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Lengthening the Minnesota Growing Season

March 16, 2010

All the rain that has hit Minnesota lately has made one thing undeniably clear: Spring is coming. Just when it seems like winter will never end, the temperatures tip above freezing and the thaw begins. It is true, however, that our winters are longer than in other parts of the country, and this cuts the growing season short. The ground doesn’t fully thaw until May and you may hesitate to plant until there’s little risk of frost. That may not happen until early June, and then after you plant, the wait begins. Seeds must sprout and leaves unfold. Cross your fingers for warm temperatures, moderate rain and sunny weather, and finally, the harvest comes in a surge of tomatoes and corn. Then, before you know it, the weatherman reports the chance of below-freezing temperatures and you have to start the cycle all over again.

Thermal Banking Greenhouse. Source: Cooking Up a Story

This brief and tumultuous growing season makes eating local produce year round in Minnesota difficult—if not impossible—for most people. For others, however, it has proved to be a challenge worth overcoming. Some farmers in Minnesota have begun to extend the growing season—and not just by starting seeds in their basement under grow lights.  Farmers are discovering that it is possible to grow many more months out of the year by using large greenhouse style structures, solar panels, and other implements. It has proved to be successful, as evidenced by locally produced salad greens in the winter and tomatoes in early July.

Two ways to do this is with thermal banking and high tunnels. Both utilize the same concept: enclosures that let sunlight and heat in, but keep much of the cold out.

Thermal banking stores the heat that accumulates in a greenhouse during the day for future use, allowing the greenhouse to be used nearly year round without needing to be heated and therefore saving energy. Most of the process of thermal banking takes place underground. Excess heat from the greenhouse’s peak is dispersed to the soil below, keeping roots warm and radiating heat. Diagrams can be found here which show how a thermal banking greenhouse works.

Similar to thermal banking, high tunnels also utilize solar heat to warm the soil sooner, and keep it warm longer. Less technology is needed as the high tunnels are made of plastic stretched over a metal frame, with plastic sides that can be opened up as needed. Unlike a conventional greenhouse, high tunnels don’t use fans or heating elements of any kind.

A high tunnel in use. Source: University of Minnesota News

The idea has been in use in Europe for some time, and has spread throughout Minnesota as more farmers discover its benefits. It allows warm weather crops to be started far earlier and continued production into the fall. In addition to extending the growing season, it allows farmers to grow crops that are not usually hardy to Minnesota (Sweet potatoes, for example.) It also improves the quality of the plants, enabling plants to grow bigger, stronger and produce more food than otherwise possible without the need for fertilizers. Need for pesticides is also decreased as the enclosed environment makes conditions more manageable. The University of Minnesota has provided more information on using high tunnels for farming in an online guide which can be found here.

Beyond simply fulfilling a need for fresh food throughout the year, it may also provide a way to keep small farmers in business, as it is more profitable than traditional farming methods. Although there are start up costs involved, they vary based on their size and scope. For the most part, high tunnels can be relatively low cost and  many farmers find that their profits increase when they are used.  Vegetables grown this way are usually of higher quality and are available for an extra 10 to 12 weeks per year, two things justifying a higher cost. It seems that people are willing to pay for it as high tunnel use continues to thrive in Minnesota.

Sources: Cooking Up a Story, Minnesota Public Radio, University of Minnesota News
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