The Art of the Minimum Hotbed

[candide]

Note: was originally titled "The Theory of the Minimum Hotbed," but I came to see that was incorrect.

My little slice of the sun belt rarely gets this cold. (I think we got down to negative 1 American; therefore -18 science).

I have a super-small bed that is mostly Brassicas that I wanted to keep alive. It is approximately two-by-one feet -- thus also American; I'm not dong the conversion to metres [sic]... Anyway, there is some clear poly-sheeting enclosing the space, and I boiled water and filled up two half gallon jugs and placed them in this mini greenhouse wrapped in cloth and between plants. I switched these out twice a day (at least on the coldest of this spell). The water never completely froze, though one time it got close and the plants looked beat to hell -- but have bounced back.

So, was that worth doing? Do you think the plants would have survived without me doing this? ... Do you agree with the assumptions chatGPT is making about the whole thing?

https://chat.openai.com/share/993eb730- ... ee36377af5

ChatGPT calculates that I gained 6 hours of being above 25 F by doing it, but the model assumes "Negligible heat transfer through plastic," but on the other hand doesn't have any inputs from the light and heat through the day, ie the "green house effect." ... So in other words, I probably could have just rolled a dice to come up with that number. But at least I tried to use my non-human resources first.

I am more filled with curiosity to learn than to defend the practice, which I am not sure I will be doing in the future, after this cold spell more or less ends -- which is predicted to be Wednesday (at least up to temperatures that I will just leave the unit unheated).

[theanimal]

I'm no engineer, but I see what could be a couple of major flaws with the assumptions.

Regarding number 1- it doesn't make sense to me why the air temperature would be the same as the boiling water. Presumably, the ambient air temperature is slightly higher than the outside temperature and rises after being heated by the pots. But I don't see how the air temperature reaches 212F/100C.

Number 5- assuming you're using greenhouse plastic with thickness of 6mm-10mm, is the heat loss really neglible at -1F outside air temp? Even at night?

[candide]

Yeah... it doesn't even work as a static model. And really, what I want is even a more dynamic model... but I have no background in any of this.

I'm sticking my neck out here in a willingness to look stupid, as a price to do some learning.

Also happy to have this pivot into a thread about hotbeds in general, such as how people heat their plants, etc. Is it worth the effort, and the like.

ETA: Also want to point out that an assumption I was making is that time under 25 degrees F as a key to killing Brassicas -- let's be more specific: mustard greens and arugula. I have some red lettuce in there that I think has bit it, and a ring green onion that I also learn toward "will make it."

[7Wannabe5]

When I've seen barrels of water used to retain heat in a greenhouse in winter, straw bales were also utilized. Maybe packing the whole set-up like a haybox would help? Few days without sun won't kill them.

[candide]

Hmmm...

Taking the responses of both @jacob and @7w5 it seems that people are in a position not even of rules of thumb, but trial and error.

It makes sense to me that barrels of water could add thermal mass and thus retain heat, but how would one know how many much water is enough?

Also, good call with the idea of just heavily insulating them. I'm sure Ruth Stout would have just plopped a whole hay bale on top, and then rolled it off when the cold snap was over.

[loutfard]

Looking at historical practice can also give some hints. https://solar.lowtechmagazine.com/2015/ ... greenhouse is fairly interesting in that regard.

Most of lowtechmagazine is recommended reading anyway. Quite a few ideas and just practical things that fit the ERE philosophy rather well. The hay box is a good example.

[AxelHeyst]

As someone who at one point professionally ran sophisticated energy analysis software to predict the thermal performance of buildings at 15min increments over multiple climate years... yeah, trial and error is the way to go there. It only makes sense to spend time on analysis if the construction of the thing is going to take a lot of money and resources and it'd be difficult to fix it in post. At the scale of a bit of plastic and maybe some bales of hay, just wing it.

It makes sense to me that barrels of water could add thermal mass and thus retain heat, but how would one know how many much water is enough?

Guess n check. Get some cheapo temperature thingies (ten bucks for a six pack on the site that shall not be named), put some barrels in your hotbed, and write down the inside and outside temp every hour overnight. Add more barrels or take away and redo. (you'll actually be able to calculate the overall Uvalue of your assembly with this data, by the way.. fun exercise.)

At the scale of a full greenhouse, I think it makes sense to do at least some back of the envelope calcs before building. But, for example, I'm going to do for my studio what you're doing for your brassicas, and I'm not bothering to do a calc even though I'm trained to do it. I'm just going to start with a 14g tank and see how it goes.

Are you familiar with the concept of coldframes? Worth an image search, you'll grok it immediately. Also a blanket you can roll over the plastic at night I should think would make a big difference for these cold snaps.

[candide]

Okay, officially changing the title to the "The Art of the Minimum Hotbed." Although maybe "the practice of..." would be best.

Also, a theme is congealing that insulation is the way to go. @loutfard, great article. It's been do long since I have checked out the low-tech site, and I need to get re-acquainted. First paragraph made it all make sense to me:

The modern glass greenhouse requires massive inputs of energy to grow crops out of season. That’s because each square metre of glass, even if it’s triple glazed, loses ten times as much heat as a wall.

ibid:

The quest to produce warm-loving crops in temperate regions initially didn’t involve any glass at all. In Northwestern Europe, Mediterranean crops were planted close to specially built “fruit walls” with high thermal mass, creating a microclimate that could be 8 to 12°C (14 to 22°F) warmer than an unaltered climate.

@Axel, all great stuff. And it seems like the idea of a blanket at night, whether in conjunction with the heated bottles or not, is the real way to go.

From a follow-up article on Low Tech:
https://solar.lowtechmagazine.com/2015/ ... the-1600s/

These greenhouse structures became larger and more sophisticated over time, but they all kept benefitting from the thermal mass of the fruit wall, which stored heat from the sun for use at night. In addition, many of these structures were provided with insulating mats that could be rolled out over the glass cover at night or during cold, cloudy weather. In short, the early greenhouse was a passive solar building.

[jennypenny]

In the future, consider putting a fresh compost pile under the bed in the fall so the heat from the decomposition will help keep the bed warm during the winter.

Other thoughts ...
*You don't have to heat with water, I've used rocks kept near the fireplace during the daytime. Pots of water also work (fill some while you shower if you take hot showers ... you can use that to water the plants too as long as it doesn't have soap in it).
*It also helps to place tiles or something similar in the the bed that will absorb heat during sunny days and emit the heat overnight, like a mini earth ship.
*I use pasta and veg water to heat/water plants (watch salt content). (I will actually cook a soup or slow cook something on my portable induction burner in my greenhouse to heat it without having to generate extra heat ... we generate so much heat during the day that gets lost, I'm always trying to funnel it to something else.)
*I'd cover the thing entirely for insulation and only remove it during truly sunny days (remove the cover, but leave the greenhouse film).

I know people who grow greens and brassicas all winter long in NE using row covers and snow as insulation. It can be done with only occasional supplemental heating as needed.

[candide]

It can be done with only occasional supplemental heating as needed.

What do you think a good cut-off for extra heating would be?

I will proceed with with more thermal mass and insulation as my go-to model. Just because I have to have some procedure, I think I will put plastic on for over night lows in the 20s, increase insulation with the teens, and then just take my lumps for anything other than that...

[jennypenny]

Why not keep the plastic on all the time? Sun + plastic cover = much higher base temps.

[candide]

The thought process, whether correct or not, was to get the highest amount of air flow for carbon dioxide and a notion I picked up somewhere that plants do well with a little bit of fighting wind, as it makes them sturdy.

I really haven't messed with a garden this time of year before, so if you think it is for the best, I will keep the sheeting on until, say, March. I'll just have to remember to vent quite often, as days of 60 degrees are right around the corner. (On balance, more days will be spent needing shade cloth than poly in my hot, hot state).

[jennypenny]

If you're in a warm state, then you can prop open the sides of opposite ends during the day and drop them at night when nights might produce a solid frost. As far a wind, you are correct that plants do better with airflow and wind/handling to strengthen the structure. I've never seen brassicas that needed that though since they don't get very tall. But in a warmer climate, airflow might be needed except on the coldest days.

A traditional hot bed with compost underneath would probably be too warm as well, and draw pests, if you're in a warm state. So ignore my advice upthread lol.

[7Wannabe5]

The most interesting designs at a larger scale also include animals for body heat. Maybe you could put a couple hamsters out there in lieu of cows.

Seriously, you might want to do the math to determine whether you wouldn't be better of growing indoors and adding light vs. outdoors and adding heat at such a small scale. The added benefit would be that greater variety of edibles would flourish under such circumstances. It was on some level depressing (chemistry for the win!), but I grew an entire Tiny Tom tomato plant to ripe perfection in my hydroponics set-up. Arugula doesn't even need that minimal level of pampering. A box of soil in the window will work.

[candide]

Well, all of the plants survived, even the red lettuce, which I really thought wouldn't have made it, and I am going to guess would not have without the extra thermal mass -- but that is only a guess.

Tomorrow is the 50s (American; 13 Science) and the 10-day forecast says 61 (American; 16 Science) by Monday.

[candide]

Post script. All the greens went through this treatment are starting to bolt. Also any greens I have filched since have been rougher and bitter.

With the brassicas I don't mind. Arugula flowers are my favorite part. Then when mustard greens flower, I'm cutting the arugula back to prevent any crossing, as I want a seed line of mustard of my own.

The only plant that is pissing me off is the red lettuces that survived. I never got any leaves from them. Probably going to pull and plant in some radishes.

The high today is supposed to be 78 degrees (26 C). I hate this state.

[7Wannabe5]

Have you tried shade cloth? Michigan tends towards a short, highly-variable spring-season, so I've struggled with similar problems with early plantings over the years. My total fresh pea yield has never surpassed the level of pitiful. Staggered seeding can also prove helpful, especially with a crop such as arugula. A planting site that gets morning sun, but is well-shaded in the afternoon might also be better suited to tender greens. In my experience, beet greens hold up reasonably well in a variety of conditions, and collard greens are an obvious choice for more southerly gardens. Maintaining fresh salad greens year-round was a luxury for only the wealthy in previous eras, like the green blender smoothies of today. Greens that are more edible when cooked or pickled are much generally much easier to grow or forage.

[candide]

@7

I think this will be the year for shade cloth. I have always had to give up on greens far earlier than I would like.

The great exception is chard, which upon looking up stuff I see is not the beets roots sibling, but rather a subspecies. Fair enough. The stuff is great with garlic and butter... Goosefoot (Chenopodium album) also has no problem here, but having had a kidney stone before which I blame on it makes me hesitate to eat it more than a few times a year... such a yummy weed.

Thank you for all your tips.

[7Wannabe5]

Yes, we call it Lamb's Quarters hereabouts, but it is delicious and grows on many vacant lot type sites. I once used it instead of spinach in the classic Moosewood Restaurant spanikopita recipe, and it was superb! Also important note that many foodstuffs can become toxic at some proportion of diet. The most tender salad greens have generally had more of the "toxic" bred out of them, but this is often reflected in their diminished nutritional profile. I think the least dangerous, most nutritious, vegetable you could eat big plates full of night after night, might be asparagus. However, this practice will eventually make your pee stinky. Well worth planting a patch, since it is also perennial.