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For those of us living in northerly regions, using an unheated greenhouse in the winter may seem far-fetched. And yet, it’s perfectly feasible. Even way up north in hardiness zone 3, a simple greenhouse covered with flimsy plastic sheeting can give you a winter harvest. Greenhouse “glazing” (be it plastic sheeting, or polycarbonate or glass panels) can help replicate the climate 1.5 zones to your south; and further coverings inside the greenhouse (such as floating row covers) can give you the leeway of another 1.5 zones. In this way, a zone 3 winter becomes a zone 6 winter. While zone 6 isn’t exactly tropical, its climate supports the growth of cold-hardy vegetables.
The key here is “cold-hardy.” An unheated greenhouse won’t do for heat- and sun-loving plants like tomatoes, peppers or melons. But with a little thought, planning and experimentation, you should be able to harvest salad greens, leeks, hardy root vegetables, and cool-weather Chinese greens during the winter months.
Building the Best Greenhouse
The location of your greenhouse and how it’s constructed will have a huge impact on your harvests. Before you start building, consider the following:
- Ideally, a winter greenhouse should be situated against the south-facing wall of a house, garage or outbuilding (such as a shed or chicken coop). That way, it will be protected from northerly winds while also benefiting from ambient heat.
- If it’s not possible to build your greenhouse against an existing wall, its freestanding north-facing wall should be opaque (ideally painted black) so that it absorbs and retains heat.
- Make sure the location isn’t shaded.
- A foundation laid below the frost line will both protect against frost heave and insulate the soil within it.
- The angle of the roof is crucial to capture the maximum amount of sunlight. Optimal roof angles differ depending on latitude. To figure out the best angle for your location, consult the Cold Climate Greenhouse Resource published by the University of Minnesota.
How Cold is Too Cold?
Depending on your hardiness zone, you may be able to get by solely with passive solar and ambient heating. In colder regions, you may need to supplement some warmth. There are a number of ways that you can generate and/or trap heat without resorting to electricity or fuel.
Especially if you are setting a foundation below the frost line anyway, a sunken greenhouse can provide the warmth plants — especially root vegetables — need. As the Cold Climate Greenhouse Resource explains, soil located 4 feet below the surface stays at a steady 40-50 degrees Fahrenheit regardless of air temperature.
Alternately, a trench or hole in the center of your greenhouse, filled with manure, will generate heat as the manure breaks down; and as a bonus, you will always have fertilizer easily at hand! Another option is to create heat sinks by filling black 55-gallon drums with water. The barrels absorb heat from the sun during the day and slowly release that heat at night. If you’re up for it, more complicated systems include solar heating and circulating water heated with solar power.
Regardless of how — or even if — you provide supplemental heat to your greenhouse, keep in mind that plants also need sunlight to thrive. During the shortest days of the year, growth will come to a standstill regardless of how warm your greenhouse is.
Choosing the Best Vegetable Varieties
As noted above, it’s essential to focus on cold-hardy vegetables for your unheated winter greenhouse. It may take some experimenting to find which varieties work best in your zone and your greenhouse’s microclimate, but to help you get started, consider these:
- Cold-hardy salad greens, including endive, radicchio, kale, Swiss chard, spinach, arugula, collard and mustard greens. Plant your greens in succession (perhaps one tray a week) and, when ready for harvest, cut what you need, leaving some green behind. The plants will continue to grow so that you get a second (and possibly third) harvest.
- Root vegetables such as parsnips, beets, turnips and carrots. As an added bonus, to deal with cold temperatures, root vegetables convert their starches to sugars, which act as a sort of natural anti-freeze. The increased sugars mean sweeter-tasting veggies.
- Pak choi, Chinese cabbage and other similar cool-weather Chinese greens.
Timing Your Seedlings
It’s best to start your seeds in late summer or early fall so that your seedlings are well-established before extreme cold hits. However, you can sow throughout the winter. If you choose to plant during the coldest months, start your seeds indoors or use a heat mat for seed propagation. Tender seedlings are more sensitive to the cold and need a solid start before they’re left 24/7 in an unheated greenhouse.
You’re not alone if you’re wondering whether digging a four-foot “basement” for a greenhouse and calculating the optimal roof slant is more work than it’s worth. But, if you’re reading this, it goes without saying that you already know the benefits of gardening, whether in winter or summer: the joy of dirt under your fingernails; putting tasty, nutrient-dense produce on your table; cost-savings compared to purchasing bland supermarket vegetables; and self-sufficiency. Using an unheated greenhouse in the winter brings extra benefits, including the cost-savings over a heated greenhouse, supporting the environment, and the joy of proving those folks wrong, who think it’s impossible to grow fresh vegetables in the depths of winter.
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