How to Build a Secret Underground Greenhouse

by Karen

One of the most important aspects of survival after the collapse of society is how to ensure you and your family have enough food. You may have a few months of food stored away, but eventually that will run out and you will need to hunt, forage, and grow your own food. The great thing is you don’t have to wait for the collapse to get started.

Growing your own food will allow you to develop the skills you will need post-collapse and it will give you the chance to save money on your grocery bill. With the cost of food on the rise, this is important. There are so many advantages of an underground greenhouse, that you might be surprised. You might also be surprised at how easy it is to build and maintain one.

Advantages of an Underground Greenhouse

An underground greenhouse offers plenty of advantages over a traditional greenhouse. The first and perhaps most important of these is the fact that you can keep it hidden, particularly if it is built into a natural hillside or slope. When it comes to survival post-collapse, keeping your source of food hidden from the roaming eyes of others is an advantage. Other advantages include:

Visual appeal: Some people feel that having an underground greenhouse looks better than having a traditional above-ground structure.  The idea of not having a large, bulky structure on their property makes it nicer in terms of visual appeal.

Insulation: When the greenhouse is set beneath the surface, the thermal mass of the earth acts as an insulator. A greenhouse set between 6 and 8 feet below the surface will keep a consistent temperature through the winter and the summer. In the winter, the temperature can easily be kept between 50 and 60 degrees Fahrenheit (10-15 degrees Celsius), which is warmer than above-ground greenhouses. In the summer, the cool earth keeps the temperature from getting too hot. This makes the greenhouse ideal for any climate and any season.

Versatility: Besides the fact that an underground greenhouse can grow food, it is also useful if you have livestock. There are a few types of livestock that can benefit from being in an underground greenhouse, particularly in the winter. This includes chickens, sheep, and goats. If you have livestock and enough space for a larger greenhouse, then it makes sense to build a greenhouse big enough for the animals.

Protected environment: An underground greenhouse is far less susceptible to pests and chemicals. Chemicals that are sprayed nearby won’t end up on your plants and pests, including insects and mammals (deer, rabbits, etc.), won’t have easy access. That said, you need to be sure pollinators have access to your crops. If they don’t and you are growing food that is not self-pollinating, then you will need to hand-pollinate.

Low maintenance: Since the greenhouse is buried, most of the structure consists of the earthen walls surrounding it and the ground as the floor. For this reason, there is very little structure that actually needs to be maintained. The garden beds have less exposure to the elements and will last longer. The only part that will require regular maintenance is the roof. the frame and plastic or panels can be replaced/repaired as necessary.

Design

There are a number of different designs when it comes to building an underground or pit greenhouse. The early sun pit was a design from 1881 that used the effect of the sun heating the ground or dark stones to heat the greenhouse. The walipini is a popular underground greenhouse design originating in South America that takes advantage of the winter sun. here is a design analysis of a walipini greenhouse. (https://www.youtube.com/watch?v=i79bE7a-f8g)

Regardless of the deign you use, it is recommended that your underground greenhouse is 8×12 feet in size. Of course, you will have to base the size of your greenhouse on the amount of space you have available and the amount of space you need for planting and walking inside the greenhouse. Here are some general tips when building an underground greenhouse.

It is important to put your underground greenhouse in the right location. When scouting a location:

• To keep your greenhouse out of sight and secret try to place it in a location that is not highly visible from major routes of travel, such as a roadway or trail.
• Make sure there isn’t too much tree cover or shade from buildings because your greenhouse needs light.
• You want the greenhouse to run east-west with the roof facing south to capture the sunlight (facing north if you are in the southern hemisphere).
• Plan your greenhouse so that you have a way in and out of the pit, whether that is a ramp or stairs.

Once you have determined where you will put the greenhouse, you need to dig a great big hole. When excavating:

• Dig down to a level 2 feet below the planned depth of the greenhouse.
• Ensure the walls have a slope of at least 6 inches between the roof and the floor.
• Clay, bricks, or any other material that will trap heat can be used to build and stabilize the walls and keep them from caving in.
• Save the deeper excavated soil to build up the north wall of the greenhouse, creating a berm, and the top-most excavated soil for the floor of your greenhouse.

Once you have your hole and your berm, you will want to plan for drainage. When planning drainage:

• Fill the extra 2 feet you dug down with gravel, courser at the bottom and finer at the top.
• Lay 8 inches of soil on top of the gravel.
• Ensure the perimeter has 2 to 3 feet of gravel, which will improve drainage.
• Ensure the bottom of the greenhouse has a slight slope from the middle to the edges to ensure good drainage.
• Open gravel wells in the corners are an optional feature that will provide a method of water collection.

If you are planning to install an irrigation system, do it at this point, digging the trenches and installing the hoses/piping. Here is one way to install an irrigation system:

• Dig trenches throughout your garden, making them a few inches in depth.
• Ensure all trenches feed into a single trench located at one end of the garden.
• Lay PVC piping and/or soaker hoses in the trenches.
• Ensure all hose ends are capped and all PVC connectors are sealed and their holes stopped up with rubber cement.
• Avoid connecting more than total of 100 feet of pipes/hoses to maintain adequate water pressure.
• Drill 1/16-inch holes into pipes and any hoses that don’t have holes and space the holes in groups about 5 feet apart.
• Install a backflow, timer, and pressure regulator to the faucet, making sure to connect the hose to the pressure regulator.

Once you have the irrigation and drainage taken care of, you can install the door(s). When planning the door(s):

• Build your ramp or stairs at the appropriate location.
• Build the door frame at the base of the ramp.
• If there are any gaps around the frame, fill them with clay or soil to prevent heat from escaping.
• Secure the door frame to the wall of soil by drilling holes into the sides of the frame at the top, middle, and bottom and using dowel, rebar, or stakes to attach the frame to the wall.

Finally, with the door(s) in place, you can install the roof. When roofing:

• The berm should be situated so the angle between the top of the roof and the berm inside the greenhouse is anywhere between 53 degrees and 73 degrees.
• Add 23 degrees to your latitude to determine the angle of inclination for your roof (the ideal is that the roof is perpendicular to the rays of sunlight on winter solstice).
• The frame can be constructed from wood, 4-inch PVC pipes, or the type of metal hoops used in hoop houses.
• The roof frame can be covered in greenhouse plastic or polyethylene or glass panels, ensuring that the covering extends past the edge of the frame by at least 1 foot on all sides.
• Plastic sheeting can also be placed on the underside of the roof frame.
• A rain gutter can be installed, placed along the bottom edge of the greenhouse, that will catch the runoff from the roof and direct it away from the structure.

Every greenhouse needs the right amount of ventilation, so it is critical that you include ventilation in your plans. The four ways to add passive ventilation (without the use of electricity or fans) to your greenhouse is as follows:

• Install two doors, one at each end of the greenhouse.
• Build a chimney into the midpoint of the wall at the back of the greenhouse.
• Build a door-sized vent at the midpoint of the top of the wall at the back of the greenhouse.
• Build drain pipe venting arms that are run from outside, through the earth, and into the greenhouse, allowing the temperature of the earth to regulate the temperature of the air entering and leaving the greenhouse.

Plants

When it comes to what you can grow in an underground greenhouse, anything that you can grow above ground in the growing season or anything you can grow in a traditional greenhouse is fair game.  This includes, but is not limited to the following.

• Tomatoes
• Eggplant
• Herbs
• Peppers
• Potatoes
• Beans
• Radishes
• Carrots
• Onions
• Broccoli
• Fruit-bearing bushes and trees (even citrus)
• Winter and summer squashes
• Melons

Through the cooler winter temperatures, you can grow cool-weather crops, such as:

• Kale
• Cabbage
• Spinach
• Peas

If you have the space, the ability, and the drive to build an underground greenhouse, you won’t regret it. This is a super way to be able to grow food all year round and keep that food off the radar because it is simply less conspicuous.  For more information on the process check out this video series:

Happy building!