I never thought I would be calling The Church of Latter Day Saints to ask about greenhouses in Bolivia. But, as with a lot of things when you start to dig beneath the surface (yes, pun intended; there will be more), you can find yourself in some surprising situations.
The impetus for this article is a picture that has been floating around the Internet so prolifically I had to investigate more. The picture is a pit – style solar greenhouse with earthen walls, called a walipini. It grabs the attention easily – greens growing prolifically underground, surrounded by four slanted walls of soil and a plastic roof – but the question kept nagging me… will it really work? Using the earth as protection from the elements and to store energy (via it’s ability to retain heat) is an ancient and common sense notion. However, nature abhors a vacuum. And when removing large quantities of earth, without proper retaining walls, it seems, a walipini will inevitably (and possibly very quickly) fall in on itself. Which sent me on a walipini hunt across the Internet, to see where and how this growing technique has been tried, and what we all can learn from it.
The first question was easy: what is up with that name? The word Walipini comes from the Aymara language, native to the people in the Andes Mountains of Bolivia. It’s said to translate to “place of warmth.” And while the style of underground growing may have already been in existence for a long time, the term walipini did not hit the American lexicon until about 10 years ago. And this is where The Church of Latter Day Saints comes in. As a part of The Church’s welfare program – specifically via an organization called the Benson Agriculture and Food Institute — volunteers travelled to many developing countries to help families “become nutritionally self-sufficient and to improve their economic circumstances” (Benson Institute). In 2002, a pilot project of the Benson Institute (then a part of Brigham Young University, and now a part of the LDS Charities) tried a new method of building a very low-cost underground solar greenhouse. Not a great deal can be found on the background of the original project, but according to Robert Hokanson, the current administrator at LDS Charities, the aim was to increase food security for local residents by teaching them farming and year-round growing methods. The walipinis were used to grow hardy crops like greens and root vegetables, says Hokanson, however an additional source claims they’ve been able to grow bananas at 14,000 feet elevation. The focus of the project was again, to keep costs low, hence creating a pit greenhouse without a foundation or retaining walls. The remaining materials for the roof and supports were all locally sourced. The resulting paper, entitled “Walipini Construction (The Underground Greenhouse)” is now the primary substantive document about walipinis, or pit greenhouses. You can download a copy here.
Pictures and the report of the Benson Institute’s pilot project have seemed to spur the term walipini into a catchall word for an underground solar greenhouse in today’s Facebook and pinterest nomenclature. But the basic philosophy is not a new phenomenon. In fact, the idea of using the earth for heat storage and protection from the elements has been around a long, long time. In the 18th century (and likely well before), American farmers used “grow-holes”: 3’ deep trenches covered by storm windows or glass. These quickly expanded into larger spaces, for someone to stand and work in. Examples of various forms of underground an earth-sheltered greenhouses have cropped up all over the world, from China to South America. A great collection of styles and types can be found on the Inspiration Green blog, here.
Why grow underground? Compared to a traditional solar greenhouse, an underground greenhouse has the advantage of being surrounded by lots and lots of thermal mass (the soil). Thermal mass is any dense matter (water, stone or soil), which has the ability to hold and store energy. So as the greenhouse heats up during the day, the soil holds that heat, and then releases it at night, preventing the greenhouse from getting too cold. The soil also insulates the airspace of the greenhouse. The walipini also takes advantage of warmer temperatures of the deep earth – the soil below the frost line, which is warmed by geothermal activity of the earth and generally stays above 50 degrees in most regions. All in all, an underground greenhouse (of any type) is a much more energy-efficient greenhouse, that stays warmer and can grow more and longer—providing year-round source of food without having to heat and cool.
Where will a walipini work? While recent info graphics and a Tree-hugger article proclaim anyone can “build an underground greenhouse for $300,” there’s a more complicated reality underneath. So before you start digging here are some very important questions to ask:
1. Where are you?
One challenge of an underground greenhouse is that unless you are building into a steeply sloped hillside, the roof will have a very low pitch. If you are located at higher latitudes, the sun will come in at a very angle, and may not hit the plants on the ground. Thus, you’ll have a greenhouse that doesn’t grow for much of the year. So, if you are in the Northern latitudes, and want to grow things well in the winter, think carefully about your roof slope and where light will come in (you’ll want to look up the angle of sun at winter solstice for your latitude and do some simple geometry to ensure there is no shading.) Often, this is overcome by building an earthen berm or insulated roof on the North side of the greenhouse, which will raise the North wall to create a steeper slope (Note: it’s not advisable to try to accomplish this by building into a hillside, where the soil will have much greater downward pressure, making collapse likely.)
2. What kind of soils to you have?
This is probably the most important question in building any underground greenhouse, and it is massively important when building an un-framed underground greenhouse. Both the Benson Institute, and any other grower with experience will tell you a walipini is only an option if your site has very stable (i.e. clay) soils. With any type of soil, re-enforcing the walls somehow is advised. The Benson Institute used rammed earth poured in forms. Creating a more stable frame / walls will obviously get you a longer lasting greenhouse (we’ll go into framing / re-enforcing an underground greenhouses in the next edition).
3. Where is your water table?
The Benson Institute paper recommends that the bottom of the walipini be at least 5’ above the water table. So if the floor of the walipini is 6-8’ below grade already, that means needing a location with water table 11-13’ down. Even if well above the water table, it’s likely water will seep in from the walls and create excess humidity. Thus, plan for proper drainage and ventilation (see the Benson report on some strategies how to handle this). It’s also important to plan water drainage / irrigation on the exterior, otherwise you’ll have a muddy swimming pool instead of a greenhouse after the first heavy rain. The Benson institute used sloping exterior walls and an irrigation ditch running the perimeter to accomplish this. In many cases, water can still run down the walls (particularly the south wall where water off the roof will drain) and cause the walls to erode, slanting into the growing space of the greenhouse.
4. How will you keep varmints out?
This is a topic the Benson report did not touch on, though I would imagine would be an important issue. A warm underground space would probably be seen as the Hilton for mice, moles, gophers, and possibly rabbits. Then you fill it with a buffet of food…. they may get to it before you do. Planning fencing or chicken wire would be a good idea. Pests and plant diseases are also going to be more of a challenge than in an above ground solar greenhouse, as there is not a way to clear out the space in case an infestation occurs (the pest or disease will likely still exist in the earth walls).
Beyond this, you’ll want to think about the usual greenhouse things, like not overheating the greenhouse, ensuring proper ventilation, and when and where light comes in.
So how do these hold up?
Sadly, I cannot not find enough case studies to really say how a walipini (again, an unframed underground greenhouse) works over time. The few videos on YouTube of a walipini in the US show someone having to re-enforce their earthen walls (the most common one, of a walipini in Utah shows some nice growth, but walls severly slanted and crumbling after 2 years). The original researchers on the Benson project have moved on and could not be found; the spokespeople now for the Benson Institute could not comment on the current state of the Bolivian walipinis. So, with that said, building an underground solar greenhouse comes with lots of red flags; building an un-reinforced underground greenhouse comes with lots and LOTS of red flags. Though infographics can tout it as simple and cheap, remember that building anything requires your time and energy, which are valuable and not usually calculated into the cost. So plan and choose carefully in your solar greenhouse design.
Next up we’ll be discussing how to frame or re-enforce an underground greenhouse. I interviewed Mike Oehler, author of the “Earth-Sheltered Greenhouse” (image below) and my personal opinion… If you’re in North America and want to grow underground and don’t want to spend every summer ramming earth onto your greenhouse walls… go this route.