Organic Broadcaster

University partners with local farmers on greenhouses for winter production

By Greg Schweser, University of Minnesota Extension

On a cold day in late November, the Organic Consumers Association (OCA) is planting a winter crop of baby greens and kale in a University of Minnesota Deep Winter Greenhouse. The OCA, located in the small northern Minnesota community of Finland, is testing whether it is possible to profitably grow crops in Minnesota winters with as little fossil fuel heat as possible.

What are Deep Winter Greenhouses? DWGs, as they are often called, use passive solar heat collected through a south-facing steeply sloped polycarbonate glazing wall. That heat is stored in an insulated underground thermal mass made of crushed river rock and is available in the above growing area during the colder night hours. Backup heat may be required to kick in when the ambient temperature drops to 35 degrees. The result is a growing environment ideal for production of brassicas, lettuces, Asian greens, and sprouts and shoots—crops that tolerate and often thrive in low heat and low-light conditions.

Through its Deep Winter Greenhouse initiative, the University of Minnesota Extension’s Regional Sustainable Development Partnerships (RSDP) and five producers throughout the state are testing a DWG prototype. (The plan for this prototype is available to the public for free download at Partners include the above mentioned OCA; the Bemidji Community Food Shelf Farm; Grampa G’s farm in Pillager; Alternative Roots Farm in Madelia; and Lake City Catholic Worker farm.

These DWGs will not only be available for producers to grow crops for winter CSAs and food shelves, but also as facilities for University research, public outreach, and workshops. With a DWG in each region of the state, Minnesota farmers interested in experimenting with the technology will be a short trip away from an operable DWG and experienced producers who will be able to answer questions and provide advice.

The DWG initiative has its roots in a 2009 UMN collaboration with Carol Ford and Chuck Waibel, a pioneering husband-and-wife team from rural Milan, Minn. Ford and Waibel sought a sustainable greenhouse production method that could withstand dramatic variations in fuel prices, be resilient to a changing climate, and withstand the brutal prairie winds that too often devastate hoophouses. They also wanted to grow crops all winter long, not just in the shoulder seasons around summer. Waibel researched old passive solar greenhouse designs made popular in the 1970s energy crisis, and constructed a small greenhouse on the south side of his garage. Ford, the green thumb of the couple, started up a winter CSA using the greenhouse. Eventually, their work caught the interest of the RSDP, which helped the couple publish The Northlands Winter Greenhouse Manual as a way to teach others how to build and grow in a DWG of their own. (The manual is $20 through the MOSES Bookstore:

This greenhouse, attached to Carol Ford and Chuck Waibel’s garage, uses a fan to push hot air from the top of the building to heat a rock bed under the soil floor.

Several years later, a growing group of DWG producers began to sprout up around the state, offering each other advice and assistance through the Sustainable Farming Association’s Deep Winter Producers Association. Ford worked with the RSDP to identify opportunities to increase awareness and support for the emerging group of winter producers. Ford and the RSDP envisioned creating a stand-alone prototype design that could be utilized, free of charge, to anyone interested in building a DWG on their farm. The University of Minnesota (UMN) could collaborate with producers in each of the RSDP’s Greater Minnesota service regions to build and field test this prototype. This innovative community-university partnership would provide an interactive and educational opportunity for anyone interested in DWG production to come “kick the tires” and meet a DWG producer first hand.

In 2015, architect Dan Handeen of the UMN Center for Sustainable Building Research visited the DWGs and conducted performance tests to identify opportunities to improve on the design. A considerable design change that emerged from Handeen’s research was changing the way hot air fills the solar mass rock bed.

The original Ford/Waibel greenhouse is a thermal mass design that collects hot air near the ceiling of the greenhouse, which is then pushed into the ground with a fan, and dispersed with a system of perforated drainage tile buried in the rock bed. Loose soil placed atop the rock bed allows the hot air to escape into the ambient above-ground space.

For greater efficiency, the new prototype seals off the rock bed with a hardpan surface, reverses the direction of the fan to pull rather than push the hot air into the ground where it fills the rock bed (without a system of perforated drain tile connected to blowers). Cool air is drawn out of a manifold near the base of the greenhouse creating a circulating system that keeps the thermal mass charged with hot air. As necessary, excess heat is vented out of windows on the side walls to prevent overheating.

With cost-share funding from several agricultural lending banks (Agribank, Compeer Financial, and AgCountry Farm Credit Services) and a newly improved DWG prototype, the RSDP released a request for proposals and received 40 applications. Five partners were chosen, one in each RSDP service region. To receive funds, producer partners agreed to allow the University of Minnesota the opportunity to host open houses and workshops, as well as provide a portion of space for research trials.

“We are excited to use the DWG to contribute to the evolution of our farm,” said Brooke Knisley of Alternative Roots Farm. “We will be changing our structure to focus less on summer production and more on fall and winter production with apples and, now, the winter greens. This will help us keep in touch with our customer base throughout the Minnesota winters.”

Construction Costs
The first project came with sticker shock. The project team had estimated construction costs by getting quotes for all of the items on the materials list from big box stores and greenhouse supply companies. The team estimated as double the materials costs. All said, DWGs should cost about $30,000 plus electric hookup and water delivery, which could vary dramatically depending on the distance from available electricity and water sources.

OCA was able to construct its prototype during the winter of 2016-17, months before others were able to start construction, and thus served as project guinea pig. After all was said and done, the price tag was slightly more than $65,000. The team poured over OCA project receipts to find a culprit for the higher-than-expected costs. Although material costs were slightly higher than expected, labor and related expenses (delivery) on the North Shore were much more than expected.

The Organic Consumer Association manages this deep winter greenhouse near Minnesota’s North Shore.

The next prototype was built by Brooke and John Knisley of Alternative Roots Farm near Madelia, Minn. They also contracted out all of the labor and had project costs just under $40,000. Still higher than expected, this price tag brings the DWG initiative closer to the goal of being a cost-effective investment in winter production. The Knisleys attribute some of the excess charges to higher-than-expected delivery charges.

The Bemidji Community Food Shelf Farm, which broke ground in mid-2017, was able to save on some of their labor costs through donations of time. Their final project costs came in at just under $22,000. The Bemidji Food Shelf Farm will also reduce costs by utilizing rain barrels filled from a nearby well, rather than have water available directly inside the greenhouse.

The final two DWGs at Grampa G’s and Lake City Catholic Workers Farm will be constructed primarily without hired labor, so project costs for those are expected to be less.

In response to those higher-than-expected project costs, the UMN team is working with established DWG producer Sue Wika of Paradox Farm to identify opportunities to cut costs from the DWG prototype. Wika constructed her DWG on the south-facing side of a milking barn, extensively utilizing salvaged materials or materials that were found on her farm. With an understanding that decreasing project costs will have tradeoffs (e.g., reduced building longevity or performance), it is essential that farmers interested in utilizing DWG technology have affordable options. What will emerge from research and experience will be a range of expected construction costs that vary depending on a site location’s available labor supply and capacity of farmers to provide that labor.

Research Trials
Now that several of the prototypes are completed, UMN researchers Mary Rogers and Claire Flavin will begin horticultural trials in the 2017-18 winter production season. While a list of powerhouse Asian greens like Tokyo bekana, mizuna, and giant red mustard grow well in the DWG, other popular crops like spinach and arugula have shown a little more sluggish growth. Although these crops may be popular with customers, filling up valuable growing space with slower producing crops reduces overall yields. Researchers will test new
varieties of such crops to determine which are suited for cold-climate DWG production.

Researchers also will tinker with seeding densities and substrates to determine if boosts in yield can be achieved. Currently, crops seeded in hanging gutter systems are densely seeded and allowed to grow to a baby green level. After a first cutting, new access to light presumably allows sluggish smaller plants to thrive and grow alongside the previously cut crops for a second or third cutting. While this works well for many crops, some crops, like arugula, have reduced yields on second and third cuttings. Trials with variable seeding density and substrate mixes will help determine whether specific growing practices can be beneficial for specific crops to boost overall production yield.

Concurrent with the horticultural research, UMN Extension Educator Ryan Pesch will survey existing DWG growers to determine production costs and revenues to gain an overall picture of the enterprise value for the DWG system. Pesch conducted a similar study in 2015 and determined that average net revenue for DWG producers was $1,862 for a season with a range of a loss of $526 to a gain of $4,491. Labor hours per week from all but one (who worked 45 hours/week in the greenhouse) ranged from 7-16 hours. A major weakness of this study was that only six DWG producers were interviewed, some at educational facilities that were not producing with a profit motive.

Existing figures and anecdotal accounts suggest that DWG production is optimal for small-scale farmers as a part-time winter add-on to an existing farming enterprise. With more DWG producers up and running since the first study was conducted, Pesch will be able to get a more detailed picture of what kinds of economic returns one might expect with DWG.

The UMN recognizes that efforts need to be made to maximize production capacity of the DWG through better use of three dimensional planting areas, boosting product yield, and identifying “off season” uses of the system (e.g., food dehydration in summer). Above all, the University of Minnesota is most interested in providing prospective DWG producers with the knowledge they need to make an informed decision about setting up a winter production system.

Greg Schweser is the RSDP Statewide Director for Sustainable Agriculture and Food Systems.


From the January | February 2018 Issue

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