How would you recommend setting up a trench greenhouse?

I’m toying with the idea of setting up a trench greenhouse at some point, in order to be able to overwinter tropical or semi-tropical perennials (cough banana plants) more easily.

I love the way the Russians did it with citrus – link here:

And I’m thinking it would be a good first stage to get seeds that are used to my soil, which I can then plant in abundance with a little less cold protection each generation, until I’ve finally bred things that can handle my winters without protection. Bananas especially, but there are an awful lot of other not-cold-tolerant fruiting perennials I’m interested in, too.

What do you think would be a good way to set one up? Would it be enough to dig down only two or three feet in order to give some perennials that are only one zone away enough protection? That seems like a good proof-of-concept to see if I really want to do it on a larger scale later.

How would I keep the walls from collapsing? What kind of lid would I use? What would be simple and easy to make with stuff I could find or make easily?

If we can come up with something like that, it would be useful to lots of us who want to start the process of working on locally adapting perennials that are usually only able to live in a zone or two warmer than our own.

Any ideas?


I think of 4~6 feet down as good, but I don’t have experience. Just have a sense that temperature would be more stable that far down.

Do you have any South facing slope? I’d choose a slope if possible, so that there’s less digging and the sun can shine in through a sloping glass or double layers rigid plastic roof, with only having roof sloping to face the Sun and have all the back wall be the slope cut-out face, acting as a heat sink. Or, ideally, an insulation layer up against the dug-out vertical ground, and inside from that, a wall made for example of earth bags. Such a heat sink may allow for less digging.

But that might be too much for you - still, even if building a more simple trench, I’d try to do it on a slope. And if I had no slope, I’d use the earth I dug out, to build up the back side to be higher than the ground level and act as that heat sink I mentioned.

I don’t really get why people build greenhouses with glass (or plastic) on all sides. It makes so much sense to me to only have that on the side facing the Sun, and use the back wall as a heat sink. I think that’s pretty standard now in China, for example, massively extending the growing season.

Also if you have the chance, insulation helps. For example…

(Badly drawn in terms of proportions - I’d rather it wider maybe!)

Here I try to show for you insulative material you could put all along the outside, to deflect frost. It’s sad to put plastics into the ground but discarded insulative material from building work or packaging even, could be used. I guess some natural thing could be used also, for short term before it decays. I don’t know the optimum angle but I would guess 45 degree like in the picture I just drew might be somewhat ideal. This deflects frost further down than the insulation extends - that’s why using such an angle, more efficient than using it vertically.

For supporting the edges I guess wood would be good. If you get a lot of rain, maybe a waterproof layer, then something… maybe the earth dug from the hole, between that and the wood to prevent it decaying quicker? Of course you could also use rocks if you have them.

Depending on your water situation, the requirements for walls and a drainage ditch around the outside and maybe a waterproof membrane, may vary.

Sorry I am quite ignorant about this stuff but these are thoughts that arise from what I have picked up along the way.

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I think a cheaper more reliable option is a geothermal greenhouse. Basically you dig down 4ft under the greenhouse put in a bunch of 4" drain pipes, and then cover it all back up. Then you blow air through the pipes. It keeps your greenhouse above freezing all year round if done right. ldsprepper on youtube has some great examples. The trench greenhouses have a lot of difficulties that this design doesn’t have.

Joseph and Holly’s monthly zoom call a few months ago featured William DeMille, who has built a lot of these - he calls them wallapinis. The recording of the call no longer seems to be working, but I think he has YouTube videos showing how they’re constructed.

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I dug a couple feet down to create a small cold frame, 4’ x 8’, a couple years back and found that it tended to be cooler and wetter than the same thing above ground. I figured it was the opposite of how raised mounds tend to be dryer and warmer since they drain better and are more exposed to the sun. I’m in zone 7a, maritime temperate. I suppose if the trenches in the greenhouse were angled to capture the most of the winter sun it might work but I think we would need to get below the frost line to get to 50*. There’s also the issue at our latitude of lower sun hours during the Persephone period and I’d wonder about that factor.

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I like that design a lot. I think you’re on to something with that being simpler to make, and using fewer materials, than a “usual” greenhouse.

It occurs to me that it might not work to dig far down because I’m pretty near a river, which means the water table in my land is high. In summer, that’s a good thing. In winter, which is when we get almost all of our water, a trench greenhouse might drown any roots. Hmmm.

The geothermal greenhouse sounds like a really interesting idea. How do you blow air through the pipes? Does it require perpetual electricity?

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I’d be interested to hear the details of such difficulties.

Then I’d suggest at least the thermal sink back wall, even if you don’t dig down - or if you can only dig down a bit, then you can use all that which you dig out, to make the back wall. Basically similar to my picture but just the ground level part or just dug down a bit. And if you use insulation on the back wall, then it should go on the outside of it, not the inside. You want the Sun hitting it and the heat being absorbed.

ya you blow air through the pipes. It uses a little electricity (like $50-100 a year for a 20x40 greenhouse.

The main difficulties i’ve heard are: 1) the angles have to be right to be passive and those angles don’t work well for here (at least in idaho they weren’t right). 2) the dirt walls have a tendency of collapsing (the greenhouses in the snow have talked a little about that).

With a low water table though that’s also difficult with geothermal, you have to use a little more pipe and it has to be non perforated. Still can work though.

I do like Justin’s idea about the thermal sink back wall. The chinese year ground greenhouses are cool, there’s still electricity requirements, and they’re big, but if I had space for one that’d be one I’d like to do (I really wanted to do a geothermal + chinese combo)

All in all when I looked into the whole thing the geothermal seems to be the best if you’re wanting to do citrus or similar

Dependence on electricity concerns me because we don’t get much electricity from our solar panels during the coldest days of winter (they tend to be covered in snow). Ideally I’d like to have something that I could rely on to keep functioning if the grid went down.

The chinese passive solars are the only ones that i know of that can be done without electricity in utah. Depending on what you want to keep alive. If you want to keep it above freezing. Although, even those you’ll want to make the blanket electric because otherwise you’re talking a serious amount of work. It’s ok if it’s your job, but for a hobby I think that the geothermals are the easiest

(Nods.) That makes sense.

How hard is geothermal to build? How expensive do you think the materials would be?

If you can get an excavator then the one that i’m planning on making is about $5000 (it’s a 20ft x 40ft). I’m budgeting $7000 since it costs about $1000 a week to rent a small excavator and im sure there are things i’ve missed. I have a spreadsheet of all the costs from about 8 months ago.

you’d also save $500-1000 if you were going for a pure hoop house. I’m planning on building one with a side profile closer to the chinese ones with a 14ft point at the back for larger fruits (bananas, other trees, etc)

Very cool! What kinds of trees are you planning to grow in it?

citrus, avocado, year round tomatoes, dwarf mangos and bananas, and anything else i can

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Very neat. It didn’t occur to me that you could grow year-round tomatoes in there, but of course you could – they’re a perennial!

Miracle fruit may be a fun one to grow, too.

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It could be awesome if you’d make a post on that Joe, and post pics as you progress with that. I’d love to see!

These videos may also be of interest. Chinese system:

Greenhouse in Nebraska, extremely good efficiency:

I think this site is selling his design:

A random sunken greenhouse video:

Ya, when I break ground on it i’ll take pictures and video and all that. It’s been pushed to this winter that we’ll start it.

I really like the chinese systems, but they’re soo much bigger. (35ft x 100ft is pretty normal). I only have 20ft x 40ft that i’m allowed to use :). But ya, i’ve been watching Dong Jianyi’s stuff for a while and it’s a really cool idea.

There is a family here in utah that is doing the greenhouse in the snow design, i’m just not a huge fan. They work ok, but they’re so much more expensive than the ones that i’m planning, and there aren’t any more benefits. You still have the electricity usage of the fans, but they just feel smaller in the same footprint.

There’s another guy online called Greencube Research Institute that I bought a while ago that has a bunch of designs. His are more expensive, like the greenhouse in the snow, but if someone was doing them commercially that’s the one i’d go with. He does normal passive solar, but uses 12" of styrofoam to make an earth battery below and says that with that you can get ~60 degrees as your minimum which would be cool.

man, looking back through all this stuff the chainlink fence top rail prices have doubled. That’s sad, that adds another $800 to the full price. So i’ll probably wait for those to go back down before building the top.

The Chinese system - wouldn’t it work on a smaller scale?

And yeah it’s possible to spend a lot of money (but still work out cheaper than the standard Western commercial systems). I think we can take design points and adapt them to cheaper ways even if maybe not quite as effective.

I had an idea for a passive air circulation system. No idea if it would work or not. Hmm, I’l try to draw a picture…

So, I do not have a good understanding of the way air moves. Please anyone correct me or add to this… My thinking is that if the lower level is gravel, the chimney will become hot, heating the air in the chimney (which will exit into the greenhouse at the top) and thereby suck air from the bottom of the gravel, forcing air to be pushed down into/through the gravel from the right side. The left using the bottom because the coolest air will be down there, giving the hot air of the greenhouse more time to transfer its heat to the gravel. So, during the day, when the Sun is shining, the air will (I hope?) circulate through the gravel, heating it, hence it will act as a heat sink.

Also even just the heat through conduction and radiation from the gravel at night should be useful if the daytime circulation works as I hope.

I’m more unsure of what happens at night with the flow. But I would hope that the hot air would simply rise out of the right hand hole. This is the part I would most like help with to get a good flow. I expect someone well versed in plumbing might have insight into this as understanding the flow of hot water in a central heating system would probably come in handy here. Anyway, this was some speculation I had with regard to creating a more stable temperature between night and day. Any thoughts?