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Eliot Coleman on The “Cool” Greenhouse

As summer winds down here on the East coast, many of us are seriously considering extending our seasons. This means, aside from preserving our summertime harvest, we’re thinking of ways to actually grow more of it during the winter. I’m talking greenhouses, here. I’m talking low-cost, efficient, and unheated greenhouses. And who better to look to for advice than Eliot Coleman.

The following is an excerpt from The Winter Harvest Handbook: Year-Round Vegetable Production Using Deep-Organic Techniques and Unheated Greenhouses by Eliot Coleman. It has been adapted for the web.

Our minimally heated greenhouses are our “cool houses.” Cool houses offer options for midwinter growing and marketing beyond those of the cold houses. However, once you begin adding heat to a greenhouse and get “the month of May,” you’re on a slippery slope. With incrementally more heat you can have the months of June or July or August and make the move into the “hothouse” realm. Many greenhouse growers have followed that path and grow only tomatoes or cucumbers. That was never our intent. We have always been interested in growing a wide range of hardy winter crops that could fully supply our local markets and using the minimal amount of energy to do so.

We began researching the potential of a just-above-freezing cool greenhouse in order to increase the variety of our winter production. As I described in chapter 1, we were using one end of a greenhouse for washing and packing produce, and we set the thermostat at 35˚F (1.5˚C) so as to protect our water supply from freezing in winter. The results we saw with the vegetables growing in the rest of that house—twice as many harvests per winter compared to the unheated houses and a wider selection of crops—caught our attention.

We have continued to set our cool-house thermostat at a just-above-freezing temperature because that has proven adequate for the cropping options we wish to explore. Yes, by adding heat we are going against Buckminster Fuller’s advice not to “fight forces,” but we have tried to determine the least costly way to do it.

We think of the minimal heat in the cool house as a different kind of protective layer. From another perspective, we can compare our minimal-heat system to the developments in energy-saving design for automobiles—the hybrid versus the pure electric model. Some auto researchers have determined that for the best overall efficiency, the combination of an electric motor with a small gasoline motor is a better choice than the electric-only model. We have been interested in determining whether, for the economic success of a four-season farm in our climate, the combination of unheated greenhouses and minimally heated greenhouses would provide a better income and a more competitive position versus the shipped-in imports.

We have taken the logical steps to make the cool houses efficient. They are double-covered (two layers of plastic with an airinflation fan). According to greenhouse research, that 4-inch dead-air space between the layers of plastic can lower our fuel consumption by up to 40 percent. Also, we make sure the houses are tight and the doors and vents fit well to prevent cold air infiltration.

Is the just-above-freezing temperature at which we set the thermostat the lowest nighttime greenhouse temperature that will assure success with the midwinter crops we wish to grow? Could they tolerate an even lower temperature without losing ground? We continue to investigate this question. Our sense based on some informal trials conducted in the early ’90s is that there may be no damage to most of these hardy crops as long as the minimum temperature doesn’t drop below 26˚F (–3˚C). (We ran these trials in an experimental greenhouse using a radiant heater and a wide-range thermostat.) This has also been our experience with outdoor crops. The occasional spring frost below 26˚F is when we have noticed cosmetic damage on hardened-off early lettuce or broccoli transplants, whereas they seem unaffected by temperatures above that level. However, on a few occasions when heaters have malfunctioned, we have also noticed that temperatures just below freezing, although resulting in no cosmetic damage, do slow down growth for up to a week after the freeze. Thus, from the point of view of increasing winter production, a dependable nighttime minimum above 32˚F (0˚C) makes sense.

Adding More Heat

Once we began to explore minimum heat we put heaters in 60 percent of our greenhouse space because the demand for our produce constantly exceeded the supply. We realized that we could achieve the equivalent growing area of a whole new greenhouse simply by adding heat to one of the cold houses, because the added heat would allow us to double the number of winter harvests in that house. The cost of a heater is much less than the cost of a new greenhouse. Furthermore, we didn’t have to worry about covering and maintaining an additional greenhouse, and we could make better use of the fertile soil we had already worked so hard to create. In addition, as tough as we may think we are, we also appreciated the better working conditions in midwinter in a house where we could raise the temperature if we wanted to.

In addition to increasing the level of production in winter, the above-freezing nighttime temperature in the cool houses also increased the variety of crops we could grow during the colder months. For example, popular crops such as turnips, radishes, ‘Bianca Riccia’ endive, and arugula are not available for quality harvest during December, January, and February in the cold houses. However, these crops are successful in the cool houses and, being cool-weather crops, their quality is outstanding. In the future, we may find cultivars and/or passive protection techniques that will be successful for these crops in the cold houses throughout winter, but we have not found them yet. Lettuces for the Christmas market in a cool greenhouse.

Heating Options

During the early trials, standard propane-fueled greenhouse heaters warmed our cool greenhouses. These heaters are smaller than would be required if we needed to maintain a 65˚F (19˚C) night temperature in midwinter for a crop like tomatoes. But are there other options that make practical and economic sense at the moment? We have recently installed a large wood furnace in our washing and packing greenhouse to replace the propane heater (except during exceptionally cold weather), and it is a reasonable improvement. We have priced a wood-fired hot-water boiler system that could warm all the cool houses, but the initial cost would be ten times as much as we have spent on propane heaters. In addition, unless it was a self-feeding wood-chip system, we would have to spend a lot of time loading the furnace at night. We continue to look for renewable solutions. There are exciting recent developments in using very hot-burning wood furnaces to heat hot water during the day. The water is stored in large insulated tanks and drawn on to heat the greenhouse at night. These systems avoid the air pollution and creosote buildup of damped-down wood fires and the need for keeping the furnace fueled for twenty-four hours a day.

We have friends who have built an ingenious system that burns used cooking oil directly for greenhouse heating. (Information on the design and their experience with the system is available at We envy growers in more temperate parts of the country where these heating concerns are not an issue.

Our experience thus far is based on the very simple unheated systems we started with and the minimally heated systems we have been trialing. Two other options for winter production are adding artificial lighting to create longer days and installing warm-water pipes in the soil to raise soil temperature. We have not experimented with artificial lighting because it would add another major use of energy and because no light bulb can truly duplicate sunlight. It seems to us that if we use incomplete, artificial light, there would be something missing in the quality of the resulting produce.

We have been slow to investigate soil heat for the same reason. We try to keep our production systems as natural as possible, and we can think of no situation in the natural world where agricultural soils are warmer than the air above them in midwinter. However, from the point of view of improving crop growth and complementing an air-heating system, soil heat has much to recommend it. The soil temperature, even in our minimally heated houses, drops to 42˚F (6˚C) at the 4-inch depth by midwinter. Some evidence suggests that lettuce growth does not slow down in winter because of shorter days—lettuce can only use eight hours of light—but because of the cooler soil temperatures. Thus, warming the soil seems a logical step for winter production. In addition, greenhouse studies have shown that soil heating can supply about 20 percent of the total heat needed by a winter greenhouse depending on the desired greenhouse temperature and the crops to be grown. We wonder whether a soil-heated greenhouse with wickets and an inner layer would keep the area under the row covers above 32˚F (0˚C) at night without having to heat the air above. If so, or if almost so, that could save a lot of money on the fuel bill for minimal heating.

All of our trials in the cool houses and all of our speculations about how to do it better are fascinating, but we remain content with our decision, as mentioned earlier, not to continue in that direction. We now add minimal heat to only one small growing area where we still do a few trials, but we may end even that. The challenge of the simple, minimalist, unheated production is where our hearts lie and where we will concentrate our efforts.

One minimal heating option we did seriously consider but haven’t tried is the “earth tube” concept. A smooth-walled, rigid plastic pipe with a 12-inch diameter is buried about 6 feet deep for 100 feet across a field and then into to the greenhouse. A fan would draw outside air through the pipe. At that depth, the air would be warmed to the temperature of the earth—45°F (7°C). The air would then be blown under the inner layer down the length of the greenhouse through perforated plastic tubes. According to research, the energy use by the fans would be about 15% of that required to create the heat artificially.

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