why are those potatoes there, now?
I know why I grow potatoes. I tried it in 2020, and I liked the results. I grow a modest amount of potatoes because:
They’re easy to grow.
Deer won’t eat them (so far).
They taste better than store-bought.
You get a lot of edible calories per square foot of garden.
They store well.
I figured that almost nobody in my area (northern Virginia) grew potatoes in their back yard. For one thing, our heavy clay soil is far from ideal. If you decide to get around that issue by going the “no-dig” route, your home-grown potatoes will end up costing more than store-bought (see Post #1073).
And it’s not as if the potato is some beautiful addition to the garden landscape. It’s a scruffy green plant with nondescript flowers. Not only are you supposed to clip the flowers off, you have to spend weeks looking at the dying foliage before you can dig up the crop.
And so, as of last year, this tiny part of my world made sense. Potatoes are cheap to purchase, expensive to grow in our ill-suited soil, and not much to look at in any case. And I couldn’t find seed potatoes locally. It was all completely reasonable.
You don’t see snowmobile dealerships in Florida. You don’t see masses of seed potatoes for sale in a Home Depot in the Virginia suburbs of Washington DC.
But now, my local Home Depot is offering several different varieties of pre-chitted seed potatoes.
Those don’t appear anywhere on their website. The only way to know that my local store has them it is to walk into the store and — surprise — they have enough bags of seed potatoes for hundreds of local gardeners to be growing potatoes this year. What I’m saying is, it’s not like they decided to test the waters with a few packages tucked away on a shelf somewhere. They have cardboard bins full of 1-pound bags of seed potatoes.
And they aren’t even cutesy, exotic spuds, the kind you’d market to the upscale gardeners in this area. Home Depot is offering your basic red, white, and gold potatoes, just as you’d buy in the grocery store.
And so my world view is turned upside down. Is this some new fad that has somehow passed me by? Has there been some viral potato-based meme? Is Yukon Gold the new black? Has some post-pandemic survivalist instinct kicked in? Or has there always been some quiet, underground population of potato fans in this area, and has Home Depot has finally decided to crack the lucrative home-potato market?
Change is bad. Inexplicable change is worse. What’s the take-home message when Home Depot is offering the seed stock for what amounts to cheap survival food, front-and-center in a mid-aisle display?
Can you grow potatoes bought at the grocery store? Answer: Buy organic potatoes.
Please note, I’m not saying “should you”. I’m answering the more basic question “can you”. Internet advice is all over the map. Some people swear it doesn’t work at all. Some people swear it works fine.
The real answer is, it boils down to the type of sprout-inhibiting treatment that was applied to those potatoes. And, as far as I can tell, the treatment that permanently prevents the potato from growing is not legal for use in potatoes labeled as “organic”.
So, yeah, in theory, you can use organic grocery store potatoes, with a pretty good chance they’ll grow just fine. You still risk importing potato viruses into your garden if you grow grocery-store potatoes. But advice that you literally can’t grow potatoes from grocery-store potatoes does not apply to to the sprout-inhibiting treatments legal for use on organic potatoes.
Detail follows.
If you grow potatoes, in theory, you’re supposed to use “seed potatoes”. The first time I tried potatoes, that’s what I did. I bought some on-line from a seemingly reputable dealer. It was an expensive failure. None of my super-duper seed potatoes sprouted. Given the cost (and the cost of shipping), I’m sure I’d have bought them locally if I could have found them.
Ever since that first flop, I’ve gone against standard advice, and simply bought eating potatoes locally and planted them.
That’s risky because you might introduce some long-lived potato diseases into your garden soil that way. For this reason, potatoes sold as seed potatoes must be certified as being more-or-less virus free, and commercial potato producers in most (possibly all) areas are required to plant nothing but certified seed potatoes (or something close to that, or deemed equivalent to that).
There is some internet chatter suggesting that it’s illegal for a home gardener to plant store-bought potatoes in (e.g.) Idaho. But if you actually read the statute, that doesn’t appear to be the case. In Idaho, it’s illegal to sell potatoes for planting that are not certified as disease-free seed potatoes. (For what it’s worth, that’s also what the law of the Commonwealth of Virginia says. And, I’d wager, that’s what the laws in every state say (e.g., Florida).) But in addition, in it’s illegal for commercial growers in Idaho to plant anything that is more than one generation removed from certified seed potatoes. But nothing in Idaho statute (or at least, that part of the statute) addresses home gardens.
But if you’re just a casual potato grower, probably the bigger risk is that your grocery-store potatoes may not grow. By the time you get to late winter/early spring, it’s a good bet that almost all eating potatoes you can buy from almost any source have been treated with some type of sprout inhibitor or “anti-spudding agent”. (Because, if not, given how they have to be stored so that they remain pleasingly edible, they’ll have started to sprout by that time, if not treated.)
In the U.S., commercial, non-organic food potatoes are most commonly treated with chlorpropham (reference). This is basically a powerful herbicide, and there’s enough concern over toxicity that the EU and Great Britain banned its use starting in 2019 (reference). It works by permanently damaging the ability of potato cells to reproduce. Even contamination with low levels of it greatly impacts subsequent growth of the potato plant (reference). And I noted that the concentration of Chlorpropham used in that last study was well below the 30 PPM limit on residues on food potatoes for sale in the U.S (reference, U. Idaho extension service.)
If you read about someone trying and failing to grow potatoes or getting a terrible yield, using food potatoes, or you’ve seen advice not to use store-bought potatoes, you’re probably reading about the effects of chlorpropham. If that’s been applied, there’s no way to reverse the effects. It fundamentally and permanently alters the ability of that potato to grow. (And, see above, it even stunts any successive generations of crops grown from those potatoes, if you can get them to grow in the first place).
Now we come to the $100 question: Can chlorpropham (CIPC) legally be used on U.S. potatoes marketed as organic potatoes? This University of Idaho publication pretty clearly says no, it cannot:
Alternatives to CIPC are needed for both organic and export markets—where CIPC is not permitted.
(But if you really want to be sure, you have to consult the USDA National List. If it’s a synthetic compound, and it’s not on the list, it’s not allowable for use in produce labeled organic. And, to be clear, it’s not on the list. So, not allowable for use in production of potatoes labeled as organic.)
Organic methods of potato sprout inhibition have turned to using substances that are Generally Recognized as Safe (GRAS), or that are themselves natural compounds, or that are on the USDA National List. The sprouting of potatoes stored under normal conditions can be inhibited with ethylene gas, or by application of concentrate essential oils such as clove or mint oil, or by use of hydrogen peroxide (reference, reference). You can also keep sprouting down by judicious choice of potato variety, or choice of varieties that can be stored at colder temperatures without loss of flavor/conversion of starches to sugars.
Here’s the key point: Most organic methods of sprout suppression do not permanently damage the potato. They will kill off the sprouts that are active. Some temporarily inhibit new sprouting. But they don’t prevent further sprouting, once they are stopped. In fact, one of the main drawbacks of these organic methods is that they have to be re-applied if the potatoes remain in storage. Here’s the key paragraph, from the University of Idaho publication cited above, emphasis mine:
These alternative compounds are not true “sprout inhibitors” that inhibit sprouting by interfering with cell division or some other biological process. Volatile oils and hydrogen peroxide are more correctly called sprout suppressants, as they physically damage developing sprouts with a
high concentration of the product in the surrounding headspace in the potato storage. Because of high volatility, these compounds leave behind little or no residue. Since new sprouts continue to develop, repeat applications are required at two to three week intervals or on a continuous basis.
Source: Organic and Alternative Methods for Potato Sprout Control in Storage, by Mary Jo Frazier, Nora Olsen, and Gale Kleinkopf, University of Idaho, September 2004.
Courtesy of the same University of Idaho group, you can see what a potato treated with clove oil looks like. The clove oil kills off the existing baby sprouts, leaving little black dead sprouts all over the potato. But this will not prevent the potato from re-sprouting.
Source: Potato Sprout Suppression from Clove Oil, By Nora Olsen, Mary Jo Frazier and Gale Kleinkopf
Based on the many, many bags of grocery store potatoes I looked at this spring, that picture looks awfully familiar. I’m not sure whether you truly can tell whether a potato has or has not been treated with a commercial essential-oil sprout suppressant. But I’ve noticed a lot of these little black dead sprouts on the organic potatoes at the supermarket. And I’m guessing that what I’m seeing is potatoes that have been treated with these sprout suppressants, and not chlorpropham.
For my first-ever crop of potatoes, I got mine at my nearest farmers’ market, after my boxes of seed potatoes failed to grow. That worked great. Since then, I tend to cruise the bags of organic potatoes at by local grocery stores, looking for ones that are showing the very first signs of sprouting. My experience is that if you can see those tiny little white (not black) sprouts forming, they’ll grow OK. They are already in the process of recovering from whatever organic sprout suppression treatment was applied. And getting them at the grocery store is both cheap and convenient.
In any case, potatoes are a crop where you have to get started early. By the time I saw I could buy seed potatoes at Home Depot, it was too late for this year. You’re supposed to “chit” the potatoes before you plant them, that is, get them to break dormancy and start sprouting. That chitting process takes a month or two. If you want to plant your spuds on the traditional date of Saint Patrick’s day, you have to buy them and set them to chit sometime around the end of January. So I already had a couple of trays of sprouting potatoes in my kitchen by the time I saw the commercially-available product at the Home Depot.
Finally, I repeat, you can grow them this way. Whether the casual gardener should do that is an open question. This does not address the issue of potential spread of viruses harmful to the potato. I’m not sure how common those are in food potatoes, but for sure, they are common enough that every state that I’ve looked at has standards that seed potatoes must meet. Further, if you introduce a particularly harmful virus into your garden soil, my understanding is that it effectively ruins the soil for potato production for years.
And, when you get right down to it, it’s not like there’s a huge cost savings, once you get your local big-box stores selling seed potatoes. Home Depot has certified seed potatoes $4 a pound. If the only trusted alternative is organic potatoes at the supermarket, you’re probably going to pay $2 a pound anyway. Had I known that Home Depot was going to be marketing these this year, I’m not so sure I’d have been buying and chitting organic grocery store spuds.
But, for now, it is what it is. I had one complete flop using certified seed potatoes, and excellent success (so far) using farmers’ market and organic grocery store potatoes. For now, I continue to grow mine from food potatoes. I’m not sure that it’s smart, but so far, it works. And that may change, now that my local big box has them ready-to-plant, right off the shelf.
Anyway, if you go the grocery store route, pick up a few bags, to spread your risks. Leave them out to chit. I figure, if they start putting out sprouts sitting in my kitchen, they’ll probably do just fine when I plant them in the garden. If they don’t, eat them.
This post is about providing light for vegetable seedlings.
Years ago, I did that by hanging a fluorescent shop light just a few inches above the plants. That’s a pretty common way to do it, and any number of reputable internet soources will tell you to do that.
As I get back into the business of starting plants while the days are still cold, I decided to re-think that. And, as it turns out, the world has moved on. What was the epitome of cheap, efficient lighting 10 years ago (linear fluorescent tubes) is now an energy-wasting extravagance compared to the latest generation of LEDs.
But after working my way through watts, lumens, and lux, and looking at cheap LED retrofits for old fluorescent tubes, in the end, the best and cheapest solution was sunlight.
This post is about constructing a simple window-hung greenhouse from a clear tote, a pool noodle, and some clear packing tape. Ten bucks, ten minutes, and a suitable window, and you’ve got the perfect temperature-controlled spot for raising your seedlings. There, a couple of hours of sunlight provides your plants with more usable energy than an entire day spent under closely-spaced fluorescents.
Here’s my journey, from fluorescents to sunshine via LEDs.
The starting point is an elderly 4′ two-tube fluorescent shop light. I’ve had it so long that I’ve forgotten when I got it. It looks absolutely no different from any other shop light: white, metal, and poorly built. But it works as well now as it ever did. It uses two 40 watt T12 tubes, and with the losses from the fluorescent ballast, probably consumes about 85 watts.
(T12? Just when you think the U.S. system of units could not get any goofier, something will come along to prove you wrong. Fluorescent tubes are measured in eights-of-an-inch diameter. Hence, T12 is an old-fashioned fat fluorescent tube that’s 1.5 inches in diameter.)
The first thing to give me pause is the amount of electricity consumed. I’m supposed to run that about 16 hours a day, in order to provide adequate light to my plants. That adds up to 1.4 kilowatt-hours (KWH) of electricity per day. Just to grow about two square feet of seedlings. If I need those lights on for a month, that’s about 40 KWH per month.
It’s not the cost of that that irks me. That’s about $6, at the rates I pay. It’s that it seems like a ridiculously anti-environmental thing to do, as a byproduct of trying to have a greener garden.
To put that in perspective, I’m pretty sure that’s more than all the rest of the lighting in my house consumes. And that’s enough electricity to drive my wife’s plug-in Prius about 200 miles.
All that, just so I can have my tomatoes a few weeks earlier. Seems kind of self-indulgent. Surely I can do better,
Next stop was an LED retrofit for those T12 fluorescent lamps. And here’s where I got my first eye-opener. A decade ago, there wasn’t a whole lot of difference in efficiency between linear fluorescent lighs and LED lights. But now, the off-the-shelf LED replacement “light bulbs” produce just about twice as much light as fluorescent bulbs, per unit of power consumed. About 120 lumens per watt, compared to maybe 65 lumens per watt for standard T12 fluorescent bulbs.
So I bought a couple of plug-n-play LED retrofit “tubes”. Some of those require you to rewire the light fixture, but others are straight-up bulb replacments. These are the exact size of a T12 bi-pin fluorescent, and, in theory, if you’re lucky, you can just literally swap the fluorescent tubes with the LED “tubes” and you’re done. It’s a bit wasteful, in that your’re still heating up that old (and now useless) fluorescent ballast (the gizmo that lights the fluorescent lamps). But it’s sure easy, and it’s sure cheap. A replacement for a 4′ T12 cost $8.50 at Home Depot — less than buying new T12 tubes individually.
Unfortunately, that easy upgrade didn’t work for my elderly fluorescent fixture. I’m not sure what gave that away faster — the erraticly flickering lights or the strong odor of burning plastic. If light bulbs could scream, these would have. (Although, oddly enough, I tried them later in a more modern fixture and they still work. Just not in the light fixture I have available for growing plants).
Time for yet another re-think. I could switch to the type of retrofit LED bulbs that require re-wiring the fixture. (So-called “ballast bypass” bulbs.) That’s not hard to do. I could just chuck the old light and buy a new one,. But instead, I got out a light meter and started assessing the situation.
Lumens and kelvins and lux, oh my.
Lumens measure the rate at which light energy is being produced. A typical “60 watt equivalent” compact fluorescent light produces about 600 lumens. A four-foot fluorescent tube produces about 2600 lumens.
Kelvin, in this case, provides a measure of the color of light that a light bulb produces. For archaic reasons, it’s called the “color temperature”. Just as you would see heating a piece of metal in a flame, a low color temperature means a reddish light, higher temperature means a blue-ish light. My understanding is that vegetable seedlings are happier with a higher-color-temperature bulb.
Finally, lux measures how brightly a surface is illumnated. One lux is one lumen of light energy flow, spread over a one-square-meter surface. Moonlight illuminates the ground at about 0.2 lux, direct sun at noon on a clear day is about 100,000 lux.
And a light meter, held 3″ from those T12 fluorescent tubes, registers about 6000 lux.
That’s pitifully dim, compared to sunlight. Direct sunlight slanting in my windows, in the afternoon, at this time of year, will easily illuminate the floor at a level of 20,000 lux.
That explains why you have to leave the lights on 16 hours a day if you want to grow seedlings. You need all that time so that the dim light of the fluorescent bulb can deliver something approaching the energy in a few hours of sunlight.
Greenhouses, cold frames, lazy gardeners, and dead plants.
There’s one obvious solution that doesn’t require any artificial light at all: Use a greenhouse or a coldframe. But in this climate, at this time of the year, that either requires heating (and so uses energy), or it requires a fairly attentive gardener.
Leave your delicate plants out on a particularly cold night and you can freeze them, cold frame or not. Leave them out on a particulary sunny day, and in a warm spell you’ll roast the plants to death.
I’ve learned over the years that coldframes just don’t work for me. One way or the other, I’ll forget to move the plants inside, or forget to prop the lid open, and the next thing you know, my seedlings are dead.
What I really need is a heated and air-conditioned greenhouse. That would be foolproof. But that would take a lot of energy to run, not to mention the cost of constructing it in the first place.
Or would it?
Hang a greenhouse out your open window.
I find that sunlight slanting in my windows is not quite adequate for growing my seedlings. For one thing, I have little south-facing window area. For another, they all spend a lot of energy leaning toward the light. And my windowsills are narrow, to boot.
But some people solve this problem by building a little exterior greenhouse, and attaching it to the interior of the house. In particular, they connect it to the heated and air-conditioned air inside the house. That way you get abundant sunlight for the plants, but the greenhouse can’t freeze or overhead, regardless of the outside weather. And it’s cheap: All the conditioning of that greenhouse air is done by tacking a tiny little additional load onto the house’s existing HVAC system.
After looking at commercially-built units (too expensive), and at plans for constructing such units out of wood (too complex), I realized that I already owned the primary component of the perfect exterior through-the-window greenhouse. It was a 56 quart clear tote.
As with my crude solar tomato dryer, if you need a clear box, it’s kind of nuts to spend a lot of time and money building one. It’s a lot cheaper and easier just to buy one. Sterilite, in particular, makes a huge range of clear plastic totes.
And so, open the window, wedge the tote firmly in place with the opening facing the inside of the house, and use a bit of tape and some cut-up pool noodles to fill the gaps and firm up the installation.
Here’s the result, which took me all of maybe 10 minutes to put together:
To orient yourself, you’re looking at a standard double-hung window. The bottom sash has been pulled all the way up, and a couple of clear plastic totes fill that gap, bottoms facing out. The pinkish things are cut-up pieces of pool noodle.
And the punchline? That’s the light meter reading showing at the top of this post. At 9:30 AM, on a clear spring day, the illumination level in that clear tot is already over 30,000 lux. That, plus the fact that the spectrum of sunlight is just what plants need to see, means that three hours in the greenhouse provides far more usable light energy to my seedlings than 16 hours of sitting under those fluorescent tubes.
And, unlike an exterior cold frame or greenhouse, the air temperature in this window-attached unit will stay at or near room temperature.
In the end, my failure to retrofit my ancient fluorescent shop light ended up generating a far better solution for growing my seedlings., And, yeah, it looks a bit redneck. But it only has to be there for a month or so. For 11 months of the year, that will just go back to being a normal window. And the totes will go back to being totes.
The only thing missing in the ten-minute version of this is insulation. And the obvious solution there is bubble wrap. So I’ll add that to the interior as soon as I can lay my hands on a big-enough piece of it. That ought to give roughly an R-2 level of insulation, not hugely different from the window itself.
Above: Used Ball lids. The one on the left clearly shows the groove left by the canning jar. The one on the right was boiled for 20 minutes, which clearly flattened that groove considerably. I picked up this tip boiling lids if you plan to re-use them from the blog A Traditional Life.
Bottom line: Ball lids appear to be widely in-stock at Walmart once again. And Ace Hardware is stocking a new brand of lids, Pur.
I bought a pack of Pur lids, thinking they had to be American-made, based on the lack of country-of-origin information on the packaging.
But after looking into it, my best guess is that I just bought some steeply marked-up Chinese-made canning lids, in packaging that managed to hide the fact that they were made in China.
I’m unhappy about that. If there’s anything worse than getting fooled, it’s getting fooled by somebody you trust. I think I can find somewhere else to buy canning supplies from now on. Continue reading Post G22-002: A Pur choice of canning lids.
To orient yourself, you’re looking at a standard double-hung window. The bottom sash has been pulled all the way up, and a couple of clear plastic totes fill that gap, bottoms facing out. The pinkish things are cut-up pieces of pool noodle.
