By Liz Morrison

The ground is frozen, but the livestock at Paradox Farm near Ashby, Minn., are still eating fresh green forage.

Otter Tail County farmers Sue Wika and Tom Prieve raise leafy greens and fodder all winter long in a specially designed “deep winter greenhouse.”

The greenhouse uses low-cost, passive solar technology to produce cold-hardy crops with no grow lights and little or no added heat.

Deep-winter greenhouses, or DWGs, collect the sun’s energy through a steeply angled, south-facing glazing wall. During the day, the sun heats the air inside the greenhouse, which rises to the top of the structure, where it’s drawn into vents and forced into a well-insulated, thermal rock bed under the greenhouse. The underground rock bed acts like a heat storage tank, or battery. At night the stored heat circulates back into the greenhouse, warming the soil and air.

This technology lets small-scale Northern farmers grow food year-round at much lower cost than in a traditional, four-season greenhouse, says Carol Ford, deep winter greenhouse project coordinator for the University of Minnesota’s Regional Sustainable Development Partnership.

“Deep winter greenhouses are a terrific fit for farmers who have already developed a clientele through farmers markets or a CSA, and would love to have a relationship with consumers in the winter, too,” Ford says. “They are small enough to go just about anywhere — alongside a community garden, attached to a retirement home or a school, on a farm, or in town. They are very versatile.”

“We are hearing a lot of interest in this concept,” says Greg Schweser, associate director of U-M’s Sustainable Agriculture and Food Systems. He estimates that Minnesota has at least two dozen deep winter greenhouse operations. The technology is catching on in other Northern regions, too, he says, including Colorado, North Dakota and Manitoba.

“We see a lot of potential for local food producers who would be able to grow products to sell in the winter,” Schweser says. “Incorporated into an existing farm business, it offers the opportunity to expand product offerings, and increase production and revenues.”

Passive solar technology on the farm
Paradox Farm’s winter greenhouse supplements a grass-based livestock operation, which includes 160 acres of permanent silvopasture, which combines grass and fruit and nut trees. Sue Wika, a sociology professor at Minnesota State Community and Technical College in Fergus Falls, and Tom Prieve, a large-animal veterinarian, manage a herd of 75 sheep and dairy goats. They also raise poultry and breed and train performance quarter horses.

Wika and Prieve sell organic lamb, pasture-raised poultry, and goat’s milk directly to local customers. The deep winter greenhouse fits beautifully into that market niche, Wika says.

“We were already in the business of selling food directly to consumers,” she says. “This is our fourth-season revenue stream, and we’re providing a product you can’t get anywhere else.”

Their 16-foot by 24-foot lean-to-style greenhouse, which they built in 2011, is attached to their milking house. The south wall is covered with clear, twin-wall polycarbonate panels, set at a 60-degree angle to catch the low rays of winter sun. Below the tarp-covered dirt floor, 24 yards of washed river rock are packed into the well-insulated foundation, forming the thermal storage bed. A small electric duct fan blows warm air from the greenhouse into the rock bed.

Inside the winter greenhouse are about 90 three-foot sections of plastic rain gutter, which serve as planters. The gutters are filled with a mixture of potting soil and a 4-4-4 organic fertilizer made from coffee bean chaff. Five levels of planters are suspended from the ceiling in rope slings, filling the vertical space. Additional raised beds rest on the warm floor.

A loft along the back wall of the greenhouse provides about 65 square feet of additional space for livestock fodder production.

The ‘seasons’ of winter
On a gray day in mid-November, Prieve mixes fertilizer and potting soil, while Wika prepares to plant arugula and dragon’s tongue, a hardy Asian mustard green. Already, lush plants spill over the hanging planters: arugula, baby kale, Asian greens, pea sprouts.

Wika begins planting around the first of November, and the greenhouse is producing full tilt by mid-December. She markets her produce through a winter community-supported agriculture cooperative (CSA). supplying a dozen subscribers with weekly six-quart boxes of vegetables from December to May.

The mix of plants in the greenhouse changes as the winter progresses. Before the winter solstice, when the days are getting shorter, Wika grows Asian greens, baby kale, chard, broccoli and many kinds of sprouts. In late January, as the days begin to lengthen, she adds more lettuces and herbs, tiny Asian turnips and cabbages, and mini romaine.

Over the 20-week winter growing season, Wika will raise about three dozen varieties of greens to feed both people and animals.

For their livestock, Wika and Prieve grow barley fodder, which they produce in 8-pound batches without soil. Prieve calls the emerald green feed “a living vitamin pill. The goats love it. It’s a treat.”

Wika plants once a week during the season, keeping detailed production notes. She harvests twice a week, cutting, weighing and packaging the greens in the early morning, while the leaves are still chilled from the night.

The greenhouse is a pleasant place in the winter, says Wika, who spends six to eight hours a week running her produce business. The temperature inside often rises into the high 80s, even on frigid winter days, she says. At night, greenhouse temps fall into the low 40s. If the mercury drops below 40 degrees F, a fan blows heat from the barn’s wood furnace into the greenhouse.

In late spring, as the winter greens season winds down, Wika uses the greenhouse to start tomatoes and other garden plants. In late summer, it becomes a giant food dehydrator where Wika makes sun-dried tomatoes, mushrooms and fruits.

Wika and Prieve teach Beginning Farmer classes for the Minnesota Sustainable Farming Association, including forage-based livestock production and a deep winter greenhouse short course.

Says Wika: “As beginning farmer educators, we encourage our students to be creative and innovative, to look for novel revenue streams, start new enterprises  and grow really good food.”

Morrison writes from Morris, Minn.