The thought of growing trees heavily laden with nectarines, peaches, figs and even persimmons in northern climates seems the stuff of fantasy. Yet, by constructing a greenhouse deep enough to utilize the earth’s consistent temperature, it’s possible to grow sub-tropical plants without an additional heat source in regions where sub-zero temperatures are common.
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In-ground greenhouse design is not new. It dates back to the late 1800s in this country, but fell out of favor until recently. Homeowners have rediscovered it as they look for ways to provide more of their own food without relying on outside sources.
Over 35 years ago, John Hemighaus wanted to see what was possible to grow in an in-ground greenhouse. Fueled by a keen scientific mind and plenty of muscle, he built his private passive solar greenhouse called the Ott-Kimm Conservatory. It boasts a Zone-8 climate in the Zone-4 area of Montana’s Gallatin Valley. “I did this for my own research and study, and to feed my family,” John said. He hand-dug the greenhouse with a base 5 feet below the ground’s surface, and poured 6-inch-thick reinforced concrete walls.
Within the greenhouse, he created an enclosed ecosystem. Pacific Northwest tree frogs are one of the tiny predators that keep insect levels in check, and the natural cycle of plants, fungi, bacteria and insects maintains the soil balance. His experiment and labor of love worked like a charm. “If it’s done properly, it will feed a modest family in perpetuity,” he said.
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After graduating with an ecology degree, Zach Weiss of Bozeman knew he wanted to help people grow their own. The realization of what could be done without creating an energy- intensive setup took shape when he worked with John in the conservatory.
“That’s the place where I learned 80-percent of my greenhouse knowledge,” Zack said.
He now uses his been-there-done-that experience to create sustainable food systems through his business, Perpetual Green Gardens. He builds in-ground greenhouses for clients, as well as providing Sepp Holzer-style permaculture and earthworks services to create perpetual foodscapes.
Anatomy Of A Geo-Greenhouse
The size of an in-ground greenhouse is an important consideration. Having adequate air volume matters because small spaces heat and cool too quickly. There’s also not enough area to maintain a viable population of beneficial insects.
“It’s nice to have 600 square feet or more,” said Zach, particularly if you’re trying to create an ecosystem within the structure.
The cost of the structure depends on the materials and level of manual labor. It can be dug by hand, although a backhoe or excavator makes much faster work of the project. The price depends on whether you have the capability to rent one and do it yourself, or whether you hire out the project.
What to use for walls within the ground is the next consideration. Pouring concrete walls reinforced with rebar on a proper footing is a feasible method in any part of the country. It’s also the most expensive.
Depending on the cost of concrete in your area, as well as the size of your walls and depth of the greenhouse base, pouring concrete might cost between $3,000 and $7,500.
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Earthbags, which are solid-weave polypropylene bags (picture a sandbag), are another option, particularly if you have a fair amount of time and manpower. Depending on the height of your walls, you might need 1,500 to 2,000 bags for a 600-square-foot greenhouse. Subject to your location, Earthbags cost between $150 to $250 per thousand bags, so you can buy these basic materials for a few hundred dollars. The fill is free since you’ll use what you excavated from the green- house base. It just takes a whole lot of time and effort to fill and stack them.
For Earthbag construction, Zach adds rigid foam insulation to the exterior. He coats the bags with cement on the interior to prevent UV rays from degrading them.
“You can also use ICFs [insulated concrete forms] made from 85% recycled material‚” said Zach. With an insulation value of R20, the recycled forms are efficient and don’t use as much concrete as solid poured walls.
The greenhouse base can also be constructed with cinder blocks, brick, native stone or any other building material used for below-grade construction that will safely hold up the sides of the excavated earth. A French drain around the base is necessary to channel water away from the walls to keep the greenhouse from flooding.
Hoops Or Wood Frame?
To build the above-ground frame, you can use metal hoops, such as those used in high tunnels or hoophouses, or you can build a wooden frame.
Coverings range from 6-mil green- house plastic to solid panels, glass, or a combination of materials. Use what you can afford or what you have on hand. Incorporating tilt-out windows is highly beneficial since heat-activated window openers can be lifesavers during the hot months.
Zach prefers Solexx greenhouse panels for the glazing. “These double-wall, flexible polyethylene panels make sense in harsh climates.” he said.
In his greenhouses, Zach uses entirely passive ventilation. He has no fans or electricity. Heat-activated venting arms along the ridge of the greenhouse allow the hot air to escape, which creates negative pressure in the greenhouse that draws air in through earth tubes made of 4-inch perforated drainpipe.
“Earth tubes are a very simple concept,” Zach explained. “Air flows from outside through the tube underground below the frost line, then into the greenhouse. While passing through the earth, the air is gaining or losing heat depending on the season. In the winter, the temperature of the earth warms the incoming air; in the summer, it cools it. Nature provides its own heating and cooling system.”
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Below the frost line, the earth maintains a constant temperature. This temperature is the mean annual temperature for a given location. For most places in the U.S., it ranges from 50 to 65 degrees Fahrenheit.
Zach emphasized the importance of using perforated pipe. “Moisture will condense in the tubes when hot humid air is drawn underground and cools, releasing water vapor. If you use pipe that is not perforated, this moisture builds, mold grows on it, and then you get all sorts of fungal problems.”
Growing Good Soil
When John built his conservatory one of the things he wanted to learn was how long it would take for the soil to collapse—to lose the nutrients and minerals—because he wasn’t bringing in outside amendments, as is the standard practice for most gardens.
By allowing the mix of vegetation within the structure to mimic the natural cycle of life and death, he discovered that the nutrients were renewed. Fruit dropping to the ground feeds bacteria, and falling leaves and tree prunings serve as mulch. The native rocks placed along the perimeter of beds react with the acidic rainwater, which he uses for irrigation instead of well water. Zach explained the benefits, “The acidic rainwater breaks mineral bonds in the soil and rock used for the beds. This de-mineralization, triggered by rainwater, releases manganese, calcium, iron and other important compounds into the soil in a plant-available form.”
Zach is following the same path. Using the topsoil from the site to start the beds, he creates more soil without bringing in anything additional.
“The first thing we want to do is to build soil,” he said. He has a poly-culture of plants with various growth habits and root depths. The clover fixes nitrogen in the soil, the corn roots reach deep in the soil to draw out nutrients and other plants add biomass that will increase the organic content.
Among the mix of soil builders, Zach has vegetables, herbs and fruit trees growing. And, just like many gardeners, he was concerned when an over-abundance of aphids started feeding heavily on the plants. Zach says he sprayed nothing, not even essential oils or organic pesticides, on the aphids to reduce their populations. Nature needed to take its course.
So when he noticed tiny worms in the greenhouse, his initial thought was that they were some other type of pest.
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“They were actually the young stage of hoverflies,” he said. “First they got the aphids on the ground, and then hatched out and took to the air, devouring the aphids on the plants.” Now the number of aphids in the greenhouse is well within balance. They don’t do significant damage to any of the plants yet provide food for the natural predatory insects.”
Part of mimicking the natural cycle is capturing the rain to water the plants. John said that if he waters his acid-loving plants, such as blueberries, with the well water, they will be dead in short order. Montana soil leans toward the alkaline end of the pH spectrum, and watering with well water compounds the situation. Rain catchment systems on both greenhouses include one or more 1,500-gallon water tanks. Zach estimates that he needs 3,000 gallons of rain to make it through the dry months of summer, when he might not see any precipitation. In 2013, John saw only 4 inches of rain until the end of August.
“Humidity is a big part of the water,” says John. Plants take up water through their leaves as well as their root zone, reducing the amount required. John uses roughly 250 gallons of water per week, which is minimal, considering the amount of vegetation and extreme heat of the summer.
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Both systems require a dormant period of two to three months during the winter. Some fruit trees require the chill period to produce, and the cold helps to keep insect populations in check. Many winters they must open the greenhouses to allow in enough cold air to chill them and create frost.
By using the natural temperature of the earth it is is possible to create a fruiting oasis even when the weather doesn’t cooperate. “If we think ahead and partner with nature rather fight it, we can create viable healthy ecosystems that are perpetually productive,” said Zach.
Fore more information on Perpetual Green Gardens, visit:http://www.perpetualgreengardens.com/