In this area of Texas, I have two problems that I encounter when growing tropical plants. In the winter it freezes – a no-brainer – and in the summer, it gets too hot for many of the tropicals I grow. It’s a pretty unfriendly environment for a plant that is used to an 80-95F range 365 and a quarter days per year. But, the land wasn’t expensive, we have family here, and our well flows good even in the depth of drought, and most importantly, my better half doesn’t want to cart the kids off to some remote Pacific island no matter how much I beg. So, determination and innovation is what I have left – and a blank slate of a property to exercise that on.
My goal is simple – grow tropical plants without going broke doing so. That means keeping the environment above freezing, and below the hundred mark if possible. And doing so without having to depend on expensive heating, be it by fuel – which is ridiculously expensive, or by electricity that, well, is produced by burning fuel currently so is similarly expensive, and without having to buy and install massive swamp-coolers that would only work until our humidity shoots up to where it usually is in the hottest, most miserable part of summer. And that means the solution needs to be inexpensive to build too. No expensive multi-layered glazing, no super-insulated walls or fancy gizmos.
Luckily, I’m a huge fan of earth-shelters and underground homes. I may never live in one and may forever be consigned to a stick-frame with a white picket fence, but that doesn’t stop me from dreaming. It just fascinates me that the simplest way to cool down on a hot day is to go a few inches down into the ground – something I’ve seen dogs do. As such, it wasn’t a far stretch for me to consider this for a greenhouse. At one time, I even daydreamed of having the greenhouse totally buried but having light wells. A major inspiration for that idea was Forestiere Underground Gardens as seen on HGTV. I was just enthralled by that show and researched it on the Internet extensively. I also found a Mother Earth News article about another DIY earth-sheltered greenhouse inspiring too. And more so, because he too grows tropicals, I found Russ Finch’s semi-pit earth-sheltered and ground-mass heated greenhouse especially inspiring.
My problem was that I had no idea what the structure of my soil was. Our property is not far from a creek, so it was easy to surmise that we were sitting on silt. So I spent an enormous amount of time researching retaining walls and re-enforcing, getting rather elaborate and ultimately fatally impractical. It was time to step back and take a breath. There was going to be an earth sheltered greenhouse here, but I just wasn’t sure what direction to take. Finally, I figured I’d put shovel to soil and see what I came up with.
It didn’t take long, tho before the shovel idea seemed less appealing. I got down about a foot then the soil was like concrete. But I had already started – so I called a friend with a back-hoe – it was time to put some money where my ideas was and step in with both feet. I was starting off small and simple – 12′ wide and about 45′ long. I had already decided to use the cattle-panel hoops from my previous greenhouse to cover this thing – going from 9′ wide 6′ tall to 12′ wide and 4.5′ tall covering an 8′ deep pit – soil composition willing.
The first couple of bucket-fulls of soil told the story – the reason why I felt like I was digging in concrete after I got a few inches into the subsoil was that it really was, after a fashion. Strip away the shallow subsoil and what I have is sandstone! A rather hard sandstone too. By the time the back-hoe had finished digging, it’s teeth where short, sharp, shiny little nubs, polished to a mirror finish. And what I had was the start of my earth-sheltered dreams – a pit that had walls of solid rock – no re-enforcing needed at all! I was beside myself – it only took an hour to excavate that, and he even carved a little ramp for me to go down into the pit.
My next quandary was how to cover this thing. It was fairly close to straight, but not perfect. I tried stakes to anchor the cattle-panels, but that didn’t work as the ground wasn’t fully even, making the forming of a perfect hoop impossible. I then figured I’d build a frame to mount it on and support it from inside the pit. That worked well, except, there was a space between the frame and the edge of the pit that I had to contend with.
I spent a winter trying to figure that out and the next Spring I decided to embark on an idea that had been hounding me – build an enclosure inside the pit for at least a temporary earth-sheltered greenhouse. The other option was to scrape the soil around the perimeter, dig a footer trench, pour a few yards of concrete then build a frame on top of that. A very good and ideal idea – but more expensive than I could afford at the time and I needed to relocate my plants pronto.
I had a corrugated-steel clad shed on the property that I was in the process of disassembling, and I had all the lumber that I used in the first framing attempt, so it wasn’t long before I had walls being made. I actually built the thing in a 2-day frenzy of nailing. First the short wall at the back end of the pit, nailed together and clad with the corrugated steel while laying down, then lifted up and leaned against the end of the pit. Then the long walls, each one assembled likewise. They were heavy too – but I managed to get them up. And finally, the door-wall. I was lucky in that our house was in the process of construction when we bought it and had a spare glassed-in screen door. I made use of that for my greenhouse. I braced the top of the greenhouse. I should have braced the bottom too – one wall has moved in a few inches, but it’s stopped where it is so I’m not too worried about it.
I couldn’t afford to enclose the entire pit – not enough lumber, not enough corrugated steel – I was on a near zero budget here. So I only enclosed a little over 17′ of the pit. That required only four cattle-panels to cover. And they went on swimmingly – very easy to do. I did everything solo – no help whatsoever, so it took a bit of creativity to make this work. The ends were a little more awkward. I figured on a couple of pieces of plywood on either end and then I’d cut it to shape to the curve. Well, I never got around cutting it, but it’s worked perfectly the way it is and I figure if I’m going to eventually disassemble this for the new and improved version I’m currently designing, why bother? But the ends really helped stiffen the entire hoop.
All I had left was to glaze it. Since I used cheap poly from the hardware – basically 6mil translucent general purpose poly – that didn’t survive long in the UV rays of the sun, I decided not to cover it right away, but rather concentrated on moving all the tropicals down in the pit. It was now Fall and starting to get cool, but I still pushed it as far as I could, waiting until the last possible moment to put the poly on.
Now, here’s an interesting thing with earth-mass. It radiates heat. It may not seem like it – but with the ground at a constant 65′ish, when it’s cold outside, the walls actually serve as heat sources. I saw the manifestation of that very clearly one morning. A morning that was only supposed to get to the low forties. Instead it dropped to the low 30′s to upper 20′s! And I didn’t have a lick of poly on the greenhouse. I went outside and the grass, house, trees, my pots and junk – it was all covered with ice. My water-hoses were solid with ice. It was cold. And I was terrified. I ran down into the pit and saw… perfectly happy tropical plants that had not a speck of ice on them – lotsa dew tho. Earth-mass at work, folks. Of course, it’s more potent in the Fall, following a hot summer, than the Spring – there is a certain amount of stored energy from the summer playing a part here.
Nevertheless, I promptly glazed the greenhouse. My next challenge was attaching the poly. Did I want to nail it, or wrap it around a 2×4 then nail that, or what? I then remembered the problem I had with my last greenhouse – where the near constant wind eventually found weak spot in the plastic that caused them to rip and caused the rips to keep on ripping. The solution presented itself to me in the spare cattle-panels I now had – why not just lay the other cattle panels right on top of the poly? And it worked. The squares matched up to the ones below the poly, forming tear brakes. Additionally, the wind couldn’t grab huge swaths of the poly, but only little squares. I did have a little rip and it stopped at the edge of the squares as I surmised it would.
Now for supplemental heat. I already had a wire out there that I used for my well, and I spliced into that to use for my heater. I didn’t think I’d need it much but figured it would take the edge off. Even tho the enclosure was completely covered now – the walls still were exposed to the outside air – both on the entrance end and in the space between the other walls and the sandstone walls. So – earth-mass or no, it was going to get a little chilly because I was not excluding the winter air effectively. It still performed wildly beyond my expectations. My heater failed at the splice early on – and I never knew it. I had a very few plants show some curious signs, but the vast majority of the plants were so happy that I chalked the few quirks to something else. I figured that the figs go dormant naturally, the boswellia sacra has a highish dormancy temperature range, and that my chocolate trees and coffee trees – all clustered tightly together – were suffering from root-rot problems from the cool, moist soil. However, no heater, and a very cold winter for these parts and the rest of my plants acted like they didn’t even know it was winter!
This winter is my second winter in this enclosure. Plans are underway for a new pit greenhouse – much larger, much longer, a little bit deeper and a taller riser giving me more headroom. I have a little more funding and am working at a place where dirt-digging equipment is easy to get ahold of. With my experiences with my current pit greenhouse, this new project is sure to be a blast. My goal is to plant right into the soil of this new pit – soil that I’ll have to make by converting the sandstone into sand then mixing with organic matter and a dash of clay and whatnot. But that, folks, is another article. For my current greenhouse – built when I was at the lowest point of being broke from scavenged components – I am growing plants that would have way outgrown my topside hoophouse, and that required a fraction of the heating that I used up there. All in all, a very economical greenhouse.
To answer a commonly asked question – yes, when it rains hard I get a few inches in the bottom. But that’s it – and the water drains away rapidly. A couple days later, 5″ of standing water becomes a little bit of wet sand. That’s the advantage of having sandstone. The new pit will eventually be completely covered and won’t flood at all, but it’s never been a problem.
Another similar project that uses similar principles that mine does is a pool that’s been repurposed into an earth-sheltered aquaponics greenhouse. Check them out at http://gardenpool.org/