Wednesday, 26 June 2019



solar greenhouse

Although several Extension bulletins are available on greenhouse vegetable production, few of these concentrate on organic production methods. This publication presents an overview of greenhouse production systems and profiles several farmers raising organic vegetables in greenhouses. It is hoped this will give new growers ideas of how to set up their systems, and provide more experienced farmers with examples of alternative methods of production.

 The term greenhouse means different things to different people. A greenhouse used to be a the structure formed of glass, with a heating (and usually cooling) system that was used year-round, but especially in winter. Then came houses built of thermoplastic (Plexiglas and others), followed by Quonsets covered with plastic, which may or may not be heated, have one or two layers, and be used year-round or for only a few months every year. The type of greenhouse you have will largely be determined by your crop and your capital and, to a lesser extent, by your management intensity and your market strategy. 

For the purposes of this publication, a greenhouse can be any of the above. Another ATTRA publication, Season Extension Techniques for Market Gardeners, contains further information on protected shelter structures such as cold frames, high tunnels or “hoop houses,” and low tunnels.

The Greenhouse Vegetable Industry

 The U.S. greenhouse vegetable industry is a mixture of small, family-run operations in the 2,500 to 10,000 square foot range and a small number of large, multi-acre facilities 10 acres or more in size. The larger greenhouses often use waste heat from a power plant or other source of cogeneration (1). 

Current U.S. production estimates are somewhere around 800 acres (2). In comparison, Mexico has about 450 acres, Canada has about 1,600 acres, and Holland has over 11,000 acres (3, 4). In the latter part of the 1990s, Canadian greenhouse vegetable production grew at a rate of 20% a year. How has Canada been able to generate this huge growth? “Significant new greenhouse vegetable production technology that was transferred to commercial producers have been primarily responsible for dramatic yield increases over the last 7–8 years, estimated at 100–120% for tomatoes and 70–80% for cucumbers” (3). Canada is strongly supporting its greenhouse growers, both with research and with investment dollars. Their research facilities at Harrow is recognized as topnotch the world over. Although most Canadian greenhouse vegetables are not produced organically, there has been an emphasis of late to use IPM strategies, rather than pesticides, to accommodate the growing market of consumers who want pesticide-free produce. Most of the organic produce imported into the U.S. is now coming from Mexico.

 Tomatoes are the leading greenhouse vegetable crop, followed by European cucumbers, lettuce, peppers, and culinary herbs such as basil, sage, and rosemary. See the ATTRA publications on these specific vegetables for more information. In addition, growers aiming at niche markets raise specialty crops, greens, and Oriental vegetables.

How Can Small Producers Compete?

With so much competition from Canada, Mexico, and overseas, how can small farmers realize a profit raising greenhouse vegetables? One issue of increasing importance to consumers is vegetables grown with minimum pesticides. The public has also become educated on the values of locally grown produce: it’s fresher, it tastes better, and it may even be less expensive since there are fewer shipping costs involved. Also, money paid to a local farmer is re-invested in the local community and helps to keep that economy strong. 

Year-round production is key to maintaining the greenhouse’s profitability. However, this does not necessarily mean that growers should be producing the same crop year-round. (Winter tomatoes bring more money than do summer ones.) Another option would be to raise a crop other than vegetables, like bedding plants for early spring sales or poinsettias for Christmas. The grower may decide that the most cost-efficient way to use his or her greenhouse during the summer is to shut it up to solarize the soil and “cook” insects (and their eggs) that are present.

 Small growers must find niche markets. It is pointless to try to compete with mass merchandisers like Wal-Mart because the small grower will always lose. What are some niche markets for organic greenhouse vegetable producers? Some of the general niches have already been mentioned: consumers are looking for organic, locally grown, early-season produce. Whatever the niche market, it is important for growers to realize that the nature of niche markets is for them to disappear after a while. Oversupply or lowered demand will create lower prices. The market will change to favor one product and disfavor another. This may happen when mass merchandisers enter the market, when the popular press promotes a particular vegetable, or when new medical evidence points to increased or decreased health benefits from certain vegetables.

Solar Greenhouses 

solar greenhouse

Greenhouses can be designed to take advantage of solar radiation and cut fuel expenses. Solar greenhouses are popular with small-scale growers. These are super-insulated greenhouses designed to collect and retain solar energy. The technology associated with solar greenhouses is rather detailed. In addition, the literature on solar greenhouses is quite large. To help growers identify some of the best resources on this topic, ATTRA compiled the Solar Greenhouses Resource List. One recent publication that features organic vegetable production in a solar greenhouse is Anna Edey’s book Solviva: How to Grow $500,000 on One Acre and Peace on Earth. Solviva is Edey’s award-winning solar-powered and animal heated greenhouse on Martha’s Vineyard. The book discusses greenhouse design, function, construction, and management. Ms. Edey includes numerous energy-efficient designs like water walls and grows tubes. She also tells how much everything costs, which is invaluable for market gardeners.

Composting Greenhouses 

Heating greenhouses with waste heat generated by compost is a second option that takes advantage of local resources and integrates different farm activities. In a composting greenhouse, heat and carbon dioxide are generated from manure-based compost contained in a special chamber attached to one side of the greenhouse.

 Compost-heated greenhouses gained a lot of attention from work undertaken at The New Alchemy Institute at Falmouth, Massachusetts. The New Alchemy Institute was one of the premier appropriate technology centers that operated in the 1970s and 80s. The Institute published widely on ecology, wind energy, solar energy, shelters, solar greenhouses, integrated pest management in greenhouses, organic farming, and sustainable agriculture. 

Though the technology to implement compost-heated greenhouses exist, they are seldom done on a commercial scale. ATTRA can provide more information on this topic on request.

Animal-Heated Greenhouses

Small animals like chickens and rabbits produce heat and carbon dioxide in addition to products like eggs and meat. A few growers have taken advantage of this fact and integrate animals with greenhouses as a source of heat. However, it can be a challenge to keep livestock in a greenhouse—the higher temperature and the humidity of a greenhouse are generally not healthy for animals. 

Anna Edey, mentioned above, uses an “earthling” to filter out the toxic ammonia gas from the rabbit and chicken manure she uses in her Solviva greenhouse. In addition, she keeps her chickens in a poultry room attached to the greenhouse where temperatures do not fluctuate from about 70°F.

Organic Greenhouse Production 

As defined by the USDA in 1980 (6), organic farming is a system that excludes the use of synthetic fertilizers, pesticides, and growth regulators. Organic farmers rely heavily on crop rotations, crop residues, animal manures, legumes, green manures, organic wastes, and mineral-bearing rocks to feed the soil and supply plant nutrients. Insects, weeds, and other pests are managed by mechanical cultivation, and cultural, biological and rational controls. 

Organic certification emerged as a marketing tool during the 1970s and 80s to ensure foods produced organically met specified standards of production. The Organic Foods Production Act, a section of the 1990 Farm Bill, enabled the USDA to develop a national program of universal standards, certification accreditation, and food labeling. Early in 1998, the USDA


 Although the process is more complicated, it is possible to obtain adequate nutrients from organic sources, but it takes more care and creative management. No single fertilizer will provide all of the essential elements required, but a combination of organic products can be devised.

 Organic fertilizers have not been well researched in greenhouse vegetable production. However, a 1999 study performed at The University of Kentucky analyzed several products for the levels of nutrients they supplied. The researchers were attempting to prove that organic fertilizers could supply nutrients at the same level as synthetic fertilizers. Products derived from algae (Again, a liquid, and Maxicrop, a powder), bat guano, and fish waste (Greenall Fish Emulsion, a liquid, and Mermaid’s Fish Powder) demonstrated nutrient levels comparable to conventional, synthetic fertilizers used for greenhouse plant production (7). The report concluded that these organic fertilizers could not be used as a concentrate for injector systems, but they would be suitable in a capillary mat sub-irrigation system. For information on how to obtain these products, see the Resources section.

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Introduction  Although several Extension bulletins are available on greenhouse vegetable production, few of these concentrate on organ...