by Tamara Scully

Right about now, the demand for local food in states with cold winters typically leaves consumers searching for farms selling their own winter storage crops, those with greenhouse or high tunnel production – or the supermarket for greens from intensive indoor agricultural operations, where the controlled environment doesn’t include soil or sunlight. But infrastructure costs can be high.

A 2019 report from the USDA (“Research and Development Potentials in Indoor Agriculture and Sustainable Urban Ecosystems”) concludes that indoor agriculture (IA) “leverages techniques in horticulture and engineering to optimize crop quality and systems efficiency to allow crop production year round.” The report acknowledges that these “fully indoor systems, IA, also require high capital investment and energy operating costs. Their successful operation additionally requires interdisciplinary coordination and expertise in business, horticulture and engineering.”

The infrastructure needed for small farmers seeking to raise winter crops need not be as intensive as that of IA operations, and can range from low tunnels, to provide season extension for cold weather vegetables, to unheated DIY high tunnels to heated greenhouses of various design. Before investing money and labor into a winter growing operation of any scale, it’s best to know your goals and make a business plan.

Do you have a viable market that wants your winter crops? What type of crops will you be able to grow in the winter? Will the return on your investment be profitable in the short-term, or will this pay off over a longer time period? How do you want to grow – in soil, with soil-less substrates, aeroponically or hydroponically? Do you want to grow in a heated greenhouse, and if so, how will you heat it?

Deep Winter Greenhouse Introduction

One source of information for those growing in cold winter areas is an ongoing study from the University of Minnesota. The Deep Winter Greenhouses (DWG) study commenced in November 2016, and newly released research results from DWG trials hold some useful data.

DWGs are passively-heated solar greenhouses, constructed with energy efficiency and low cost in mind. They are east-west oriented, and built with a south-facing glazing wall. The glazing wall is built at a specified angle, depending on latitude, and the solar-heated air is directed into the underground bedrock by a fan system. This allows the bedrock to retain the heat, and function as a battery, releasing the solar energy at night, when it is needed to maintain temperature.

Crops which can thrive in low light situations (lettuce, brassicas, herbs and sprouts) are target crops for DWG growing. Plants can be planted directly into the soil, or soilless media can be used. Suspended planter or shelf systems can be utilized to maximize space, and raised beds or pots can be used for plants requiring more space.

A Minnesota statewide initiative has supported the building of five DWGs across the state. Resources and research to help small growers build their own DWGs, select the best crops, growing media, temperature and relative humidity parameters, seeding rates, spacing and more have been conducted or are underway.

Economically, DWGs average $18,000 to build (about $33/square foot), according to an enterprise analysis of eight Minnesota DWGs. Annual expenses varied widely, but averaged $906, and an overall return of investment of around 7% was seen. Growers are encouraged to research low cost construction options, maximize production space and find ways to decrease annual operating costs in order to maximize profitability. Although individual profitability varied widely, an average operating revenue of $2.52/square foot of growing space was calculated.

Production Concerns

Greg Schweser, director of sustainable agriculture and food systems for University of Minnesota Extension Regional Sustainable Development Partnerships, and researcher Claire Flavin Hodge have conducted trials to assist growers in selecting the best crops and conditions for DWG growing.

When growing in DWGs, light availability and day length restrict plant growth. This changes throughout the season, which is split into three sub-seasons. From October to the winter solstice, both daylight and day length decrease, a period known as diminishment. The solstice sub-season stretches from the solstice until mid-January, when both factors hold stable. From mid-January through the spring – the extension sub-season – both qualities increase.

Four DWGs in Minnesota, located between latitudes 44.15º and 47.46º N, were studied to see how different variables affected crop yields. Salad greens were found to be twice as high in the expansion period when compared to yields in the diminishment period. Solstice yields were 1.5 times higher than during diminishment. Cultivars were selected to have characteristics thought to be advantageous for DWG growing, including early maturing, slow to bolt and low temperature germination varieties.

Greens were grown in gutters, with custom media growing mixes, and seeded at an average rate of 144 seeds/square foot. Greens were harvested when four inches high, up to three times per sub-season. The best grower yield was 305 grams/gutter, while the lowest yield was 52 grams/gutter.

Researchers found that gutters closest to the glazing wall had higher average yields, and that growers who optimized temperature and water to match the needs of the plants had the best yields. Rotating the gutters, and selecting high-yielding cultivars, particularly in the diminishing and solstice periods, is important to maximize yields. Spinach, chard and sorrel were the lowest yielding of the greens tested.

Using heating mats to start seedlings did not result in any statistical yield differences. Experiments with the cold and hot thresholds – where temperature ranged from 35º to 50º F, or 70º to 85º F, showed that colder temperature thresholds resulted in lower yields, especially in spinach. Higher temperature thresholds resulted in much quicker crop growth overall, and a significant decrease in days to maturity was seen.

On sunny days, ventilation is required to keep temperature and humidity levels optimal for plant growth, and a backup heating method should be available if the temperature goes below 35º F.

The best crop and cultivar selection for growing greens in a DWG depends on the sub-season and the growing media. A variety of lettuce, Asian greens, beets, kale, chard, mustard greens, Claytonia and cress cultivars are suggested by farmers in Minnesota already growing in DWGs.

Most Minnesota DWG growers are utilizing a standard growers’ soil mix, either homemade or bagged. Researchers found that eliminating blood meal in the mix resulted in yield losses. Concerns about prolonged overcast skies and shorter nighttime hours, under which conditions plants will convert unused nitrogen into stored nitrates, have led to recommendations of reducing the amount of blood meal by 50% during the solstice period as a safeguard. Researchers have not yet found any toxic levels of nitrates in DWG greens grown in the growers’ soil mix under any growing conditions, and nitrates from leafy greens have actually been linked to health benefits, but remain controversial.

DWGs, which have a small carbon footprint, are built with relatively inexpensive and easily accessible materials and produce in-demand winter crops and can enhance small farm resiliency and local food system development. For farmers in cold winter regions, DWGs might offer a viable opportunity to make winter a productive and profitable growing season.

See the full DWG production manual at