The Idea Behind Unheated Greenhouses

In the last few posts, we have related how we became interested in and started looking into unheated greenhouses.  In the process we encountered The Winter Harvest Handbook, a book describing how a Maine grower has been successfully using unheated greenhouses for years.  We gleaned some very useful information from this book…

(By John)

            Reading through Eliot Coleman’s book, I was fascinated.  I also learned some of the limitations of unheated greenhouses.  Overall, however, it appeared that an adaptation of his basic method would be highly doable.

             One important point to stress from the start is that we’re not talking about growing tomatoes or watermelons here.  A major reason that going unheated was so appealing was that no snazzy or expensive equipment was needed.  Obviously, using plants that don’t mind the cold as much means that you can still get a decent harvest without maintaining a July-like temperature inside.  This leaves mainly hardy, leafy vegetables like lettuce, spinach, kale, chard, kohlrabi, and the like, as well as root crops such as beets and carrots.

            A second point was that maintaining a slightly warmer temperature was not the only way the greenhouse would protect the plants.  Although cold temperatures are clearly part of the reason that plants can’t normally survive the winter, wind and humidity are also huge factors.  Without the effect of the dryness and the wind, our vegetables would do much better even without the benefits of a warmer climate. 

As of today (Nov. 13th) these marigolds have spent 10 days in the greenhouse

            Back on the problem of temperature, it seems hard to believe that plastic wrap would do that much to keep off the cold.  This is why having two layers is so important (right now we’re covering the young plants with sheets at night; later, we may need another layer of plastic) .  In an unheated greenhouse, the combined effects of no wind, increased humidity, and slightly elevated temperatures make it like the plants have been moved 500 miles south.  Adding a second layer to protect against the bitter cold of the winter nights moves them another 500 miles south.

These marigolds, from the same bed as above, stayed outdoors (Nov 13th)

            Ideally, we would have started the planting in August.  Still, although October is definitely not the ideal time for planting, we should still get something.  The project could work; now all we had to do was actually get it going…

In the next post, follow us through the construction of the greenhouse.