Watching the snow blow and hearing the wind gust during the winter months, it’s hard to imagine growing vegetables outside. Sure, a greenhouse might make it possible, but that greenhouse would have to be a sturdy structure with a reliable heat source, right?
Actually: No. Winter hardy vegetables can be grown in unheated greenhouses–even those covered with flimsy plastic sheeting–into Zone 3. I live in Zone 3. Trust me; it gets cold here. The minimum average temperature is -30 degrees to -40 degrees Fahrenheit, and that does not include the windchill factor.
The idea of growing our own vegetables outside in those kinds of temps is pretty amazing, isn’t it? If building an unheated greenhouse for winter use piques your interest, check out this excellent in-depth guide from the University of Minnesota (UMN). Much of the information that follows can be attributed to that guide. As northern Minnesota is in hardiness zones 3a/3b, the information in the guide is widely applicable to most regions of the USA that experience freezing winter temperatures (except Alaska!).
How an Unheated Greenhouse ‘Works’
If you have an existing greenhouse, it may or may not be suitable for winter use. Typically, unheated greenhouses used in the winter are designed to capture as much sunlight and heat as possible. These structures can’t be used after late spring because they get too hot.
To capture that sunlight and heat, the structure needs to follow careful design and placement principles (more on that below). When positioned correctly, the structure’s outer covering moves the plants within it one and a half hardiness zones to the south. If a second layer of covering is added inside the greenhouse (such as with a floating row cover or cold frame), an additional one and a half hardiness zones are gained. In this way, gardeners in Zone 3, for instance, can replicate the climate of Zone 6.
A double layer of covering also increases humidity levels within the greenhouse, which offers further protection against frost.
Even with all these considerations, an unheated winter greenhouse is best used to grow leafy greens and cold hardy vegetables. While the cool temperatures are an issue for more tender vegetables, the real issue is the lack of sunlight. Tomatoes, for instance, need a minimum of eight hours of full sun a day (not to mention pollinators!) in order to produce tasty fruit.
Location, Location, Location
The location of your greenhouse is crucial. It’s ideal to place the structure against the south-facing wall of your house so that it gets full sun as well as ambient heat from your home. Another option is to place it on the south-facing side of an insulated garage or shed.
But before you run out and start framing in a greenhouse against an existing building, there are a few other things to consider. A south-facing wall won’t be of much use if it’s heavily shaded by trees or nearby buildings. The UMN guide recommends walking around your property at different times of the day to gauge how much shade your prospective site gets. Particularly with hoop houses, make sure surrounding trees are in good shape and unlikely to lose limbs that would damage the structure. Additionally, since frost tends to settle in dips and hollows, don’t set your new greenhouse in one. And, finally, make sure the site you choose has good drainage, particularly if you intend to plant directly into the ground.
If you don’t have any suitable south-facing walls, another option is to use an earth berm on the north side of your structure to provide support and insulation. Freestanding greenhouses should have an opaque and insulated north-facing wall, as heat is lost rapidly through clear glazing on the north side.
The UMN guidebook points out that the way a greenhouse is oriented and the angle of the roof/top glazing are key to a successful winter harvest. However, both things are often overlooked in the design stage.
The angle of the roof affects how much sunlight (and heat) permeates the structure. The UMN guidebook offers advice on how to determine the best angle for your greenhouse roof, which involves looking at the sun path chart for your latitude and at the angle of sunlight in your part of the world on the shortest days of the year.
As well, it’s crucial that the opaque back wall face due north, with the short sides of the structure aligned in an east-west direction.
As with any foundation, the one for your greenhouse should be laid below the frost line. In Zone 3, that means going at least four feet down. Setting the foundation that deep not only protects the structure from frost heave but helps maintain warmer soil. As the UMN guidebook explains, at that depth, soil temperature stays at a steady 40-50℉ year round, regardless of temperature fluctuations in the air. The foundation will also provide insulation for the soil it surrounds.
Frame and Walls
There is no cut and dried advice regarding the best types of materials to use for your greenhouse frame and walls. There are pros and cons to metal versus wood frames, as well as to plastic sheeting versus polycarbonate versus glass glazing. Consider your budget and the intended use of your new greenhouse and consult the UMN guide to figure out your best option.
While it’s a good idea to invest the time and energy into designing the best greenhouse for your needs and location, be prepared to tweak things over time. As you make use of your greenhouse in the winter and learn what works–and what doesn’t–you’ll likely make some changes. Gardening, whether in a garden or in a greenhouse, is typically a learning process anyway, isn’t it?
Do you have an unheated greenhouse that you use in the winter? How is it working for you? Please share any tips in the comments below.