Jerome Osentowski, director and founder of the Central Rocky Mountain Permaculture Institute, shows how to design and build successful greenhouse projects in The Forest Garden Greenhouse (Chelsea Green Publishing, 2015). Learn how to use passive and active solar heating strategies to create tropical or Mediterranean climates even at high altitude or in cold climates. The following excerpt is from chapter 2, “Expanding the Possible with Season Extension”.
You can buy this book from the GRIT store: The Forest Garden Greenhouse.
In the sections that follow, we describe cloches, hoop houses, domes, gutter-connected greenhouses, and four-season greenhouses. By combining solar collection, thermal mass, and insulation to capture and store energy, we can effectively grow a much wider variety of plants, even in heavy snow country.
We are also learning how the underground soil mass can even out the temperature swings of both cold and hot climates to provide a supportive growing environment for the food plants we want to eat. As we write this text, we are designing a climate battery for a greenhouse in the hot desert Southwest that uses the same principles in reverse. That greenhouse will draw daytime ventilation air from a shaded orchard area through cooler underground pipes before it enters the greenhouse, with added water-fogging evaporators on really hot days to help maintain moderate inside temperature and humidity levels. For a future dependent on renewable energy and constrained supplies, we need to conserve as much as we can. Growing a wide variety of food crops locally will save transportation energy, keep resources within the community where they are generated, and help create unique regional cuisines and artisan cultures.
Let’s get growing!
The simplest form of crop protection is a cloche, or fabric-covered hoop, erected over a bed of plants. The simplest form of cloche is just a layer of floating row cover, a porous, nonwoven fabric, laid over an area of seedling plants and held down around the edges with rocks or tent stakes. Sunlight passes through the white fabric to nourish the plants, and heat builds up in the soil to keep them warm at night. This can even provide 4 degrees of frost protection (at 29 degrees outside, the plants inside the cloche will remain around 33 degrees). The fabric can be supported on sticks in the soil when the seedlings are young. And as the cover is very light, the plants will lift it as they mature and spread out. Using heavy wire or thin plastic pipe, you can create a “hoop” cloche that will support the fabric above the seedlings, providing them ample sunlight and air space for growing.
When the danger of frost is past and your plants are ready to be in direct sunlight, you can remove the fabric during the day, tucking it in again for the night. Eventually you can leave it off at night as well, until the danger of frost returns in the early autumn. Here in the Rocky Mountains, we are typically able to keep summer squash and even melons protected from frost well into October using cloches. This allows us to extend the normal three- to four-month growing season to five or six months.
These structures are like cloches except that you can walk under them. They are often used over cloches to gain another layer of protection. In our Rocky Mountain climate, this would push the growing season out to seven or eight months, or even to twelve months with a climate battery and winter planting of cold-hardy greens. Sunny sites in USDA zones 6 and warmer will be able to grow year round using this combination. In warmer outdoor zones, hoop houses support an ever wider variety of crops through the winter.
There are many examples of hoop houses, or “high tunnels” as they are also called, which you can buy as kits and erect yourself. The “Rolling Thunder” greenhouse, made by Rimol Greenhouse Systems in New Hampshire, is the high tunnel inspired and favored by Eliot Coleman of Four Season Farm in Maine because it rolls on rails, allowing him to start a crop under the structure, move the hoop house to the next space when the first crop is ready to grow outside, and start another crop under cover again. Instead of propagating plants in a greenhouse and moving the plants outside, the Rolling Thunder greenhouse makes it easy to move the structure.
Most hoop houses use natural ventilation by providing roll-up sidewall covers, but a few of them have vent openings along the top where the heat collects. If you use one without a top vent, the structure will be cheaper to buy, but you may have to install high-volume fans to keep the hoop house cool in the hottest months of summer. Shade cloth and other light-scattering covers can also be applied to reduce summer solar heat gain.
Hoop houses may be fitted with double-inflated poly greenhouse film over the frame for additional insulation value in cold climates. This method involves stretching two layers of plastic film over the hoop house frame, sealing all edges of both layers together with continuous clamps to the frame, and installing fans to inflate the air space between the layers of film. The still air between the layers increases the insulating value of the cover. You may also cover the roof of your hoop house with rigid polycarbonate panels. These are more expensive, but they last longer and can provide a better insulating value than film. Even on a hoop house covered with double-inflated poly, the end walls are often finished with polycarbonate panels because these flat, rigid surfaces are easier to fit with swinging or sliding doors than is film. You can easily mix double-inflated poly film roof covering and rigid polycarbonate panel end walls on the same structure.
A farmer, realizing success from longer growing seasons with a hoop house, might want to buy another but would soon discover one of the disadvantages of simple hoop houses: They can’t be connected side to side to make a large space like gutter-connected greenhouses can. In snow country, hoop houses and high tunnels must be separated far enough to allow snow to collect between them when it slides off the roofs. Lightweight greenhouse structures must be designed to shed snow, which could otherwise collapse them. And this snow must go somewhere on its own. Even if they could support the weight, simple hoop houses don’t have gutter connections that would collect snowmelt.
The result is that the grower can only enter each hoop house from an end. It may be possible to gang multiple structures end to end, but this makes for very long working paths. Even with lateral separation, the architecture of the hoop house makes it much more like a tunnel than a large sunlit warehouse. We dream of a hybrid system between the economy of the hoop house and the flexibility and water catchment of the gutter-connected greenhouse system.
In spite of these limitations, there are serious four season farmers, like Eliot Coleman and Barbara Damrosch in Maine, who raise beautiful produce and flowers around the year with movable high tunnels, supplemented by row covers inside.
Buckminster Fuller invented the geodesic dome, an extremely efficient form from both energy conservation and structural performance perspectives. The dome relies on triangular and spherical geometries to create an enclosure that will support heavy loads of snow with a great deal less material than a conventional rectilinear structure. A spherical shape also has the advantage of enclosing the maximum volume of space with the minimum surface area, making the dome the most naturally energy-efficient shape possible. This combination of structural and energy-conservation efficiencies makes the geodesic dome an ideal greenhouse.
In the 1980s, at a research institute in western Colorado called the Windstar Land Conservancy, Buckminster Fuller designed a greenhouse dome, a structure he called the biodome. The Windstar biodome was 50 feet in diameter and, in addition to the ground area, contained two lofts for growing food and two large water tanks for raising fish, holding fertilizer, and thermal storage. That greenhouse experiment went on for most of a decade until the grant funding ran out. The people operating it then applied what they had learned to develop better indoor growing systems in other settings. One of these is Udgar Parsons, who was the production manager at Windstar. He began building greenhouse domes in 1989 in the Roaring Fork Valley under the company name Growing Spaces, LLC. He and his wife Puja moved their operation to a factory in Pagosa Springs, Colorado, in 1995, where they continue to build six sizes of their Growing Dome. These range from 15 feet to 42 feet in diameter, satisfying year-round growing needs for families, schools, and community groups.
Domes have significant advantages in the real world of energy conservation. I like to think of the material savings this way: a 42-foot-diameter dome covering 1,400 square feet of growing space uses less wood than is in the bottom two layers of logs in a 1,400-square-foot log home! And the material it does use is of smaller dimension, making it much more economical to build.
A dome is extremely wind resistant because of its aerodynamic shape. The force of consistent high wind is distributed uniformly over the triangulated frame, holding the structure down rather than blowing it away. Rectangular framed greenhouses require heavy trusses and significant cross bracing to maintain their structural integrity under large wind loads; this construction makes rectangular greenhouses inherently more expensive.
Finally, the dome geometry provides a cover with minimal surface area and maximal volume, making it the most energy-efficient enclosure we can create. Any other shape will have a higher surface-area-to-volume ratio, resulting in more heat loss to the outdoors. One disadvantage is the potential for increased breakdown and maintenance because of the large number of seams between triangular glazing panels.
In spite of the many advantages, though, dome structures come with a cultural disadvantage we call the “hippie stigma” because of their prominent use among young counterculture practitioners in the 1960s. If Buckminster Fuller had invented the geodesic dome well after the emergence of hippie culture in the 1960s and ’70s, the geodesic form might be more widely accepted as an efficient structural solution to many space-creation challenges, including greenhouses. Fortunately, though, school-age children of the early twenty-first century carry none of this cultural baggage, making the growing dome an ideal greenhouse solution for Edible Schoolyard projects.
We have helped organize our own community to raise the resources to build growing domes and outdoor gardens at two local high schools. These domes are functioning beautifully as science labs, and they double as local sources of fresh produce for the school cafeterias. In the Rocky Mountains, we like to say there are two ways to teach agricultural biology in our schools: either change the school year to run from February to November or build greenhouses. Schools in most temperate regions face the same choice, and the only acceptable choice compatible with our cultural calendar is to build greenhouses.
To include every kind of plant in our AgBio greenhouses, including tropical perennials, we build climate batteries underground before we lay the foundations and erect the domes over them. When we experienced our first deep cold wave this past winter, with several nights in a row dropping below -10 degrees F, the solar heat we had stored underground helped to keep the climate inside the domes in the high 30s, nearly a 50-degree difference from outside, and nearly warm enough to keep bananas, papayas, and everything else from suffering damage. We do use some backup heat when outdoor temperatures dip below 10 degrees F, but the climate battery handles weather extremes down to that point, saving over 75 percent of the energy conventionally provided by fossil fuels. As with all greenhouses, these efficiencies depend on the realities of the site and the elements included, which are specific to each greenhouse and can be endlessly refined.
Reprinted with permission from The Forest Garden Greenhouse by Jerome Osentowski and published by Chelsea Green Publishing, 2015. Buy this book from our store: The Forest Garden Greenhouse.
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