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!
I have always been convinced that there is nothing new in agriculture. Growers are so tirelessly ingenious that someone, somewhere is sure to have visited an idea at a previous time. I am fond of the passage in Antoine de Saint-Exupery’s Wind, Sand and Stars, where he suggests that the most delightful design solutions were not invented by anyone “but simply discovered, had in the beginning been hidden by nature and in the end been found.” As we progressed along, investigating our discov- ery, we were sure that someone must have wandered this route before us. And so, while researching some old abstract journals in the library a few years ago, we were not surprised to find a reference to a study published in the 1950’s by E. M. Emmert, a professor of horticulture at the University of Kentucky. That study, detailing his system for winter vegetable production without supplementary heat, led us to his other publications and into the history of the early use of plastics in horticulture.
The more we learned about Professor Emery Myers Emmert (1900-1962), the more we were impressed by his accomplishments. Although originally a biochemist, respected for his work on tissue analysis and plant hormones, he was almost solely responsible for developing the main applications of plastics in vegetable cultivation. He is acknowledged as the father of plastic greenhouses in this country and was belatedly recognized for his pioneering work at the recent 25th Congress of the American Society for Plasticulture. He began trials in the 1930’s by using the largest sheets he could find of the cellulose acetate that covers cigarette packages, stretching and fastening those sheets to lightweight frames, and combining numerous frames on a superstructure to create a greenhouse. Once polyethylene was available after World War II, he built his first plastic covered house in 1949. He designed inexpensive field houses which are the precursors to today’s high-tunnels. He pioneered low-tunnels as well. He developed black plastic mulch. He used tubes of poly lying on the soil to conduct hot-air heat down the length of the greenhouse and deliver it to the root zone through evenly spaced holes in the tube. But the item of most interest to us was his use of an inner layer of plastic inside his plastic greenhouses for unheated winter production.
The drawings of Emmert’s winter greenhouses (above) show an inner layer of polyethylene held about a foot above the soil. Pictures in another pamphlet reveal the plastic was supported by wire wickets. Emmert double-covered his field greenhouses and also used a double layer of plastic for the inner cover. He noted that a protected lettuce crop didn’t freeze inside when the temperature was 10 degrees F (-12 degrees C) outside. He experimented with blowing air under his sheet plastic inner covers to vent them during the day, a concern we don’t share because of the self-venting ability of today’s spun-bonded fabrics.
Much of Emmert’s effort was directed toward using plastic covers to get as much as he could for free from the natural world. In addition to interior layers for his winter greenhouses, Emmert dug ditches 3 feet deep and 12 inches wide that ran alongside the inside walls of the greenhouse and extended outside for 50 feet down a slope. He covered the ditches tightly with a semi-circular plastic hood, so that ground heat, warmed by the sun during the day, could flow up into the greenhouse. He recorded as much as 5 degrees F (2.75 degrees C) warmer nighttime temperatures with this system.
When I read Emmert’s pamphlets I am impressed by the extent of his ingenuity and imagination. There is hardly an idea he didn’t try and usually successfully. But it is dismaying to realize that few people know of him, even in his home state of Kentucky, and no one seems to have followed up on his inner-layer concept. That situation may be explained by the fact that in addition to working for the university, Emmert ran his own commercial vegetable farm where he applied all of these ideas to his own small farm production. I suspect his developments seemed too home-made, too practical, and too small farm oriented when compared with the large scale, technological, and agribusiness bias which was beginning to dominate US agriculture in those days. An agribusiness mentality was unlikely to take note of new methods to improve the competitiveness of small scale local growers. Whatever the reason, other countries were not so blasé. One correspondent told me that Asian and European growers at that time were paying close attention to Emmert’s work.
The continued development since Emmert’s time of high tunnels for unheated production by Asian and European growers obviously reflects their appreciation for simple, less expensive solutions. During a January 1996 research trip along the 44th parallel in France and Italy we saw tunnel greenhouses everywhere, many with smaller tunnels inside them. The growers were adding that second layer to get a slightly earlier start with tomato and pepper crops. They told us it was well worth the trouble because an unheated greenhouse with inner tunnels gave 6 – 7 degrees F (3.5 degrees C) of freeze protection. The temperature records we keep at our farm agree with that conclusion. But our records also show an additional benefit that would not be apparent to those who are only concerned with maintaining above- freezing temperatures for tender crops. The effectiveness of the inner layer increases progressively, as the ambient temperature drops. That 6 to 7 degrees F difference between inside and outside when the temperature is 25 degrees F (-4 degrees C), increases to a 30 to 35 degrees F (16 to 19 degrees C) difference when the outside temperature drops to –15 degrees F (-26 degrees C). The protective blanket of the two layers becomes dramatically more effective just below the point at which tender crops freeze and, consequently, below the point where tender-crop researchers would lose interest.
Out of curiosity we always asked the European growers whether they thought there was sufficient sun to produce salad crops in winter. They were surprised by the question, as might be expected, because their salad crops were beautiful. They told us the difficulty was not day length but cold weather. We laughed at that because compared to their 40 to 45 degrees F (4 to 7 degrees C) average January temperature, we are growing in a climate with a 21 degrees F (-7 degrees C) average January temperature. They were astonished. It has been interesting to note, when we have visited parts of the US with the same (or warmer) January average temperature as the Provencal growers enjoyed, that there is almost no winter vegetable production in high tunnels in those areas. We have wondered if it is fear of cold, the lack in the US of a winter vegetable growing tradition anywhere but Florida and California, or the fear of competition from those traditional winter vegetable areas. It may also be the knee jerk association in this country between the word “green-house” and the word “tomato” to the almost total exclusion of all other crops. Whatever the reason, I think Dr. Emmert’s pioneering low-tech ideas for hardy winter vegetables deserve a following here as well as abroad.
GREENHOUSE COVER – All of our greenhouses but one are covered with just a single layer of plastic. We made that choice for two reasons. First we wanted to maximize light input. Each additional layer of plastic, as, for example, with a double-plastic air-inflated house, cuts out an additional 10% of light. And, second, we prefer to work with systems that are inexpensive and simple. Thus, we decided to forgo the expense of both the second layer and the electric blower required to inflate the space between the layers.
For experimental purposes, we have trialed one small (17 feet by 36 feet) air- inflated house. The results from our temperature records are interesting. The night- time low temperatures averaged 4 degrees F (2.2 degrees C) warmer in the air-inflated house. On a cold night when it was –8 degrees F (-22 degrees C) outside, the temperature was 1 degree F (-17 degrees C) in a single layer house, 19 degrees F (-8 degrees C) under the inner layer, and 7 degrees F (-14 degrees C) in the air inflated house, 24 degrees F under the inner layer. Although there did seem to be slightly faster growth of new seedlings in the air-inflated house, we could detect no difference in the quality of the crops of harvestable size. For the moment we have decided in favor of simplicity and better light input and remained with our original single-layer greenhouse decision.
We are interested in comparing greenhouse plastic from different manufacturers to find the cover that lets in the most light and keeps in the most heat. We presently use a 92% light transmitting plastic that is advertised to have an eight-year life span. In our cold climate we want to increase daytime heat-gain and light-levels, so we favor covers that maximize those inputs. The plastic covers with an anti-drip coating which causes the condensed moisture to form a thin film instead of droplets not only let in most light but the thin film of moisture also acts to reflect back the heat waves radiating from the soil at night thus helping to keep the greenhouse warmer. Growers in the southern states where cold is not as intense may want to look at some of the plastics designed to block infrared input and thus help to keep the greenhouse from overheating.
GREENHOUSE LAYOUT – The majority of our greenhouses are 30 feet wide. Some are 50 feet long and some are 96 feet long. They are laid out with growing beds running lengthwise. There are four beds, each 30 inches wide and separated by 12 inch access paths, on either side of a 16 inch wide central path. A second access path running side to side in the center of the 96 foot houses divides the growing area into four equal quadrants. There are two equal size areas in the 50 foot houses.
With this layout 72% of the greenhouse is actually growing crops. (The edge beds are slightly wider than 30 inches.) We have experimented with 48 inch beds (a space efficiency of 79%), but they are not as easy to harvest because it is a long reach to the center. I have visited European heated greenhouses where every square centimeter of the floor area was planted. I envied their efficiency. But they were clearing the whole house with a single harvest and then replanting again. We make multiple harvests of multiple crops and we need enough working space to remove and replace the inner covers. We are always looking for ways to increase the planted percentage but, for the moment, we have settled on 30 inch beds as the most efficient overall.
ROW COVERS – Years ago, when we first began exploring these winter-harvest ideas in our home garden, we used coldframes inside the greenhouse. They gave excellent protection to the crops. The success of those early efforts convinced us of the benefits of the inner and outer layer concept. But when we began planning to transform our backyard-scale winter-harvest experiments into commercial winter production, we knew cold frames were too costly and too labor intensive for a commercial-scale operation. We thought about placing small tunnel greenhouses inside the larger ones like the Europeans but, on further consideration, the management and ventilation seemed complicated and the use of space seemed inefficient. We considered the motorized night-curtain systems used in heated greenhouses but they were very expensive. We even experimented with adding just the minimal amount of heat to keep the temperature from dropping below 32 degrees F, a practice we still use in one greenhouse. However, after exploring all of the above, we reverted, as we usually do, to starting with the simplest, least expensive option – single layer greenhouses with a floating row cover inside them. If we had gone with more elaborate systems, we never would have known if they were really necessary.
Although we worried that floating row covers as the inner layer might be considerably less protective against cold than our original glass cold frames, the self-ventilating ability of the row covers and their availability in large sizes were overwhelming advantages. And, further, we did not know if we had yet pushed our crops to the lowest temperatures they would tolerate in a protected micro-climate. Our opinion, after many years of practical experience with winter-harvest systems, is that the protected micro-climate we have created is successful principally because it protects against wind (think of wind-chill readings and the desiccating effect of cold dry winds on winter vegetation) and, secondarily, because it protects against the fluctuating wet-dry, snow-ice conditions of the outside winter. In this micro-climate, a few degrees of temperature one way or the other does not appear to be the crucial determinant of survival for most of our crops.
One of the delights of using row covers inside a greenhouse is the ease of management. Since there is no wind, there is no need to bury or weigh down the edges. Even large pieces can be removed and replaced frequently, for harvest and other access needs, with no problems. Our interior covers are 20 feet wide by 50 feet long, large enough to cover one quadrant of the greenhouse. The covers are supported, 12 inches above the soil, by flat-topped wire wickets. We make the wickets from 76 inch long straight lengths of #9 wire. The flat top is 30 inches wide and each leg is 23 inches long. When the wickets are in place over the beds they do not block the one-foot wide access path between the beds.
We space the wickets every four feet along the length of the bed. That provides sufficient framework to support the row cover fabric. When the fabric is in place, we pull it taut and clip it to the end wickets of the quadrant with clothespins. That prevents the fabric from sagging under the weight of condensed moisture which can be substantial at times. We have noticed occasional frost damage whenever the fabric has dropped down and frozen to the leaves below, as opposed to no damage when it is held up above them. The edges of the fabric drape down and touch the soil.
In order to learn how much protection we would be sacrificing by shifting to fabric covers from our original cold frames, we began trials in the winter of 1996-97 using two row cover fabrics, one lightweight and one heavyweight. We compared them with Ludwig Swenson aluminized cloth, which, since it is totally opaque to sunlight, we put on over the fabric every evening for protection and removed every morning to let in light. The same aluminized cloth is used as night-curtain material in heated greenhouses because it reflects back 100% of the long-wave radiation coming from the heat in the soil. Plastic materials such as the row cover fabrics are known to be transparent to long-wave radiation. We expected the aluminized cloth to be more effective and it was, averaging 4 degrees F warmer nighttime minimums than the row cover alone. However, since we detected no differences in crop quality from the warmer minimums and, since the cost of the aluminized material is very high and the twice daily spreading and removing made for a great deal of extra work, especially on mornings when it was frozen to the row cover material beneath it, we decided to stick with the row cover material alone.
The difference with and without the aluminized cloth would be greater without some help from the natural world. The film of moisture which condenses on the floating row covers at night is nearly opaque to long-wave radiation and provides a reasonably reflective surface. There is usually plenty of humidity under the covers when they are covering crops of harvestable size. It is when we have cleared a crop, worked in more compost, and reseeded, that the soil surface can dry out and the air can be less moist. We find we can raise the night temperature a few degrees and benefit the young seedlings by moistening the surface soil if it looks dry. A buried water line with frost-proof hydrants make water available within 100 feet of all the greenhouses.
The heavyweight fabric we trailed is three times heavier than the lighter. Because the extra weight adds a little insulation, the nighttime temperatures at the start under the heavy fabric were a degree or two warmer than under the lightweight. However, the heavy fabric only allows 50% of the light to pass as opposed to 83% to 85% for the lightweight varieties. That means the important daily rewarming of the protected area by the winter sun is inhibited. The end result was colder average temperatures and more soil freezing under the heavier fabric. In this case less is more and simpler is better.
We tried a fabric with reinforcing fibers but they came loose and caught on the support wickets. We then compared the standard spun bonded fabric with two models, 750N and 3000N, of a Japanese row cover called Tuff-Bell. The Tuff-Bell has many pluses and some minuses. Tuff-Bell is extremely durable and should last up to eight years under our protected conditions against only a year or two for the spun bonded fabrics. The Tuff-Bell allows 90-92% light transmission. It has also averaged one degree warmer nighttime temperatures than the spun bonded fabrics. When we gave extra protection, by placing a layer of light fabric over the Tuff-Bell at night, it averaged 3 to 4 degrees F (2 degrees) warmer. Unfortunately Tuff-Bell costs 17 cents per square foot compared to 3 1?2 cents for the other. Also, Tuff-Bell is only available in a 78 inch width. We solved that problem by taking it to a local sail maker where, for a reasonable price, we had it sewn into pieces wide enough to cover our quadrants. The price differential between the two materials balances out in the much longer life of the Tuff-Bell. Under our winter conditions the 750N is preferable to the 3000N because it is not as porous as the 3000N and is warmer at night. However, growers in warmer winter areas, who need less nighttime protection, may prefer the superior ventilation of the 3000N.
Throughout our experimental investigations of greenhouse plastic and inner layers we have been interested in pushing the limits of the system. We have opted for less expense, better light transmission, and simplicity over better temperature protection even though we are in Maine. Those decisions may seem backwards for a winter harvest system without supplementary heat, but the crops have been surprisingly resilient. Nevertheless, we don’t think we have found the answer yet. Some of the floating row covers tear easily and seem too light a material for adequate protection. Under the lightest we have experienced interior temperatures of 16 degrees F (-9 degrees C) when they might have been 24 degrees F (-4 degrees C) or higher with air inflation and more effective night covers.
At the moment, we are comparing Agribon and Covertan, two equally successful inner layers based on performance and cost. Using them, in our climate, we have to give extra protection to three of our salad mix ingredients during the coldest months. We continue to search for more adequate protection while trying to determine where the balance lies between the extra effort to be expended in management and labor and the improved results to be gained in growth and quality.
We expect there may also be a cumulative effect from more substantial night protection. The warmer soil temperature, resulting from more effective nighttime cover during the cold months, should maintain both higher air temperature and a higher rate of plant regrowth. However, it may be possible to achieve a similar effect by allowing higher interior temperatures during the day. Normally we begin to vent our houses when the daytime temperature under the inner layer reaches 70 degrees F. We have actually discussed venting at a lower temperature so that plants would experience a narrower range of temperatures. But we have one small house with inadequate ventilation that gets much warmer (85 – 95 degrees F) during sunny winter days. That house also records higher nighttime temperatures which may be attributable to increased soil heat storage during the day. Thus far we have not detected any difference in crop quality from the higher daytime temperatures. Some plant physiology studies suggest that plants do not respond to normal highs and lows but to the average temperature over 24 hours. If that is so then, following a 20 degrees F night, a 90 degrees F daytime temperature might be better than a 70 degrees F daytime temperature. (A 55 degrees F average over 24 hours rather than a 45 degrees F average.) We have a lot of hunches but no hard options yet on any of these variables. We plan to investigate these ideas further in future years.
The eventual solution may come either through finding ideal inner and outer layer materials or through managing some greenhouses warmer and some cooler as we have been doing experimentally. I can envision a situation, with our present materials and present knowledge, where 60% of our production would be from the very simplest systems growing the hardiest crops, 20% from greenhouses managed for higher nighttime temperatures by using air inflation and/or reflective covers and 20% from greenhouses heated just enough to keep the temperature above 32 degrees F. We still think increasing light transmission by a few percentage points is more important than raising nighttime low temperatures by a few degrees but, to be honest, we do not really know whether light makes a difference when temperatures are low in mid- winter and the crops are basically hibernating.
A most striking introduction to the winter-harvest is provided by making two visits to our greenhouses – one at dawn after a cold night, and the other a few hours later. During the dawn visit all the crops are frozen solid. Raising the inner covers, difficult because they too are frozen stiff with dew, reveals a spectacle of drooping, frost-coated leaves bleak enough to convince anyone that this idea is foolhardy. Yet a few hours later, after the sun (even the weak sunlight of a cloudy day) has warmed the greenhouses above freezing, the second visit presents a miraculous contrast. Under the inner covers lie closely spaced rows of vigorous healthy leaves that stretch the length of the greenhouse. The leaf colors in different shades of greens, reds and maroons, stand bright against the dark soil. It looks like a perpetual spring.
Over the course of devising, developing, and improving this winter-harvest concept, we have amassed a collection of technical studies on hardy crops and the effect of freezing temperatures. Copies of research papers fill our file cabinets. Yet none of them offer as much information (or inspiration) as those two visits to the greenhouse. In the natural world, hardy crops like spinach and chard inhabit niches where resistance to cold has been a requirement for survival. Winter annual crops, like mache and claytonia, have found their space to grow by germinating in fall, growing over winter, and going to seed in spring. Whereas the outdoor winter climate here in zone 5 Maine is too harsh for even the hardiest of them, the twice-tempered climate under the inner layer of our greenhouses offers them conditions within the range in which they have evolved.
With the exception of the true winter annuals, which do not seem inhibited by the short days of mid-winter, the rest of the hardy winter-harvest crops need to make most of their growth in the fall – thus the importance of precise timing of planting dates. In our experience, and that of other amateur and professional greenhouse growers, the point at which plant growth slows way down is the point at which the day length becomes shorter than ten hours. We call this period the Persephone months after the Greek myth of the earth goddess Demeters’ daughter, Persephone, who would spend the low sun months with her husband, Hades, in the nether world. Demeter, saddened by her daughter’s absence, made the earth barren during that time. For us here on the 44th parallel, the winter days are shorter than ten hours from November 7 to February 7. For those on the 39th parallel, like Washington, DC, the dates are November 16 to January 24. For Charlotte, NC, on the 35th parallel, the period of day lengths under 10 hours extends from December 1 to January 10. You can determine the Persephone period for your location from the “Calendar” and the ‘Time Correction Tables’ of the Old Farmer’s Almanac.
Many crops with well established root systems, like spinach, claytonia, and minutina, continue slow regrowth right through the Persephone months. New sowings from mid-November on, however, although they germinate, will just sit there waiting expectantly. Once the sun returns, they are off and growing. Those sown first will mature first as the season progresses.
We keep replanting right through the winter as spaces open up following harvest. That is another bonus of the twice-tempered micro-climate. The soil under the inner layer of our greenhouses experiences no more than light surface freezing on the coldest nights. That is as one would expect from its zone 8 climate. With the possible exception of one or two days each winter, when an extremely cold night is followed by a heavily clouded day, the soil is always unfrozen and ready to rake into a fine new seedbed for replanting. As I mentioned earlier when discussing our choice of inner covers, that diurnal influx of sun heat, bringing the covered area above freezing during the day, is the key to successful winter production.
Another significant biological reality is that too warm is more damaging to these winter-harvest crops than too cold. For us here in zone 5 Maine the transition month is March. We heed the soothsayer’s caution to Caesar and “beware the ides of March” because by March 15, at the latest, we need to begin folding back the inner covers on sunny days to prevent overheating of the crops underneath. Those in warmer climates should plan to begin one month after the end of the Persephone period. March is also the month when we start irrigating again, whereas, because of low evaporation and high water tables, we have had to provide little supplemental moisture during the winter months.