Walipini is an Aymara word that means “place of warmth.” Originally constructed in the 1990s by the Benson Agriculture and Food Institute for a site in La Paz, Bolivia, the walipini is a low-cost greenhouse, sunk into the earth, that’s designed to extend the growing season without relying on conventional heat sources.
These structures use solar energy to bring light and heat into the growing space and thermal mass to retain that heat. The result is a greenhouse that can keep relatively stable temperatures without using additional energy sources.
The idea of sunken greenhouses, also called pit greenhouses, is much older than the ‘90s, of course. Growing food in earth-sheltered spaces has a long history that spans the globe. As a charity working for food security, the Benson Institute was especially interested in capitalizing on the idea to make low-cost structures that could be sustainably built using local materials and operated entirely through passive design principles.
Their goals have resonated with a lot of gardeners. But a lot of gardeners are wondering whether the design is a good option for their growing needs.
Walipini Sunken Greenhouse Design
A walipini is constructed by digging a rectangular greenhouse footprint somewhere between 6 and 8 feet deep. Once the topsoil has been removed and saved for a growing medium, the deeper soil is used to construct a berm. The roof of the greenhouse will be angled to take maximum advantage of daylight hours, so the taller wall is typically built to face the winter sun (i.e., south, if you’re in the northern hemisphere; north if you’re the southern hemisphere).
In many cases, the soil won’t be sufficient to create a stable earth wall. In areas where the soil can’t be compacted enough to be structurally sound, the walls of these sunken greenhouses can be supported by stone, earthbags, bricks or concrete blocks.
Many growers use rainbarrels filled with water to support the wall and to increase heat absorption; since water is denser than soil, it has better heat-storage capabilities. Others will use concrete piers and frame the greenhouse in to stabilize it.
The roof is usually made from a double layer of plastic sheeting of some kind. The roof lets light and heat in, while the layering keeps heat from escaping too quickly overnight. In extremely cold conditions, insulated shutters might be installed. Ventilation is provided by some combination of doors, windows and/or roof vents.
What Can I Grow?
In general, vegetables seem to thrive in these greenhouses. The stable temperatures of a walipini greenhouse make them especially useful for veggies with a long growing season. Cool-weather vegetables like kale or cabbage can survive nicely during the winter months. Hardier greens like chard are a good choice for cooler temperatures, as are root vegetables, broccoli and cauliflower.
Benefits of Going Underground
The biggest benefit of a sunken greenhouse is its use of passive energy. Conventional greenhouses require energy to heat air and move that air around. A sunken greenhouse does that work without the need for mechanical intervention.
John H. Lienhard of the University of Houston explains that temperatures lag below ground, since it “takes time for the soil to cool in winter.” Even at a depth of a few feet, temperatures can also be more stable because they’re not subject to the air temperature variations that the earth’s surface is.
This means that in winter, the underground structure will already be warmer than the surrounding area, even before the sun gets to it. The sun can work much more effectively than it does in conventional, surface-built greenhouses because if has a lot less work to do to heat the greenhouse to ideal growing temperatures.
Cons of Sunken Greenhouses
The biggest downside to a sunken greenhouse might be cost and it might be labour, depending on your resources. If you have the equipment (or the friends and the back strength) to dig this out yourself, it will be very inexpensive to build but you will owe your friends a lot of beer. If you don’t have those resources, excavation costs could be high. Much higher than a conventional greenhouse.
Durability could also be a significant downside. This style of greenhouse is especially vulnerable to soil instabilities and water damage. If these aren’t accounted for in the design and building process, your pit greenhouse could easily just become a pit.
Things to Consider
Water. Water is the enemy of a sunken greenhouse. You’ll need to avoid groundwater running into your structure as well as water seeping up from below. Incorporating appropriate waterproofing and drainage techniques is essential. It’s also essential to build your greenhouse so that the bottom is at least 5 feet above the water table. If your water level is higher than 13 feet below surface, it’s time to consider other options.
Thermal mass. The denser the mass of your wall, the more heat will be stored and released overnight. If you only have the space to construct a small greenhouse, you might not have the mass needed for this building technique to create the stable temperatures you need. The Benson Institute recommends at least 8 feet by 12 feet. Anything smaller than that and an above-ground greenhouse might be simpler and easier to control.
Soil type. An underground greenhouse that doesn’t have a foundation requires highly stable soil with low permeability. If you’re set on this structure but don’t have ideal soil, you’ll want to consider adding piers and framing your structure in.
Aligning with the sun. In the northern hemisphere, a southward-facing structure will allow the most light into the greenhouse and absorb the most heat. A sunken greenhouse could still extend your growing season if you can’t make that alignment happen in your space, but you might have a more limited range of plants you can grow.
Lindsey Schiller tells Mother Earth News that gardeners who are interested in building a walipini-style greenhouse should make sure they “design the structure for the solar angles at your latitude.” For those of us who live far, far away from the equator, where the sun is (often depressingly) low in the sky, sinking the greenhouse entirely won’t result in good growing conditions because sunlight won’t reach the greenhouse floor. A partially sunk greenhouse could allow much more light to penetrate.
Where to Find Plans
The Benson Institute shares detailed information on the design and construction of their walipini.
Owen Geiger, at the Natural Building Blog, shares these plans for earthbag pit greenhouses.
Susanna Raeven explains how to build an earth-sheltered greenhouse here.
Or you can check out Homesteadonomics, which features this video series on building a sunken greenhouse.
Feature image: Mary Ray; Image 1: Marta Reis
