Year-Round Gardening in an Underground Greenhouse
How would you like to have fresh vegetables and fruit year round? Sounds pretty good right? Having an underground greenhouse will keep the temperatures hot in the winter and help prevent overheating in the summer; making it possible to grow your garden vegetables through the cold winter months.
How it Works
We all learned in school that under the earth’s crust is magma which heats the entire sphere. Surprisingly, if you dig down 4 feet, the heating process becomes apparent. For the vast majority of the planet, 4 feet below the surface will stay between 50° to 60°F even if the weather above the ground is 10°F with a cold wind! This phenomenon is called the thermal constant and it’s what the underground greenhouse thrives on.
The original design for an underground greenhouse was invented in Bolivia and was called a walpini, an Aymara Indian word which means “a warm place.” Typically, a walpini is a rectangle shape that is 6-8 feet deep in the ground. The longest area of the rectangle will typically face towards the south (in the Northern Hemisphere) to take advantage of the most sunlight.
Since the walpini is taking advantage of the thermal constant, it requires less help from the sun to heat the greenhouse.
The design of the underground greenhouse isn’t that complicated. It can be as simple as a hole with plastic sheets laid on top. The roof seals in the heat and insulates the area to keep a warm, moist environment for your fruits and vegetables.
The location of your walpini will depend on how big you want it to be. You’ll need enough space to grow your plants and have a small area to walk into your greenhouse. The bottom of the greenhouse will need to be at least 5 feet above the water table in your area. The recommended size for an underground greenhouse is 8 x 12 feet.
When planning where your greenhouse will be located, remember that your roof will need to be receiving light during the winter too. That means that nearby trees or buildings can’t block your greenhouse during the winter time when the sun will be in the south.
Typically, an underground greenhouse will run east to west with the roof slightly facing south to take advantage of the winter sun. This is also an advantage during the summer (more about this later).
You’ll obviously need a way to get in and out of the underground greenhouse. Many people will carve a ramp into the ground on a side of the building. You’ll want to take this into consideration when you’re planning the layout and location of your underground greenhouse.
Once you’ve decided where you’d like your underground greenhouse to be located, you can start digging. Plot out the area above ground to keep track of where you should be digging. While you’re excavating, dig at least 2 feet deeper than your desired depth (more on this in a minute). Be sure to keep the soil you excavate close by too. You’ll be using that to prop up the roof.
The walls of your underground greenhouse should also have a minimum 6-inch slope from the roof to the floor. This will greatly reduce the amount of crumbling and caving that will occur with the soil. Many times, people will even layer the walls with a clay to prevent erosion or use bricks to stabilize the walls of the building.
Irrigation & Water Management
While you’re digging the hole for your walpini, dig an extra 2 feet below the desired depth. You’ll fill this area with stone or gravel and then 8 inches of soil. Ideally, you’d lay larger stones and gravel on the bottom layer and the gravel would become progressively smaller until you reach the soil.
The bottom of the greenhouse should be slightly sloped from the center to the edges. Along the perimeter, you should leave a space of 2-3 feet just filled with gravel. This is designed to help the water drain more easily. Many people have also created open gravel wells in the corners of the greenhouses that allows them to collect the water. This will allow you to draw a bucket into the hole and pull out water if you find you have too much.
Installing a Door
Once you’ve filled in the floor with the drainage system and the soil required for growing, you can install the doors.
Place the door frame at the base of the ramp and fill in the areas around the door as much as possible with dirt and clay. Filling in these gaps will prevent heat loss in your greenhouse.
Many times, people will use 2-inch door frames that have holes drilled into the top middle and bottom of each side. They will then use wooden stakes, dowels or rebar to secure the door frame into the soil wall.
Angle of the Roof to the Sun
The angle of the roof will make a big difference on the sun’s ability to heat your greenhouse. Ideally, the roof should be facing directly at the winter solstice at a 90 angle. This angle will maximize the heat during the winter solstice and minimize the heat during the summer solstice.
The angle of your roof will depend on your latitude. Find a reliable map and figure out your latitude and then add 23°. This number will be the angle of decline that you will need on your roof. The majority of the United States runs between 30-50° Latitude. That means that the internal angle of your roof will need to be between 53° to 73°.
This is where the extra soil that you excavated comes in. Use that soil to create a berm. The berm is basically an extension of the north wall of the greenhouse. This allows you to control the angle of the roof by adding or taking away dirt. Build up the berm continuing the slope that you used on the wall. If you’re using bricks – continue using them on the berm.
Building the Roof
Recheck your berm to ensure that the angle of the roof will be between 53° to 73°. Once you’re sure that the angle is right, you can begin laying your roof.
The most economical, durable material for your roof is 4-inch PVC pipe. Using PVC elbow pieces, joiners, etc, you can create a flat roof frame that will cover your underground greenhouse.
After you’ve created a PVC frame, lay it in place on the top of your hole. Then lay plastic sheeting across the top of the frame and make sure that it extends past the edge of the frame by at least 1 foot. This flap will prevent run off water from the roof from running back into the greenhouse itself.
You might even consider installing a rain gutter along the bottom edge of the greenhouse to catch the water that runs off the roof and direct it into a different location that won’t affect your greenhouse.
Once you’ve laid the plastic material on top of the roof frame, move inside and tack another layer of plastic wrap along the inside of the roof frame. This internal plastic sheeting will create a 4-inch barrier between the inside and outside of the roof. This will act as an insulator that will keep the heat in more effectively.
You’ll want to make sure that you leave a few inches of plastic hanging down on the lower (south) end of your roof. This will force moisture that collects on the roof to drip off above the drainage system or on top of your plants instead of at the base of the roof. If you allow the moisture to run to the base of the roof frame, it may affect the soil at that location and break down your wall, etc.
Ventilation is crucial to maintaining the proper environment for your greenhouse. You have several options to solve your ventilation dilemma such as: Installing two doors, one at each end; installing a vent roughly the size of the door at the top of the back wall; or installing a chimney at the center of the back wall.