Posts about gardening and related topics, such as canning and maybe other aspects of food preservation.
Edit: See Post G22-015. Skip the drying racks, just place the food directly on the floor of the tote. Replace the ventilation “chimney” with a computer fan. With those changes, two days in the sun produced perfectly dry potato slices.
Last fall I came up with what I hoped would be a cheap and simple solar food dryer capable of drying tomatoes in the humid climate of Virginia. Continue reading Post G22-014: Plastic tote food dehydrator, version 2: Construction.
A parthenocarpic plant is one that produces fruit without fertilization, that is, without pollination. The resulting fruits are sterile and lack fully-developed seeds.
Without getting into the deeper theological aspects, the word derives from the Greek “parthenos”, meaning virgin. And “carp”, meaning to complain. Thus, the Parthenon is a temple to Athena, who was virgin who had few complaints.
(Technically, carp means seed. So parthenocarp means “virgin seed”. I like my version better.)
Of course, now that you know the word, examples crop up everywhere. The banana is almost surely the most familiar example of a parthenocarpic fruit. If you’ve ever wondered why bananas are seedless, now you know. It’s due to their parthenocarpic nature.
Every parthenocarpic fruit is more-or-less seedless, but not every seedless fruit is parthenocarpic. Some still require fertilization, they just don’t (or rarely) produce fully mature seeds. Seedless watermelons fall into that category. Unlike true parthenocarpic plants, seedless watermelons must be pollinated to bear fruit. The term of art there is “stenospermocarpic”, which seems to be Greek for narrow fertilized seeds.
This is also not to be confused with plants that require pollination, but not pollinators. Those include plants that are “wind pollinated” (like most cereal grains), and plants that may be “self-pollinating” due to perfect flowers containing both male and female parts, so that simply shaking the flower may sometimes pollinate it. (This is the source of the electric toothbrush hack for ensuring good tomato pollination.)
Greenhouse and poly-tunnel farmers provide the commercial demand for parthenocarpic varieties of common garden plants such as cucumbers and squash. In those enclosed environments, without bees, those crops would otherwise have to be pollinated by hand. That’s an obviously labor-intensive step, and may be a practical impossibility for crops grown under low “hoop house” type row covers.
Several different varieties of parthenocarpic cucumbers and squash are available to the U.S. home gardener. I’ve been compiling a list, but I’ve limited it to the small subset of fruits that appear more-or-less identical to their seeded, pollination-requiring cousins. The subset of interest to me includes:
Cucumbers: H-19 Little Leaf, Corintino, Dive, Excelsior, Piccolino, Quirk
Squash: Venus, Part(h)enon, Burpee’s Sure Thing, Defender, Duntoo, Dunja, Cavili, Golden Glory.
(Parthenon or Partenon, sure. But Venus? Singularly inappropriate.)
As far as I can tell, these are exclusively F1 (first-generation) hybrids. (Because, seedless, right?) So if you will only grown heirloom plants, or those from which seeds can be saved, this is not for you.
To determine which varieties to grow I will apply the Tomato Paralysis cure from Post G22-001. List in hand, I’ll cruise the seed racks at my local garden center and grow whichever of those they carry locally.
I ended up here because I had such a dismal time trying to grow cucumbers and summer squash for the last couple of years.
The squash vine borer is present in this area (Virginia Zone 7) for a couple of months. That is, more-or-less for the entire squash growing season. If you restrict yourself to relatively short-lived pesticides (I used spinosad), controlling it requires careful spraying at five-to-seven day intervals. See Post #G27, A Treatise on the Squash Vine Borer.
The cucumber beetle was essentially absent from my first year of gardening, and I had a bounteous crop of cukes. But by my second year I had built up an unstoppable population of them, and got almost no cucumbers whatsoever. I never found a way to control the cucumber beetle that a) worked and b) was acceptable to me, in terms of environmental impact.
The damned things are like vampires: All it takes is one bite. Cucumber beetles spread bacterial wilt. So it’s not the actual leaf and blossom damage from their feeding that matters. It’s that any feeding at all infects the plant and kills it. As far as I can tell, a) once bacterial wilt starts, it’s just a short while until the entire cucumber plant is dead, and b) “wilt-resistant” cucumber varieties aren’t, they end up just as dead as non-resistant varieties.
But if I don’t need pollinators, I can grow summer squash and cucumbers under insect netting/row cover. In theory, if I can sterilize the soil under the plants (with a neem oil soil drench, perhaps), and keep a bug-proof enclosure over the plants, I can physically prevent those pests from reaching the plants. And yet have a crop, because barring the bees entry does these plants no harm.
I’ve been hesitant to try this. Not just because it seems like a lot of work to set up, and a lot of hassle to maintain. But because of the “vampire” nature of cucumber beetles. It’s not their feeding that matters directly, it’s the disease they carry. If a single beetle breaches the defensive perimeter, it’s game over for the cucumbers. Do I really think I can (e.g.) lift the cover off to pick the ripe fruit and set it back again without letting in a single cucumber beetle?
It seemed to be a fairly non-robust setup. I understand that insect netting can greatly reduce insect damage. But because of the nature of the beast — bacterial wilt — I really need to eliminate it entirely. If the endpoint is going to be a bed of deceased cucumber plants, I know ways to achieve that with a lot less effort.
But if the bees and butterflies can’t get in … then nothing bars me from making that enclosed garden bed an arena of death. All of those highly-effective (i.e., deadly) pesticides that I normally won’t touch due to bee toxicity are now back on the table. Subject to some constraints, nothing need stop me from hosing the bed down with (e.g.) pyrethrins on a regular basis (subject to controlling runoff). This means I can install a secondary, chemical line of defense beneath the primary (physical) barrier. If need be.
I’m looking for parthenocarpic, not carcinogenic. So it’s not like any pesticide is fair game. But cheap, short-lived and effective organics like pyrethrins would seem to be plausible. Once I screen in the bed, I no longer have to worry about killing off my local bees and butterflies. More-or-less any bug that gets through the outer defenses is fair game.
Anything worth doing is worth over-doing. Given how much hassle it’s likely to be to do this at all, I think I’ll go for more, rather than less.
My plan is to dedicate one entire raised bed to parthenocarpic cucumbers and squash. Roughly 4′ x 16′ or so.
Plausibly the major expense will be for the requisite statue of Athena, so that I may dedicate my parthenocarpic garden appropriately. And some large-economy-sized Bucket-o’-Death, to ensure that any bug making it past the cover will die ASAP.
Otherwise, for me, this requires no investment in materials. I already own a more-than-lifetime supply of thin floating row cover. As well as a pile of loose PVC pipe and fittings, which is to adults what Tinker-Toys are to kids.
A year ago, I didn’t even know that such a thing as parthenocarpic squash existed. This year, I’m going to grow a bed of it. I’ll let you know how it turns out.
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 flattened that groove considerably. I picked up this tip boiling lids if you plan to re-use them from the blog A Traditional Life.
One of the many U.S. shortages that occurred during the COVID-19 pandemic was a shortage of lids for use in home canning. I’ve posted extensively on that here. Continue reading Post G22-011: Canning lids, from shortage to wide-mouth surcharge.
Source: Wayfair
The vacuum sealer is that rare device that serves as both a kitchen appliance and a source of entertainment. Every time I run my new Nesco VS-09, I practically want to applaud when it finishes.
I don’t normally give much thought to air. Until it’s all gone. Then the arithmetic of 15 pounds per square inch leads to the realization that this goofy little countertop appliance generates a literal half-ton of crushing force on a 6″ x 10″ pint-sized bag.
But I digress. I actually bought this for the serious purpose of preserving garden produce. The fact that I find the process and results to be so entertaining is just icing on the (perfectly flat half-inch thick piece of) cake.
In any event, there is a serious purpose to this post. And that is to show that if you have a freezer that’s already running, then freezing your tomatoes is by far the most energy-efficient way to preserve them. The only method that would beat that is solar drying, and I haven’t quite figured out how to do that well in my humid Virginia climate.
The last thing I need is another kitchen appliance.
But I bought this vacuum sealer anyway, after thinking through all the food preservation I did last year. Of all the pickling, canning, drying, and freezing, by far, the tastiest, most garden-fresh results came from freezing. With drying (dried tomatoes) a close second, due to the intense flavors that produces.
And so, purely from a quality standpoint, for tomatoes to be used in soups and stews, my wife and I agree that freezing is the best option. It preserves that fresh tomato taste. But how does it stack up in terms of energy use?
Freezing gets a bad rap, as a means of home food preservation, for its relatively high energy use. But I think that’s not entirely correct.
If you run a freezer expressly for the purpose of preserving garden produce, then, sure, I’d bet that freezing has a fairly high energy cost. In that case, you’d have to pro-rate the annual electricity use of that freezer over the pounds of produce preserved. (Because, by assumption, you wouldn’t be running that freezer if you weren’t using it to preserve your garden produce.)
Just tossing out some round numbers, based on past experience, I’d bet that a typical 15-cubic-foot chest freezer has enough space to store 300 pounds of produce, and consumes about 300 kilowatt-hours (KWH) of electricity per year.
So, roughly speaking, if you run that freezer because you use it to preserve your produce, you’d consume about 1 KWH of energy for every pound of produce preserved.
By contrast, if you are already running a freezer, and will continue to run it regardless, and you have the space, then freezing your produce only costs you the energy needed to freeze it in the first place. The cost of running the filled freezer doesn’t count, because you’d bear that cost in any case.
My fridge comes with a big freezer. It’s not like I’m planning to unplug that any time soon. And so, I’m perfectly happy to let my frozen garden produce be a free rider here — taking advantage of the fact that the freezer is running, but not being asked to “pay” for it.
In that case, the only additional energy cost is the cost of getting the room-temperature produce down to the 0 F temperature of the freezer. Given that (e.g.) tomatoes are 94% water, that’s more or less the energy required to bring one pound of room temperature water down to 0 F. Including the one BTU per pound required to cool the water, and the 144 BTUs per pound required to convert to ice, that works out to (70 + 144 =) 214 BTUs, or (at 3.4 BTUs per watt-hour) 63 watt-hours. So, if you are just tossing your produce into a freezer that is going to be running in any case, freezing it takes 0.063 KWH for every pound of produce preserved.
You might think that’s a bit of a cheat, because one way or the other, you’ll want to peel those tomatoes before you use them. The most typical methods for peeling them involve heat (either boiling water, or holding them in the flame of a gas stove). But — surprise — it’s actually a snap to peel them after they’ve been frozen, per this YouTube video.
Take a look around 47 seconds into that video. My jaw dropped just after the tomato did. I know the term life-changing is overused, so let’s just say this was a tomato-life changing revelation for me. As in, I’m never going blanch and peel a tomato ever again. Arguably, it may actually take less energy to freeze-and-peel than to blanch-and-peel, what with the energy costs required to boil the water and cool the tomato afterwards.
I have already tracked the energy costs of preserving by canning or drying, in various earlier posts. Let me bring all of that together in one place, below.
Drying tomatoes in my four-tray Nesco dehydrator consumed 8 KWH of electricity (per Post G21-049). That was in the humid outdoor Virginia summer. I am fairly sure that each tray can hold less than a pound of quarter-inch-thick tomato slices,, but a) I could stack up to 12 trays at a time for drying, and b) those were very “wet” slicing tomatoes, not the paste tomatoes that are normally used for drying. That said, for illustration, let me just assume one pound per tray, four trays, yield 2 KWH for every pound of produce preserved.
Canning tomatoes in a water-bath canner consumes a considerable amount of energy as well. I did the full workup on the energy cost of home canning two years ago, in Post #G22. I had to do that because, as far as I can see, the rigorous research literature on this crucial topic looks like this:
In any case, the all-in energy cost for canning five quarts of pickles, on a gas stove, in an air-conditioned house, was 5528 kilocalories (kcal).
Source: Post #G22.
Per the USDA guide to home canning, quarts of pickles require a much shorter processing (boiling) ,time (15 minutes) compared to quarts of tomatoes (45 minutes) in a water-bath canner.
Based on my prior calculation (shown above), I need to add another 800 Kcal to account for that, bringing the total up to 5300 Kcal for 5 quarts (= 10 pounds) of tomatoes. At 1.16 watt-hours per kilocalorie, that works out to be 0.6 KWH for every pound of produce preserved.
I should note that this is a little conservative, because you have to peel the tomatoes first. That’s going to involve a little additional boiling time. But with all the boiling that’s taking place with the canning, I figured that was more-or-less rounding error.
Finally, I can take a rough guess at the energy cost of my crock-pot spaghetti sauce. Crock-pot spaghetti sauce (Post #G21-048) absolutely minimizes the labor input, and is idiot-proof to boot. But it requires processing tomatoes in both a pressure cooker (briefly) and a crock-pot (overnight). For four quarts (eight pounds), the crock-pot portion uses about 4 KWH. But the pressure-cooker portion (20 minutes at pressure) likely used almost as much energy as canning, so for four quarts I need to add one-third of my pickle canning estimate above, which, by the time all the arithmetical dust has settled, adds another 2 KWH. Or a total of 6 KWH for 8 pounds of tomatoes, or 0.75 KWH for every pound of produce preserved.
Edit, fall 2024: In hindsight, that’s much more energy-intensive than a more traditional reduce-it-on-the-stove approach to making tomato sauce. A crock pot is, in fact, a terrible (but idiot-proof) choice if you want to evaporate water out of a sauce. I’ve gone back to making my spaghetti sauce by boiling down tomatoes on the stove, like a normal person. I still briefly pressure-cook, dump in a strainer to remove the liquid, pass the solids though a Foley mill to remove the skins, then reduce. This allows me to use all types of tomatoes, including salad and cherry tomatoes, without having to peel or seed them first.
There’s no additional energy cost for peeling in this method, because the entire batch of tomatoes is run through a Foley mill after pressure-cooking. That takes out the peels and (most of) the seeds.
Let me now produce the nice neat table of energy required for food preservation, all of it expressed in terms of KWH of energy per pound of produce preserved.
All of that comes with some caveats. The canning was done on a gas stove in an air-conditioned house. The drying was done outside, in humid air. I could dry up to twelve trays at once, instead of the four that I already owned. Maybe there’s a little more energy required for the blanch-and-peel step in some methods. And so on.
Nevertheless, the results are so clear as to be undeniable. (So clear that I double-checked that freezer math a couple of times). If you have space in your freezer, and you’re going to run that freezer anyway, by far the most energy-efficient way to preserve tomatoes is to toss them in the freezer. And, per that YouTube video above, peel them as you thaw and use them.
I surely need to mention the one common method that isn’t on the list, solar (or open-air) drying. Plausibly that has zero energy cost, but I have not (yet) figured out how to do that in my humid Virginia climate. I’m already working on how I’m going to improve my simple $18 plastic-tote food dryer (Post #G21-049). The solution might be as easy as “don’t overload it”.
Two minor factors make this conclusion somewhat less that complete. Those are coefficient of performance (COP) of a freezer, and the different rate of greenhouse gas (GHG) emissions for natural gas and electricity used in the home. Near as I can tell, neither of these results in any material change in the relative efficiency of the various preservation methods.
First, this calculation isn’t complete because it doesn’t factor in the energy conversion efficiency or coefficient of performance (COP) of refrigerators or freezers. The coefficient of performance for a heat pump is the amount of heat energy it can move, for a given amount of electricity supplied to it. Almost all commercially-used heat pumps have a COP greater than 1.0. That is, they can move more than 1 KWH of heat energy for every KWH of electricity they consume. COPs for modern AC or heat pump units typically run around 2.5 to 3.5 (per the link above).
The estimate above — 0.063 KWH — is the amount of heat that needs to be (re)moved from the interior of the freezer. It will actually take less than 0.063 KWH to do that, because fridges and freezers are just another form of heat pump with a COP greater than one. While Wikipedia (cited above) assures me that they have a COP greater than 1.0, I have yet to find a source that will pin that down further. The best I’ve found is a passing reference to a COP of around 1.0 for a deep freeze unit (per this reference).
The bottom line is that a typical home freezer might use somewhat less than 0.063 KWH to remove 0.063 KWH of heat energy from its interior. But how much less, I can’t find the source that will let me pin that down. I suspect that, given the large temperature differential between interior and exterior, the COP of most freezers isn’t much higher than 1.0 or so.
Finally, KWH is not the same as GHG (greenhouse gases). This only measures energy consumed within the home, and does not differentiate between natural gas and electricity. Fossil-fuel based electrical generation is far from 100% efficient, so the actual amount of fuel consumed (to generate the electricity) is a low multiple of the energy actually delivered to the house. But in addition, electrical generation consists of a mix of generation sources, some of which create greenhouse gases, some of which do not. If the ultimate question is one of carbon footprint, we’d have to modify this calculation, treat electricity and natural gas separately, and then redo it for some assumed electrical generation mix.
That said, when I take a rough cut at the difference between natural gas (burned in a stove) and electricity (produced with a typical U.S. generating mix), I’m not sure that adjusting for each fuel type separately would make much difference.
Natural gas releases 100% of its energy within the home. But a typical natural gas stove is only about 40% efficient. That’s the energy that goes into whatever you are trying to cook, with the rest simply serving to heat up the kitchen. Basically, for every 100 units of C02 produced, you get 40 units of usable energy from your gas stove (Whatever units might mean, in this case).
For electricity, by contrast, the amount of fuel burned at the generating plant is far more than the amount that makes it into your home. But once it gets to your home, I’ve either directly measured 100% of what was consumed, or the theoretical calculation (for freezing) should be close to that. And so, as with natural gas, for every 100 units of C02 produced in generating electricity, you get X units of usable energy in the home.
The problem is that X depends on the generating mix that feeds your particular section of the grid. Even so, let me do the arithmetic for Virginia’s electrical grid.
Last time I checked, Virginia’s electrical grid released 0.7 pounds of CO2 per KWH of electricity delivered. Starting from that, I’m going to compare C02/KWH of usable energy for the Virginia grid versus a 40 percent efficient gas stove.
The EPA shows that burning a therm of natural gas releases an average of 0.0053 metric tons of C02. At 2204 pounds per metric ton, that’s 11.7 pounds of C02 per therm. A therm is 100,000 BTUs, and there are 3.4 BTUs per watt-hour.
Slapping that all together, burning a therm of natural gas produces 11.7 pounds of C02 and 29.4 KWH of (heat) energy, or 0.4 lbs C02 per KWH.
But a natural gas stove is only 40% efficient. A stove has to use (1/.40 =) 2.5x as much natural gas to deliver that usable KWH of heat. The bottom line is that a 40 percent efficient natural gas stove releases 1.0 pounds C02 for every usable KWH of heat delivered in the home.
And so, per KWH of usable energy, in terms of GHG emissions, electricity (in Virginia, at 0.7 lbs C02 per usable KWH) is slightly cleaner than natural gas burned in a (typical) 40 percent efficient stove. But only slightly. So the electrical options actually perform a little bit better than shown in the table above, relative to the gas-stove-intensive canning.
There’s nothing in any of that to change the conclusion that tossing your tomatoes into a freezer that would be running in any case is by far the most energy-efficient way to preserve them.
All of the above brings me back to my new toy, the vacuum sealer. If I’m going to freeze my tomatoes, the binding constraint is now the space they take up in the freezer, and secondarily, the length of time they’ll last once frozen. Both of which will be best addressed by vacuum-sealing them.
Most sources suggest that you freeze the tomatoes before vacuum-sealing. But at least one source shows tomato chunks that were vacuum-sealed and then frozen. That’s what I’m now aiming to do, only using whole tomatoes, not chunks. Given the literal tons of force that one of these sealers can generate, I’ll have to use the setting that allows the strength of the vacuum to be controlled manually. In the end, I’m aiming for a freezer stocked with nice, flat, well-preserved packages of energy-efficient frozen tomatoes.
With any luck, we’ll see how that all plays out in a few months.
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.
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.
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:
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.
Source: Figure 9, “Identification of black microplastics using long-wavelength infrared hyperspectral imaging with imaging-type two-dimensional Fourier spectroscopy“, Kosuke Nogo, Kou Ikejima, Wei Qi, et al., DOI: 10.1039/D0AY01738H (Paper) Anal. Methods, 2021, 13, 647-659
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.
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.
Now, let’s use a physics law, an insulating R-value, and a bit of math, and show that this works in theory. Continue reading Post G22-007: The math of the mason jar cloche (corrected!).
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.
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.
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.
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.
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.
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.
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.
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.
