Winter Production of Fresh Vegetables
by Eliot Coleman of Blue Hill, ME
An Excerpt from THE WINTER HARVEST MANUAL
The winter was not given to us for no purpose. We must thaw its cold with our genialness. We are tasked to find out and appropriate all the nutriment it yields. If it is a cold and hard season, its fruit, no doubt, is the more concentrated and nutty. – Henry David Thoreau
Any claim about winter production of fresh vegetables, with minimal or no heating or heat storage systems, seems highly improbable. The weather is too cold and the days are too short. Low winter temperatures, however, are not an insurmountable barrier. They can be sufficiently moderated for cool season crops by the simple greenhouse protection described below. Nor is winter day-length the barrier it may appear to be. In fact most of the continental US has far more winter sunshine than parts of the world where, due to milder temperatures, fresh winter vegetable production has a long tradition.
Day-length is determined by latitude. Our farm in Maine is on the 44th parallel of latitude, the same as Eugene, Oregon and Oshkosh, Wisconsin. On the other side of the Atlantic the 44th parallel runs across the far south of France and the upper-middle of Italy. Although Provence in southern France and the Ligurian coast and northern Tuscany in Italy have a milder winter climate than Maine thanks to the influence of the Gulf Stream and the Mediterranean, a 40 degree to 45 degree F (4 degrees to 7 degrees C) average January temperature compared to 21 degrees F (-7 degrees C) at our farm, those areas of Europe have the same winter day-length as we do since they lie along the same 44th parallel. Because the French and Italians appreciate the value of fresh food, commercial production of winter hardy crops is a common affair in those regions. Whatever they can do with the help for the Gulf Stream, we can duplicate with a little ingenuity.
That ingenuity involves combining four familiar ideas – tunnel greenhouses, row covers, hardy vegetables, and succession sowings. In a world of ever more complicated technologies, our winter-harvest is refreshingly simple because all of those ideas are well known to most commercial vegetable growers. What is not well-known is the synergy created when they are used in combination.
GREENHOUSES – Our winter-harvest greenhouses are standard, plastic-covered, gothic-style, hoop-houses, also known as high tunnels. Most of our houses are 30 feet wide and either 50 or 96 feet long. They are aligned on an East-West axis and are covered with a UV resistant plastic. They have no heating or heat storage systems.
ROW COVERS – Almost any of the lightweight translucent fabrics that allow air and water to pass are suitable. Floating row covers, like greenhouses, temper the climate beneath them. By placing row covers over crops inside the greenhouse, we create a twice-tempered climate. Since the double coverage also increases the relative humidity in the protected area it offers additional protection against freezing damage. Below-freezing temperatures still occur, but nowhere near as low nor as stressful for the crops as they would be in unprotected outdoor conditions. The climate modification achieved by combining inner and outer layers, like wearing a sweater under a windbreaker, is the technical foundation of this low-input winter-harvest concept.
COLD-HARDY VEGETABLES – The list of chilling-resistant vegetables includes the well-known – spinach, chard, carrots, scallions – and the novel – mache, claytonia, minutina, arugula. At present there are some 30 different vegetables – arugula, beet greens, broccoli raab, carrots, chard, chicory, claytonia, collards, dandelion, endive, escarole, garlic greens, kale, kohlrabi, leeks, lettuce, mache, minutina, mizuna, mustard greens, pak-choi, parsley, radicchio, radish, scallions, sorrel, spinach, tatsoi, turnip greens, watercress – which at one time or another we have grown in our winter-harvest greenhouses. Those with which we have the most experience are discussed later in the section title “Crops.” These chilling-resistant vegetables are far hardier than growers might imagine and many can tolerate temperatures as low as 12 degrees F (-13 degrees C) without damage as long as they are not exposed to the additional stresses of outdoor conditions. These cold-hardy vegetables are at their best during the cooler temperatures of fall, winter, and spring. They are more tender and more flavorful without the heat stress of summer.
SUCCESSION PLANTING – We begin planting the greenhouse sites on August first, the start of what might be called the “second spring.” We continue planting through the fall. Timing the sowings takes some getting use to because the seasons are
reversed. Day length is contracting rather than expanding; temperatures are becoming cooler rather than warmer. Those conditions dramatically affect time to plant maturity. Thus the choice of precise sowing dates for fall planting (see Appendix E) is much more crucial than for spring planting. The dates are also very crop specific. In order to be able to harvest throughout the coldest months the goal is to get the plants up to a certain size before the length of day drops below ten hours. From then on growth slows way down and the plants basically hibernate until we harvest them, or until they begin vigorous growth again in response to increasing day length. Younger plants from a succession of fall planting dates will be hardier during the winter than older plants from summer plantings. The combination of hardy vegetables and precise planting dates is the biological foundation of this low-input winter-harvest concept.
The winter-harvest can be practiced successfully by combining those four familiar ideas. In our case we have added one new twist by revising an old European practice – the mobile greenhouse.
MOBILE GREENHOUSES – Instead of being attached to a foundation of short pipes driven into the ground, the hoops of our tunnels attach to a pipe-rail the length of the greenhouse. These pipe-rails, one supporting each side of the greenhouse, sit on the ground like sled runners. They allow the greenhouse to be moved in a straight line, like a sled, to an adjacent site. When in place, the pipe-rails are held down by a series of ground anchors.
This innovation has a number of advantages. First, it allows us to avoid the cost of expensive greenhouse cooling systems when starting our winter crops in August. The winter crops are planted out-of-doors on the adjacent site. Heat-loving summer crops such as melons, eggplants, tomatoes, peppers, cucumbers, and sweet potatoes can be grown in the greenhouses during the summer. When the summer crop season is finished, sometime in mid-October here in coastal Maine, we move the houses over to cover the winter crops. If, instead of growing summer crops, the houses have been sown, directly after the end of our winter harvest, to a long-term green manure for soil improvement, we will move them in late September so as to cover the winter crops slightly sooner. The following year, the same process happens in the reverse direction.
The second advantage of a mobile greenhouse is the avoidance of the pest and disease build-up and excess soil nutrient problems which so often arise in a permanent greenhouse. For one year out of every two our greenhouse soil is uncovered, exposed to the cleansing powers of sun, rain, wind, and snow. As an additional advantage, the uncovered year encourages us to grow a long-term, deep-rooting, leguminous green manure crop on the uncovered section. This green manure occupies the soil of the alternate site for either 13 months (June through the following July) if sown in June or 9 months (October through July) if sown after a summer crop. Its virtues of protecting, enriching, and aerating the soil are an important part of our soil fertility maintenance program. We turn the green manure under three to four weeks before the start of planting of the next winter season crops.
On any given December/January/February day, as I walk to the greenhouses across a typically cold, snow covered, winter landscape in Maine, it does seem highly improbable that I am about to harvest fresh salads or work the soil into a fine seedbed for replanting; especially, since we don’t add supplementary heat to the greenhouses, and the low temperature the previous night was near 0 degrees F (-18 degrees C). Even though I have been working with these systems for many years, I continue to be amazed by the daily miracle. When I enter the protection of the greenhouse, I can take off my parka because the micro-climate I encounter is that of a location approximately one and one-half USDA zones to the south. When I reach my hand under the row covers I have moved another one and one-half zones south where the Maine winter definitely does not prevail. Outdoors the climate is zone 5; under the inner layer, the climate is zone 8. (See the USDA zone map.)
We prefer not to call our structures “unheated” greenhouses because that makes it sound as if we are not doing something – heating – that we should be doing. At times we have even avoided the word “greenhouse” since many people assume that green- houses, if unheated, are super-insulated technological marvels or complicated heat storage devices. Ours are neither. The best short statement to describe our approach is a quote from Buckminster Fuller in his book Shelter (1932) – “Don’t fight forces; use them.” Instead of bemoaning the forces of winter and trying to fight them by adding heat, we have limited our intervention to the climatic protection provided by two translucent layers. Instead of trying to grow heat-loving crops during cold weather, we have said, “So, it’s cold, great! What likes cold?” The answer is some 30 or more hardy vegetables.
Fighting force requires energy and energy costs money. Our approach is to take advantage of everything our two translucent layers can get for free from the sun and the residual heat of the soil mass and the work within those limits. In our minds we have created inexpensive “protected micro-climates” and then found the plants that will thrive in those micro-climates. The same applies in reverse during the summer. When those protected micro-climates are extra warm, we don’t fight that warmth with motorized greenhouse cooling systems. We use it to grow heat loving crops.
Obviously, those growers in climates with less severe winter weather can grow a wider range of crops in unheated greenhouses than we can because the duration and the depth of temperature drops will be less severe. They can similarly create protected micro-climates for the same winter crops we grow with fewer resources than we require. Growers in zone 8 and 9 could get by with row covers alone in areas with little snow. Growers in zone 7 might find a double-covered, air- inflated tunnel sufficient protection without the row-cover inner layer.
OVERCOMING PRECONCEPTIONS – When we started experimenting with the winter-harvest, many people assumed freezing temperatures would kill all the crops. Obviously that is not so with these cold-tolerant vegetables. Others predicted we would fail because of inadequate sun in winter. They were astonished when we informed them that the city of Portland, Maine on “the sun-baked Atlantic coast of Maine,” as we now jokingly call it, lies on the same parallel of latitude as the warm sandy beaches of St. Tropez, France; that New York City shares the 41st parallel with Naples, Italy; that Washington DC on the 39th parallel lines up with the Mediterranean island paradises of Majorca and Corfu. By contrast, the renowned winter cold-frame vegetable production around Paris in the 1800’s was up on the 49th parallel, the same as Gander, Newfoundland. Today’s impressive, state-of-the-art, year-round greenhouse industry in Holland, which presently supplies winter vegetables to much of Europe, is on the 52nd parallel, the same latitude as Battle Harbor, Labrador. The truth is that despite the much colder winter temperatures in the northern half of the US compared to Europe, we enjoy a major advantage over growers in those more temperate climates where winter vegetable growing in unheated high tunnels is a common practice. The continental US lies further south on the globe and, consequently, we have more winter sun!