In the dead of winter when the rolling hills of Ashby, Minnesota are blanketed by stark white snow, vibrant green plants are growing at Paradox Farm as if it were a warm summer day.

“We’re able to provide customers with delectable plants fall, winter and spring until they are able to plant their own gardens,” said Sue Wika, owner of Paradox Farm. “People get hungry for good, fresh greens in the depths of winter and with our ‘deep winter’ greenhouse we can provide them with the fresh greens they are craving,” she added.

Increasingly, greenhouses are being used for more than getting a head start on plants destined for transplant in fields or gardens. Instead the structures are being used to produce vegetables during the traditionally off months of winter.

Extending the growing season in your greenhouse can provide an opportunity to offer a desired, potentially higher-priced crop during otherwise dormant months.

Special considerations

Bountiful crops require a delicate balance of sunlight, heat and air circulation to produce well. Growers in cold and northern climates will likely need supplemental lighting and an artificial heat source to be successful.

Rather than investing in lighting fixtures, strategic plant selection is a workable alternative. Specific plants can survive during shorter daylight hours and are a good choice for greenhouse growers opting not to provide an additional light source. “We primarily grow Asian greens, lettuces and some herbs because they can tolerate the limited natural lighting,” Wika said.

Greenhouse growers have several options for heating their structure during the frigid, winter months. Fuel is necessary, either in the form of oil, propane or gas or solar heat. At Paradox Farm, Wika opted for a specialized “deep winter greenhouse (DWG). The DWG design uses fans to force the rising heat from the sun into a rock storage area.

The passive solar heating system captures the warmth from the daytime sunlight and strategically releases it at night. On particularly cold nights, a gas heater maintains a minimum temperature of 42 degrees Fahrenheittemperatures. “We liked the small energy footprint this design offers,” she said.

Air circulation is as important in active winter greenhouses as it is in traditional, seasonal operations. Good air circulation encourages the plants to develop strong woody tissue while at the same time decreasing the opportunity for fungus to attack plants. Fans force air to move throughout the house while replenishing much needed carbon dioxide.

In general, it is recommended that producers in northern and cold climates wait until February 15 to plant the first crop. Through research and careful selection, Wika has found plants that will work well in her geographic region and begins planting in the fall.

“We prep the greenhouse and begin growing in the hanging gutters in early to mid-October. We may start plants for the raised beds as early as September,” she said. By late November, the greenhouse is in full production. Deep winter greens production begins waning in March, just in time for the structure to transition over to garden transplants. We clean out the greenhouse in mid- to late May and let it “solarize” until late July or early August.

Crop selection

Not every crop thrives in a greenhouse. The greenhouse, how it is heated (solar vs. fuel) and available sunlight will determine which crops can be successful.

Cool season vegetables such as beets, broccoli, cabbage, cauliflower, carrots, chard, leaf lettuce, peas, radishes and turnips are good candidates for greenhouse crops when natural temperatures are too cold for outdoor growing.

Cool season vegetables thrive when daytime temperatures range between 50-70 degrees Fahrenheit and overnight temperatures stay between 45-55 degrees Fahrenheit, though some can tolerate even cooler nighttime lows. “We select plants that can handle night-time temperatures that may sink into the 30s. Many Asian greens and arugula can shine in these conditions. As the nights begin to shorten (days lengthen), we plant more lettuces,” Wika said.

Conversely, warm season crops including beans, cucumbers, eggplants, cantaloupe, peppers and tomatoes require greater light intensity, warmer temperatures and more space to do well in greenhouse operations. The crops can be lucrative options for production, especially during off-seasons, but require additional care.

Getting creative

With some creative thinking, greenhouses can be productive in other ways too. “We also use our deep winter greenhouse to grow fodder for our livestock, goats and milk cows,” she explained. “We supplement our livestock ration with barley fodder November through mid-April.” Not surprisingly, the livestock enjoy the fresh rations in the winter as much as their human caretakers.

Typically, many greenhouses “close down” in the summer months when the outdoor temperatures soar, making it blazing hot indoors. “We have found the intense heat provided by the deep winter greenhouse, which is too hot for plants during the summer, is excellent as a “walk-in” dehydrator,” she said. To keep the greenhouse working she places halved paste-type tomatoes on large trays throughout the greenhouse. “In a day or so, we have a large amount of delicious dried tomatoes,” she said.

Getting started

The majority of year-round greenhouse growers are currently large-scale operations providing vegetables to retail and wholesale markets. The facilities include sophisticated temperature control and ventilation systems and are built from modular kits. Even small-scale producers can get involved with year-round growing with structures that are less complex and reliant on passive heating and ventilation systems.

Managing a year-round greenhouse operation can be a costly venture. Before jumping in, spend time learning about the crops viable for your region of the country and which supplement heat/light sources are most efficient for the climate.

Once you have done your homework, you can forge ahead with a year-round greenhouse without regret. “This (the greenhouse) offers us a way to feed ourselves, our livestock in the depths of winter,” Wika said. “Our deep winter greens are fresh, delicious and of a sort that you just can’t find in local grocery stores.”