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How to Build a Geothermal Subterranean Greenhouse

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High Times recently showed you Dragonfly Earth Medicine’s 15-foot plants in British Columbia; now check out how they did it!

A subterranean greenhouse is a great way to have continuous temperature and relative humidity levels throughout the whole year in your garden space. Native to Bolivia, the technique’s original name, walipini, means “place of warmth.” By digging a greenhouse down into the ground you gain access to what’s called the thermal constant. At five to seven feet below the frost line, anywhere on the surface of the planet, the soil will always be at a constant temperature. No matter how freezing or scorching the climate is the soil below you never changes come winter or summer.

The sun creates a dynamic when it hits the surface of the Earth called the flywheel effect. A thermal mass will collect heat from the sun all day, and release it at night. To capture the desired flywheel effect in the walipini you will want to watch the sun’s path throughout the year on your projected building site. You will need to map out the path and angle the sun follows during the winter months to help position your greenhouse so it receives the maximum amount of sun; the long ends directed east to west is optimal.

The deep, earthen side and back walls of the greenhouse will act as a thermal mass. Solar rays transmit heat and store it in the interior back high wall. Incorporating basalt rocks on the back wall will collect and hold the heat more efficiently for colder climates, while using white flagstone for warm climates will reflect the sun.

Before you hire a backhoe to dig the hole, make sure you are familiar with your water table and how deep it reaches at its highest point. Dig at least five feet above the water table and a minimum of seven feet down for the back wall in order to reach the thermal constant. Northern climates and desert heat climates can be dug deeper so the garden can withstand extreme surface temperatures.

Once your hole is dug, prepare the base of your soon-to-be raised garden beds with water pipes. Pumping heated water in the winter or cool water in the summer through the piping you create a regulated environment in your greenhouse. Do this by snaking black polycarbonate plastic tubing or galvanized piping up and down the base level of the beds. This technique can work in cold climates with a rocket mass heater, wood furnace with a water jacket or solar water heater to heat the greenhouse and reduce humidity.

Next, it is time to build the raised trench-beds, your wooden walkways and (our favorite part), build your soil. We really encourage gardeners to make long, wide trench beds. They are easy to build, easy to fill and are a great, no-till solution that will keep your soil healthy and nutrient-rich forever. Use planed wood or rounds to create the raised beds. Our planting beds are at least four feet deep with layers of soil material. We suggest a method of layering instead of tilling your beds. A cover crop of microgreens and grasses will replenish beneficial microbes and nutrients, keeping your soil healthy.

A few things that can go into your soil mix include straw, alfalfa, soil from other parts of your land, manure from Holy Cow and Madonna Cow, fall leaves (including those from cannabis), homemade cultures for beneficial microbe colonization, compost, composted grass clippings, ash and biochar from a wood stove, chipped cannabis stalks, rocks, forest floor materials, brown rice, our Dragonfly Earth Medicine inoculants, homemade bokashi, burdock and rhubarb root mash and apple pumice.

If you have to buy your soil, plan for layers of peat, coco, worm castings, alfalfa pellets, green material from your property, last year’s cannabis leaves, glacier rock dust, Himalayan salts, compost, green sand, pumice stones, nematodes, mycorrhizae, deciduous leaves from fall cleanup, EM (effective microorganisms), bokashi and DEM bacterial-rich herbal inoculants.

Building the beds deep will help to layer and cover crop, and creates a more defined cold sink in the aisles. Cold sink paths are important to have in between the beds. This keeps the cold, damp air from lingering around your plants. Jack Herer is good in your beds, but Jack Frost is not. These pathways should be dug at least two feet deeper than the bottom of the greenhouse. This work can be done with a pulaski and a flat shovel. Then, build a mini deck walkway at least 2 feet above the bottom of the aisle. A deeper trench is optimal for colder climates. Install fans blowing through your cold sink to circulate the cool air upward during hot days.

Now that you have the beds, soil and the walkways complete, you need to build a proper entrance. An easterly entrance is beneficial for colder climates. The entrance can be dictated by the lay of the land for warmer climates and flat topography. Prepare for the entrance to be along the side of the wall and dip lower than the bottom of the garden beds. This allows cold air to settle at the entrance, rather than inside the greenhouse. We molded the ground to create stairs descending into the growing space.

With your entrance put in place, it’s time to start your interior walls and roof. The roofline has to be at a 25°-35° angle (with a steeper pitch if you get snow), and its angle will dictate how long your posts have to be. Dig postholes and use wood, bamboo, brick or metal, depending on local resources and wind speed. If you have strong winds, bamboo will not be an option.

Four to six-inch round conifer saplings make for great posts. If you are going to use whole saplings, make sure to de-bark the whole sapling and char the last three feet that will go into the hole to protect the base from rot. To char, build a mini bonfire and burn the ends in the coals to create a black coating. Dig post holes two to three feet deep. Reinforce the pole with concrete or heavy rock in the hole before you fill with topsoil to make a solid anchor for the entire greenhouse. Posts should be about 10 to 12 feet apart down the interior center for large greenhouses and four feet apart on the exterior walls.

When you are building your base frame, build it two to three feet above the ground for airflow and snow pile.  Once your sides are built, roof rafters are the next step. Areas that get a lot of snow need more rafters; in this case make them three feet apart. Zero snow areas can be four feet apart so the roof lets in maximum sunlight.

Investing in polycarbonate for the sides and roof will offer a higher insulation value and last longer than its poly-weave counterpart. For extra insulation, you can make a semi-permanent second layer of poly-weave you can use in late fall. Roll up the double layer after the nights are at a minimum of 50°F in the early spring. With this added protection you can make year-round growing a reality, even in the most northern climates!

When you lay out the plastic, make sure it’s clean, pulled taut and fastened with a greenhouse interlock system to reduce any holes. Do not fasten down the overhanging plastic on the sides; this prevents warm weather airflow. Use alfalfa or hay bails along this outside short wall for extra insulation in the winter. Throw weathered bales on top of the garden beds in the spring. We do a simple roll up of the poly weave for air flow and keep it open during the whole summer growing season and close it up in the colder months.

The greenhouse will look very inconspicuous because it only stands two to three feet off the ground. If you want to cover it with hard polycarbonate, make sure your dimensions fit for easy roll-out fastening. You can easily create a blackout system inside for light deprivation (forced 12 hour dark and 12 hour light cycle to force the plants to flower) or pull a tarp twice a day to get several harvests a year. Plants in their vegetative cycle can be moved outside and transplanted into the garden beds for natural light cycle flowering. Many regions can get five full flowering cycles a year with proper blackout technology and low-cost supplemental lighting during darker seasons. Northern climates need supplemental light for healthy growth in the dead of winter.

Not only does geothermal solar technology cut out all the high costs associated with flowering plants, it decreases your footprint on the earth. The cost of running lights keeps  medicine too expensive, especially considering this ever-growing market. Sun-grown herb creates higher-quality medicine at a lower cost, because nothing grows plants better than the sun. Geothermal subterranean greenhouse technology will be the future of this industry because of its ability to gather natural resources and provide optimum growth. 

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