The doldrums of winter are upon us and gardeners are drooling over seed catalogs and dreaming of fresh vegetables and riots of flower colors. But spring planting is still several months away. This is an excellent time to research gardening season extenders such as greenhouses, cold frames, and low tunnels. They could help you start your growing season sooner in the spring and last longer in the fall. To determine which one(s) will best meet your growing needs, there are some key questions you will want to ask yourself and information you’ll want to consider. 

A greenhouse can be described as a place to grow plants inside. Greenhouses can be heated or unheated. They are usually made of glass or plastic to enable light to shine through and they have a roof. Using the sun, the greenhouse captures heat and light. This then generates warmth for plants to grow. Greenhouses also allow gardeners to control the environment because plants grown inside the greenhouse are not subject to the wind, variable rain amounts, and temperature extremes. You may be able to manage pests like insects more easily, and that means you can use fewer pesticides. And if you, like many of us, are plagued by rabbits or deer, you may be able to keep them away from your plants, too!

Unheated Greenhouses

An unheated greenhouse allows a gardener to start plants earlier in the spring and extend the harvest season longer into the fall months, but it will not be usable during the winter. To use a greenhouse all year in Minnesota, it needs to be heated. Heated greenhouses increase the costs not only of construction but also for operation. (More on heated greenhouses, known as “Deep Winter Greenhouses” below.) Determining the purpose of the greenhouse will help you decide which option is best in terms of function and costs. 

Greenhouses may be standalone or may be an added structure to your house, garage, or sheds such as a lean to. They need to be placed in a location that receives full sun. Next, you need to determine the building materials, which will be based on whether you want a heated or unheated greenhouse. Size may be dictated by your available space, availability to sunlight, and how you hope to use your greenhouse. Sizes can range from tabletop greenhouses to a tent-like shelf structure to large buildings with room for several people to work simultaneously. 

Greenhouses need to be anchored so they don’t fly away or tip over. They need ventilation to provide air flow but also to vent excessive heat so plants don’t burn or keep warmth in on colder days. Flooring needs to allow for drainage and prevent weeds from growing. And once the structural items are addressed, shelving and benches to hold the plants and other supplies will need to be selected. 

If a greenhouse doesn’t fit in your budget, you may consider these options, which may be less expensive, to extend your growing season. For individual plants, you could use a cloche, which is sometimes called a bell jar.

 It protects individual plants from cold temperatures and other weather extremes. Cold frames, which are essentially mini-greenhouses, can be created using old windows, doors, or other building materials, with glass or plastics. They are an excellent method for starting some seeds, especially the cool weather loving plants like lettuce, broccoli, spinach, cauliflower. They can also be used to get seeds started indoors used to being outside without full exposure to the cold, wind, or rain. 

Another option you may be less familiar with is low tunnels, which are usually temporary structures that basically create a barrier to the elements like wind, cold, excessive rain, and pests. They can be built with PVC pipe, electrical conduit, or rebar that are used to create hoops that are covered by plastic. The plastic can be purchased at home improvement stores or local hardware store and is recommended to be between 0.8-2 

mil. 

Deep Winter Greenhouses

Greenhouses in Minnesota must be heated to be usable year-round, unless they are constructed using Deep Winter Greenhouse (DWG) methods. This is a new approach to creating a year-round growing environment in the cold North, enabling gardeners and producers to grow vegetables in a non-heated structure. Building a greenhouse with heat and operating the heating source, as well as the maintenance costs, can become costly and often make them unobtainable for the majority of home gardeners. The University of Minnesota Extension calls DWGs passive-solar greenhouses that use energy from the sun to generate heat. This lessens the need to use traditional heating methods. 

This is accomplished by careful alignment of the building structure in an east-west position. The south facing direction is built out of a glazing wall, which can be angled based on the latitude of the site to maximize solar energy. This is advantageous for northern climates especially on very cold days. Inside the DWG, the air heats up, is blown underground, and stored in a thermal mass made of soil or rocks, essentially acting like a battery.   

While this does create a great growing environment, DWGs are not suitable for all plants. Plants that do best grow in minimal light and include lettuces, herbs, Asian greens, sprouts, and brassicas e.g., bok choy, broccoli, cauliflower, cabbage, choy sum, kohlrabi, napa cabbage, rutabaga, turnip. 

The initial set-up costs for DWGs, with an estimate of $18,583 to build ($33 per sq ft) with annual costs ranging from $35 to $2,148 depending on what fuel sources are used, how many seeds are purchased and quantity and type of soil and amendments are used. There Regional Sustainable Development Partnerships (RSDP) and Center for Sustainable Building Research have created a DIY version of the DWG Starting a DIY Deep Winter Greenhouse Operation on a Budget. Aimed more toward farmers than backyard gardeners, it may still provide some information that may be useful on a smaller scale. The RSDP also provides blueprints, construction manuals, resources, and information on other types of solar greenhouses.   

This has been a very brief overview of greenhouses and other growing season extenders. To learn more about different types of greenhouses, structural options, building materials, and alternates to greenhouses like cold frames, low tunnels, and cloches, explore these links: 

Very Small Greenhouses for Backyards from University of New Hampshire Extension

The Hobby Greenhouse from Oklahoma State University Extension

Extending the Growing Season in Your Garden from University of Minnesota Extension

Extending the Garden Season from Utah State University Yard and Garden Extension

References

Garden Gate, March 2024

University of Minnesota Extension 

University of New Hampshire Extension 

Oklahoma State University of Extension 

Utah State University Yard and Garden Extension 

University of Minnesota Extension

• Deep Winter Greenhouses: https://extension.umn.edu/growing-systems/deep-winter-greenhouses 

  • This article contains numerous additional resources, guides, cost-benefit analysis, and information

• Winter Greenhouse Enterprise Analysis: https://extension.umn.edu/community-research/winter-greenhouse-enterprise-analysis 

  • A report detailing eight enterprises in the Upper Midwest 

• New deep winter greenhouse design available for download: https://blog-fruit-vegetable-ipm.extension.umn.edu/2024/12/new-deep-winter-greenhouse-design.html 

• Deep Winter Greenhouses grow fresh greens year-round: https://extension.umn.edu/source-magazine/deep-winter-greenhouses-grow-fresh-greens-year-round 

National Center for Appropriate Technology: https://attra.ncat.org/topics/greenhouses/ 

Photo credits: Oklahoma State University Extension (1), University of Utah Yard and Garden Extension (2,3,5), University of Minnesota Extension (4,6)