I have a lovely greenhouse situated adjacent to an irrigation pond, in a micro-climate protected from the worst winds by trees. Between the pond and the North side of the greenhouse, I have a lemon and an olive tree, winter greens, and seed starts. I manage to keep this area above freezing about 11 months out of the year.
For several nights during the coldest months, though, we have to heat our greenhouse. We do it by heating up a woodfired hot tub which we also enjoy. So, it's not a real hardship. But, if I lived in a colder climate than my USDA zone 7a, I'd have to do a lot more work to keep my greenhouse from freezing.
Truth be told though, as much as I love my greenhouse (particularly the hot tub), if I'd taken a bit more time researching season extension, I would have made a different choice. I would have gone with a walipini.
Unfortunately, I didn't even hear of a walipini until a year or so after I spent a small fortune on a greenhouse. If you are currently considering options for year-round growing, then hopefully this post is reaching you in time so that you can consider a walipini before your greenhouse is built!
What is a Walipini?
A walipini is basically like a greenhouse crossed with a root cellar. The growing area is dug into the ground. This gives you the thermal benefits of being cool in warm weather and warm in cold weather similar to a root cellar. The earth acts like your insulation.
Unlike a root cellar, though, you'll be putting a greenhouse top on your dug-out space. That will allow the sun to come into the area and make it even warmer. Plus, that sunlight will be used to grow your plants.
To build a walipini, you dig a 6-8 foot deep area in the ground. Then you install a greenhouse type roof angled toward the sun. If you have earth moving machinery or live in an area where this kind of work is inexpensive and can be performed without a permit, you can save a lot of money building a walipini instead of a full-blown greenhouse.
Now before you spend all your time planning your perfect walipini, there are a couple of things to consider. Your soil type, for instance, will have a significant impact on whether your location is suited to constructing a walipini.
One of the soil's many functions is to hold or drain water. Clay soils, for instance, have excellent water holding capacity. However, this makes them very heavy at times and susceptible to collapse if your walipini is not properly engineered.
Sandy soil provides too much drainage and is hard to stabilize. So, it might not make the best choice for a walipini without structural support for the walls. Bedrock, loam, silt, and other soil conditions may also necessitate special construction techniques to ensure the security of the walls.
Similar to installing an in-ground pool there are options such as back-filling, wall-building, and other methods that can be used to create a structurally sound walipini. However, the more additional engineering necessary, the more work and potential cost involved.
Water Flow Considerations
Beyond soil type, the amount and way that rain falls in your location may have an impact on your building methods. For example, in areas with limited rainfall, building a berm around and covering the perimeter of your walipini with a non-permeable membrane may be sufficient to keep rain from penetrating and collapsing your walls.
In areas with continuous heavy rains where rain can penetrate deep into the subsoil, it creates more challenging circumstances. In those cases, retaining walls, drainage routes, and more may be necessary to redirect water flow from the walls of your walipini.
Also, like root cellars and basements, radon can be an issue in some parts of the world. I am not an expert on radon but I considered buying a few properties with cellars that turned out to have radon issues. So, I know it can be a challenge to manage when building underground in parts of the Northeastern US.
Given the challenges detailed above, each walipini design will be a bit different based on the demands of your location. In general, there are a few key features that all walipinis need to address potential concerns.
The walls of a walipini are sloped in the same manner as irrigation ponds are. In the case of a pond, these slopes are used to create a bowl-like effect that keeps water trapped inside while reducing the amount of silt and sediment that washes into a pond. For a walipini, this shape serves the same function, except that rather than catching water other design features whisk it away.
Floor Level Drainage
When water runs into the walipini, a self-draining floor and water evacuation channels are often used to pass water back out of the walipini. Simply by grading the floor in a slightly downhill direction, creating low points for water to drain, and then installing pipes to send the water well away from the walipini can do this.
In some case, similar to basements, some people also use sump pumps. When the water hits a certain level, a float valve then turns on the pump to lift the water out and away from the walipini.
Bermed Back Side
As another measure to reduce the amount of water that flows inside the walipini, the non-sun catching side (e.g. Northern side in the US), is bermed to cause water to flow away from the walipini. This is basically the same concept we use on houses with sloped roofs.
Just like with roof coverings, depending on the amount of rain, snow, etc. the angle and area of the berm can be used to adjust for regional precipitation flow rates. A longer berm slope, that drains further away from the walls of the walipini, might work better in areas with continuous, penetrating rains. A steeper slope might help in areas with heavy flows that need to be moved quickly.
Sun Catching, Rain Diverting Roof
The point of a walipini is to allow for winter growing and in some cases more comfortable summer growing conditions. Since light is essential for plants to grow, a roof that allows for maximum light is needed.
Similar to the reasons for sloping walls and berming around the walipini, having a sloped roof is necessary to control water flow too. The slope should angle toward the sun for optimal light exposure in the underground area. Yet, it should also divert water down and away from the interior of the walipini.
Rain Collection Methods
Since you will need water for plants inside the walipini, many people use the roof area to catch rain in tanks. Similar to the way that you use rain barrels on a house roof, gutters or channels catch and redirect the rain to downspouts. In this case though, instead of running down outside a house, they channel water back to containers stored inside the walipini.
For rain barrels on a house, most people have an overflow valve at the top of the tank that connects to a hose that redirects water away from your house foundation. With a walipini, it's a bit trickier to safely redirect excess overflow since you are underground.
Planning adequate collection tanks is important. Using linked tanks to allow overflow from the primary tank to a secondary tank can help. Out-letting to the drainage areas in the floor and planning sufficient flow rates for extra rain drainage are important.
Also, adding a way to redirect flow outside the walipini can be critical during heavy flows. So, when tanks approach full, just stop the flow into the walipini and redirect flow to some location well away from your walls.
Non-permeable Perimeter Protection
Adding non-permeable surfaces such as a roofing membrane, pond liner, or plastic sheeting over your berms and integrating that with your roofing design can really cut down on the amount of unplanned water that flows into your walipini.
This material is not usually very attractive. So, you may need to consider design ideas to make them less obvious. Techniques like making a sloped living roof can be integrated with your walipini design. These things will increase work, cost, and time to build but can make for a much more aesthetically pleasing walipini.
Similar to root cellars, a walipini can get dank, humid, and overly warm without proper venting. Having the ability to open windows in your roof area can help. Also, using chimneys to draw warm air out and using solar vent fans to draw cool air in can be lifesavers on sunny, warm days or in heavy humidity.
Depending on your design, access can be as simple as having a hatch with a latter to climb down into. Or, you can dig an exterior stairwell with a reinforced door jam.
Stairs are much easier when trying to haul in plants and materials and carry out your harvest. But if you plan to harvest daily rather than all at once, then carrying a basket up a ladder isn't terribly difficult. Things like bags of compost can just be dropped down your hatch since they are non-breakable.
If you are already implementing a more elaborate design, then having a staircase entry is totally worthwhile. But if you trying to keep things simple, the hatch can also work.
Since the point of the walipini is ultimately to grow things out of season, planning a proper growing area is key. Generally, good quality soil is brought back in after excavation and the completion of the drainage in the floor.
To get the most thermal benefits, keeping the beds in contact with soil will help. Often raised beds are installed. Alternatively, the entire floor can be used as the planting area with small pathways left open, similar to a garden.
Whichever method you use, make sure to bring in sufficient soil. Since excavation has removed all the topsoil and at best your beds are being built on subsoil or drainage rocks, I suggest at least 8-12 inches of high-quality soil as a good starting point. You will need to continually replenish this with nutrients and compost for good growing results.
Alternative Growing Methods
Some people also use a walipini as a climate-controlled environment for alternative planting methods such as aquaponics or hydroponics. These do require electricity and methods for occasionally flushing out the water systems.
The drainage floor or sump pumps may also be used for draining these systems in the event that whole system cleaning is needed.
Why Go to All the Trouble
Now, I know the list of details above sounds like a lot of work and possibly expense. So, you might be wondering why I wish I had built a walipini instead of a greenhouse. Well, let me tell you the benefits of a walipini.
Making a walipini is more work at the outset, to be sure. However, it has some serious benefits where I live.
Better Temperature Control
It can add up to a huge amount of work and resources to heat a greenhouse during the cold months depending on your climate. Also, in the heat of summer, even with all the windows and doors open, my greenhouse gets so sweltering that it becomes difficult to grow anything. At times, I have to turn on fans just to keep my heat hardy olive and lemon trees alive.
With a walipini, the earth helps regulate your temperatures. The climate is much more regulated in a walipini than in a traditional greenhouse on both the hot and cold front.
Less Panel Cleaning Maintenance
Also, most people don't realize, but greenhouse walls and roof panels need to be cleaned periodically because pollen, dust, and more begin to limit the light passing through over time. So, having a design that only requires cleaning of a roof area and not all the walls as well, means less maintenance work.
Potentially Lower Cost
In my area, we can get someone out to dig an area the size of a super large septic tank for just a couple hundred dollars. Bulk gravel, soil, and lumber are also fairly cheap. Plus, the cost of the roof and other materials would have added up to much less than the cost of our greenhouse kit.
So for us, our savings would have been pretty significant to make a walipini instead of a greenhouse. This won't be true for everyone, but it's worth exploring cost savings in many locations.
Avoid Wind Worries
We also live in a windy area. Even though we chose a somewhat wind protected area for our greenhouse, we still have issues with wind. As such, we had to build pretty significant concrete footers to secure our greenhouse against winter winds.
Also, on windy days we can't open our greenhouse windows or wind-facing door because we run the risk of the wind ripping them off. So, we have to find other creative ways to keep plants cool on excessive hot and windy days like running a fan over an ice chest.
An in-ground walipini would simply not have the same wind issues. In our case, our wind side is also the North, so the drainage berm would have effectively acted as a wind barrier too.
Now, I am not just about the thermodynamic properties of a cool walipini on a hot day. But, can you imagine showing your friends and family your walipini?
I love showing off all the other non-traditional homesteading methods we use like keyhole garden beds, hugelkulturs, exotic fruits, and more. Adding a walipini to that line up would just be over the top on the cool, off-grid homesteading front!
(Yes, even us self-sufficient homesteaders like to display egoistic pride now and then!)
The idea of using a walipini for season extension is starting to gain a bit of popularity. In fact, there are even some pretty cool guides out there to help you plan your construction. These ideas are even being incorporated into house designs and modified using other alternative building techniques like rammed earth homes, rocket mass stoves, and other passive solar techniques.
Even if you don't decide to build a walipini, know that there are a lot more options for season extension than a traditional greenhouse these days. So, make sure you do your research and find the season extension design that will work best for your conditions.