We are in the “depths” of winter in northern Minnesota, even though we have just passed Groundhog Day (or Imbolc if you follow earth-based nature festivals), which is the half-way point between the Winter Solstice and the Spring Equinox. We still have plenty of winter left. Our outdoor growing season is long gone but leave it to Minnesotans to invent a way to grow greens in the winter using materials you can easily get up here: lumber, glass, big rocks, dirt, and sunshine. The Deep Winter Greenhouse (DWG) has gone through several re-designs, but it all started with a book by Carol Ford and Chuck Waibel in 2009: The Northlands Winter Greenhouse Manual. They lived in Chippewa County, Minnesota, several hours south of here, but the idea caught on with growers in the far north too.
As part of a statewide initiative, the University of Minnesota Regional Sustainable Development Partnerships (RSDP) have worked with the College of Design’s Center for Sustainable Building Research to develop construction documents for three DWG designs. UM Extension has conducted ongoing studies of each successive design. The RSDPs funded the construction of five models of the first design, DWG 2.0., in conjunction with community partners. They were completed in 2017-18, so they’ve been operational for awhile. The closest ones to us are in Finland (at the Organic Consumers Association’s Agroecology Center) and in Bemidji (at the Bemidji Community Food Shelf).
So, what IS a DWG? It is a passive solar greenhouse that captures the sun’s light and heat during the day and stores the heat it in the earth to recirculate at night. The greenhouse is oriented east-west with a large south-facing glass or polycarbonate wall built at an angle that will catch as much of the sun’s energy as possible, given the latitude. The other walls are solid and very well insulated, often with reflective interior surfaces, and the north wall is sometimes earth sheltered, if the landscape permits. It is the dirt, and sometimes gravel or large stones about four feet deep UNDER the greenhouse that act as a battery to store the heat that’s captured during the day. That heat is blown underground with a fan and then ducted out and up into the growing area at night.
DWGs usually have a back-up heat source in case of prolonged cloudiness, but the stored heat from the sun usually lasts three days without recharging. When I visited the DWG at the Agroecology Center in Finland, I saw damage where the stored heat was so intense that it melted some vinyl vent covers. Wow, that northern sun is powerful! One of the challenges with building this DWG was drainage—it is on a wet site, so they had to add fill around the insulated four-foot basement to prevent moisture from getting in. And there are still some problems with mold in the back room. All challenges that designers are working to solve.
Stefan Meyer who manages the Finland DWG has been harvesting all winter. Lettuce, Asian Greens, salad mix, rainbow chard and napa cabbage. By the time you read this, they will have planted spinach, arugula, and more kinds of lettuce. These all need the lengthening days which tend to become noticeable by early February. The greens are sold at the Finland General store, the oldest Cooperative in Minnesota.
The DWG at the Bemidji Community Food Shelf is located on the food shelf farm, just south of the food shelf building. The farm manager and volunteers run the greenhouse and grow microgreens, radishes, chard, peas, and salad greens which are given away at the food shelf. The farm produces an amazing amount of fresh produce for the food shelf, too. But the DWG allows production to continue all winter while the farm fields lie frozen under the snow. The food shelf is within the industrial park in Bemidji, which has a high water table---not good for underground heat storage. They raised the foundation 12 inches, but now wish it would have been more. They installed a berm around the foundation and added insulation but keeping the rock bed dry so that it absorbs the heat optimally is an ongoing challenge. On a positive note, the Bemidji DWG has a metal exterior which has worked out very well and is maintenance-free.
Friends in Linden Grove have built a modified DWG as part of a complex that also houses their sauna, beekeeping storage room, work room, and root cellar. Their greenhouse is on the south side and its south-facing wall is polycarbonate but is built like a regular greenhouse with a vertical wall and roof rather than the full wall at a 60-degree angle that is typical of DWGs. It is built over a four-foot soil and sand heat storage area with pipes zigzagging through the sand, bringing the heat from the top of the greenhouse near the roof down into the soil/sand. This greenhouse does not have a mechanism for blowing the stored heat back into the growing area. But it stays at 40 degrees during the coldest part of winter, offering some warming to the greenhouse above. They use mini-tunnels and space heaters to supplement heat to grow greens at this time of year.
The main benefit of this design, according to the owner, is that it can be used year-round. It has side doors that can be opened in the summer and, for added heat in the winter, the sauna vents into the greenhouse. And, of course, the structure houses multiple work areas for the farm as well as a root cellar to store crops beyond their harvest.
The newest DWG design is called a Farm Scale Winter Greenhouse. It is like a hybrid between a DWG and a hoophouse. The intent was to make a structure that is affordable to construct and operate. It might require a little more added heat than the early DWG models. The new design can be found here https://extension.umn.edu/growing-systems/deep-winter-greenhouses#design-and-construction-2066621 Is a deep winter greenhouse in your future? A University of Minnesota Extension publication might be helpful. You can find “Starting a DIY Deep Winter Greenhouse Operation on a Budget” here https://conservancy.umn.edu/bitstream/handle/11299/199881/Starting-a-DIY-DWG-Operation-on-a-Budget.pdf?sequence=1&isAllowed=y And your local University of Minnesota Regional Sustainable Development Partnership can help. Ours is the Northeast RSDP https://extension.umn.edu/regional-partnerships/northeast-rsdp .