Nothing should be growing this winter day on these frozen, rolling hills.

Yet here are green vegetables, kale and lettuce, growing in near-90 degree temperatures. They're thriving in a specialized "deep winter" greenhouse, letting farmers Tom Prieve and Sue Wika grow fresh vegetables year round -- without a crushing electric bill.

Their plants survive largely on natural winter light. Fans force rising heat down into a rock storage area, part of a passive solar heating system that captures the day's warmth and releases it at night. On cold nights, a gas heater kicks in to help keep the temperature at 42 degrees. There are no banks of artificial lights.

It's a different kind of greenhouse, mixing technology and old school ingenuity to create an energy efficient winter farm. University of Minnesota researchers say the idea is starting to take off. About two dozen deep winter greenhouses can be found now in Minnesota. Many more are in the planning stages. A deep winter growing association will soon give winter gardeners a place to share what they're learning.

A small loft offers a bird's-eye view of the deep-winter greenhouse at Paradox Farm in southern Otter Tail County not far from Lake Christina on Tuesday, Jan. 28, 2014. Temperatures inside the greenhouse vary by elevation with this thermometer indicating around 85 degrees near the peak. Paradox Farm is owned by Sue Wika and Tom Prieve. (Ann Arbor Miller for MPR) Ann Arbor Miller/For MPR News

The small operations can be put up and run without spending a lot of money. Wika and Prieve's $5,000 winter greenhouse near Ashby is built like a lean-to against the south wall of the barn. Clear plastic panels cover the south wall, which is slanted at a 60 degree angle to best catch the midwinter sunlight. Next year a wood stove will help fight the overnight chill.

• Link: More about the greenhouse

"I want people to know that this is a definite reality for people in northern climates," Wika says. "They can have greens in the depths of winter."

This is the second winter green plants have filled her greenhouse. Wika, the chief gardener, keeps careful records on everything growing. She says she's still learning. What plants are best suited to winter production? What soil works best? What's the most efficient way to heat the greenhouse at night? Who has the best ideas for affordable construction?

Dozens of 3-foot-long pieces of plastic roof gutter filled with soil hang from the ceiling. Rows of them are suspended from just above the floor to head high. Thick green vegetation spills over the sides.

Sue Wika's enthusiasm for sustainable food production including a deep-winter greenhouse is evident when she talks about Paradox Farm and its farming practices. Building and using a deep-winter greenhouse, which combines passive solar energy with underground heat storage, is both durable and doable, says Wika. Ann Arbor Miller/For MPR News

On the floor are plastic bags of soil with holes cut in them. Chinese cabbage, turnips, radishes and beets sprout from the bags. The heated rocks under the floor keep them warm.

"Today we'll harvest some red Russian kale and we'll also harvest some of this komatsuna which is another really fast growing and productive Asian green," says Wika, who praises the Asian greens as the "the real star in these deep winter greenhouses."

There are also trays of barley, looking like squares of lush green grass - a treat for the goats and cows the couple milk as part of their sustainable farming effort.

"When it comes to feeding time you just simply peel it out of there and chunk it up and they gobble it down," says Prieve, who trained as a large animal veterinarian. "It's candy, they look for it first thing when they come in and it's a more healthy form of energy than the straight grain."

In about a month, the greenhouse will be filled with young tomatoes and other plants getting a head start on the outdoor gardening season. In summer, Wika uses the greenhouse as a giant dehydrator to make sun-dried tomatoes.

Rain gutters in assorted colors and sizes provide growing space for a variety greens for both human and animal consumption. Leafy vegetables like chard, red Russian kale, Chinese cabbage and komatsuna are among those being grown in a deep-winter greenhouse at Paradox Farm. Ann Arbor Miller/For MPR News

Wika and Prieve eat fresh greens every day and they sell or trade produce to six families in the area.

There are a lot of varieties that do very well in the winter with little care. Still, Wika spends about eight hours a week planting, watering and harvesting.

"I'm in here more time than I'm working," says Wika, who holds a doctorate in sociology. "I always spend a lot of time just enjoying the sun and temperature. A lot of it is leisure and therapeutic."

The plants will keep growing. Wika expects another harvest in three weeks. "They have amazing ability for recharging," she says. "Some you might only get two harvests from, but there are some of these greens I might get four harvests from. They can be very productive."

Wika places a tray of kale on a bench and starts trimming the thick bushy plants with a sharp knife, dropping the greens into a plastic tub. As she's working, she sees Buddy Kasper peering into the greenhouse through fogged-over sunglasses. The 69-year-old drives 30 miles each week from Fergus Falls to get his fix of winter greens.

"You don't even worry about salad dressing or anything. You reach in and grab a handful and stuff 'em in your mouth," he says. "It's just all of these different flavors come together at once. It's all you need in the dead of winter."