This year our experimentation has continued with our first-ever cold-weather winter garden. Eliot Coleman has been developing the use of unheated greenhouses for winter gardens for many years now, and his book, The Winter Harvest Handbook, provides much useful information. Basically, by adding two layers of protection from the cold and wind, using fabric row covers inside a polythylene-film covered high tunnel or hoophouse, the garden's climate is effectively moved south. Each layer results in a climate similar to that 500 miles south or 1 1/2 climate zones warmer, so now we're essentially gardening 1000 miles south of New Hampshire, about three zones warmer, or at least that's what the plants think. Coleman lays out the pros and cons for using a single layer versus double layer of poly on the greenhouse (insulation vs. light transmission, and simple vs. more complex), but since ours was already double-covered with a blower, that was a done deal. The row cover can be lightweight or heavyweight, with the same pros and cons. We elected lightweight, since, again, that's what we already had. Also, he uses a wide row cover that completely covers the ground. Our fabric only covers the individual crop rows. On sunny days, when the greenhouse warms up to quite comfortable temperatures, we have been uncovering the rows, both to let more light in to the plants and to warm the soil. However, with the aisles not covered, they are staying colder. I suspect that the straw mulch we use helps moderate that.
This past fall, after the tomatoes, peppers, cucumbers, eggplants, and basil were played out, we rolled our greenhouse to the next position in the field. We had a row of kale and chard left from the summer garden that we elected to maintain, and had planted 5 other rows of various cold hardy vegetables for the fall and winter garden. Since it's all a new experiment for us, we decided to see how long the crops that are more "cool weather" than "cold weather" would last.
Everything did exceptionally well until early January. The coldest nights we'd had up to that point were about 12 degrees (F) outdoors. All the plants were doing well (except the scallions), and we were still harvesting lettuce, broccoli, arugula, chard, kale, spinach, and cilantro. The carrots, mache, minutina, endive and beets were all growing, but not ready for harvest.
A little over a week ago, the weather turned noticeably colder. We had overnight lows at zero two nights in a row, followed by a night at 18 below zero (as personally observed on an outdoor thermometer). Then there were several nights back in single digits. I didn't check the crops after the two zero degree nights, but after the minus 18 night, most of the crops were wilted. The recording thermometer inside the greenhouse (but above the row covers) registered a low of minus 5. The carrots, spinach, mache, minutina, endive, and some of the cilantro seem to have survived. The experiment is continuing, but our take so far is that we definitely need to add some supplemental heat when outdoor temperatures drop below 10, if we want to grow any of the "cool" crops, but that the "cold" crops will do ok.
Check out the photos of the winter garden below. Steve