GREENHOUSE HORTICULTURE

GREENHOUSE HORTICULTURE. Plant cultivation is influenced by various factors, such as soil quality, water availability, and climatic conditions. Techniques have been developed either to adapt food crops to their environment (as by breeding and selecting plants more resistant to drought or with shorter production cycles), or to adjust the environment (for example, temperature, nutrient supply) to meet plant needs. Practical means of modifying the environment surrounding the plants have involved methods such as the use of windbreaks, mulches, plant or row covers, and cold frames. These methods of protecting plants may be described as passive methods since they only raise barriers between the plants and their environment, and do not control the environment. Some types of garden frames (hotbeds, heated frames) may be heated by artificial means, but do not actually provide for any control of the environment. The only method of food crop production that makes use of control of the environment is greenhouse production. Modern greenhouse production is also referred to as controlled environment agriculture (CEA). With the use of a greenhouse, it is possible to cultivate food-producing plants in locations and at times when climatic conditions would adversely affect them or even prevent them from growing. Also, when climatic conditions allow outdoor plant cultivation, greenhouses can be used to protect crops against weather phenomena (such as wind, excessive rain, or hail) that would negatively affect them. For the purpose of this article, the term "greenhouse" is defined as a structure covered with a transparent or translucid material, in which environmental conditions can be modified or controlled, for the cultivation of plants. Tunnels are also used to modify environmental conditions for plant production, but are not usually considered greenhouses. Since the distinction between greenhouses and tunnels is not always clear in the literature, both structures, when high enough for people to move and work freely in them, will be considered together.

Food Produced in Greenhouses

Although greenhouses have been in existence since 1800 (or earlier), and greenhouse food production started to develop as an industry in the second half of the nineteenth century, the largest growth and expansion of the greenhouse industry occurred throughout the world following World War II. Today, food production in greenhouses can be found in all continents. Most popular food crops grown in greenhouses are tomato (beefsteak, cluster, Italian, cherry), cucumber, and sweet pepper. Other greenhouse grown vegetables include watermelon, muskmelon, summer squash, zucchini, lettuce, eggplant, snap beans, celery, cabbage, radish, Welsh onion, and asparagus. Fruits such as grapes, strawberry, banana, pineapple, papaya, orange, mandarin, cherry, and fig, as well as culinary and medicinal herbs, are also grown in greenhouses.

Today's Greenhouses

Covering materials. The main greenhouse covering materials are glass and polyethylene (PE). Glass has been used since the early days of greenhouses. The introduction of PE film after World War II was the main reason for the expansion of greenhouse production around the world, and it is now the most widely used covering material in the world. Glass-covered greenhouses are concentrated mainly in northern Europe and North America. The low cost of the PE greenhouse is the main reason for its high popularity, especially in developing countries. In recent years, the use of PE-greenhouses has even spread to northern regions. Research has shown that, under Canadian climatic conditions, heating costs of a double-layer PE-greenhouse are 20 to 30 percent lower than for a glass-covered greenhouse. Most of the greenhouses built now in Canada are covered with PE. Standard PE film blocks the ultraviolet, but not the infrared radiation, and has a short durability. However, improved PE films retain the infrared, but allow the ultraviolet, radiation (necessary for the bees, used for pollination of plants, to orient themselves)in the greenhouse, and are more durable. Polyvinyl chloride (PVC), another plastic film used to cover greenhouses, is used mostly in Japan. Other covering materials for greenhouses include rigid plastic acrylic, fiberglass, polycarbonate, and PVC panels, but their use is generally limited because of their high cost, compared to PE. Beside glass and PE, polycarbonate is often used on the sidewalls of polyethylene greenhouses in northern regions because of its good insulation, durability, and reasonable cost.

Technology in the greenhouse. Greenhouses come in many styles and sizes, from the original houses with minimal climate control (furnace and vents) to the modern 10-ha (25-acre) or more, multispan greenhouses with high-tech climate controls (sophisticated and powerful heating system, CO ₂ enrichment, evaporative cooling pads, exhaust fans, roof vents, thermal/shade curtain, computer controls, light sensors). Most sophisticated greenhouses are generally found in the developed, northern countries. Phytotrons are highly sophisticated structures that allow for accurate control of environmental conditions including light, and are generally used for scientific research in universities and research institutes. However, phytotrons cannot be considered greenhouses since they are not covered with a transparent material.

The degree of environment control needed depends on various factors. The first factor is the location of the greenhouse (local climatic conditions). Northern regions are characterized by cold winters and warm summers. If the objective is to grow plants all year long, then such large differences in climatic conditions between winter and summer require a high-tech greenhouse. In regions such as the Mediterranean (Spain, Italy, Morocco, Greece), the mild winter climate does not require the use of powerful heating systems, and low-tech greenhouses are sufficient for winter production. However, these regions have very hot summers, and the use of a low-tech greenhouse may not provide satisfactory temperature control to grow plants during summertime.

The production schedule also affects the level of environment control and thus the level of technology. A greenhouse in northern regions may require a high level of climate control if the objective is to grow crops all year long (or long-season crops). If the objective is only to extend the production season (e.g., one early crop in spring), then a less sophisticated greenhouse could be satisfactory.

Optimal growing conditions differ from one species to another. For example, lettuce prefers cooler temperatures than cucumber. Thus, the crop grown in the greenhouse may influence the level of environment control needed or desired. A low-tech greenhouse may provide

sufficient climate control for lettuce but not for cucumber, depending on the location of the greenhouse and the production schedule.

Economic development also plays a role in the level of technology used in the greenhouse. In developing countries, growers may not be able to afford the most sophisticated equipment, and may lack technical expertise and technical support.

Greenhouses in desert regions. Although greenhouses were developed in northern regions as a means of protecting crops against cold temperatures, and are therefore generally associated with cold climates, they are also used in arid regions such as Saudi Arabia. In such regions, the objective of the greenhouse is to protect plants from the excessive solar radiation and temperature, and to prevent excessive water loss by plants (especially since water resources are generally limited in those regions). Therefore, technology in greenhouses in these regions is directed toward cooling.

Artificial lighting. In northern countries, high-tech greenhouses can provide optimal growing conditions (temperature, humidity, carbon dioxide) for vegetable crops even during the coldest winter months. However, even with excellent climate control, yield and quality of crops grown during these months are low due to the low light level available. Research has shown that it is possible to produce good yield of high-quality produce during the winter months by using artificial light to supplement the natural radiation. The most common artificial lighting is the high-pressure sodium lamp. The high cost of electric energy in many regions is the most important factor preventing an increased use of artificial light.

Production Systems

Growing in soil. Since the early days of greenhouses, plants have been grown in soil or in soil-filled containers. The first technique for fertilizing plants, which is still in use today in organic production, was the use of manure. Today, fertilization of plants can also be accomplished by incorporating chemical fertilizers in the soil, or by distributing fertilizers dissolved in water (so-called fertigation) to plants with a drip (trickle) irrigation system. Intensive and repetitive cultivation of crops on the same soil generally results in a degradation of soil properties and fertility. Salt accumulation may be another problem in soil cultivation. Incorporation of manure, compost, and other organic materials into soil can be used to improve its structure and replenish its fertility. However, ensuring perfect fertilization of plants grown in soil is still a difficult task. Furthermore, intensive and repetitive cultivation of crops on the same soil can also result in insect or disease infestation. Soil replacement and soil fumigation are two solutions, but the first technique is expensive and the second is not always successful. Greenhouse production in soil is still used widely.

Growing without Soil

In order to better control fertilization for optimizing plant growth and yield, and also to avoid the problems occurring in soil, growing systems that do not use soil (soilless) were developed for the cultivation of greenhouse crops. These soilless systems can be classified in two groups: liquid (water) and solid (artificial substrates that are either inorganic or organic). Systems using water as a growing medium are the nutrient film technique (NFT), deep flow technique (DFT), and aeroponics. Common inorganic media are rockwool, vermiculite, perlite, and clay pellets. Organic substrates are peat, coconut coir, sawdust, and straw. Inorganic and organic substrates are usually contained in bags, and plants are irrigated with a complete nutrient solution distributed by a drip irrigation system. The excess of nutrient solution can either be allowed to leak into the ground or is recuperated and recirculated (after treatment) to plants. In liquid systems, plant roots are continuously exposed to nutrient solution, which is not leaked into the ground.

Growing methods using artificial substrates or water are known as soilless culture or hydroponics. Hydroponics is literally defined as the growing of plants in water, but the plants are actually grown in a complete nutrient solution. Ideally, the term hydroponics should be reserved for water culture, and the term soilless culture for plant cultivation on artificial substrates. In practice, the terms hydroponics and soilless culture are used indiscriminately to describe water and substrate-based systems.

Although official statistics are unavailable, hydroponic systems are known to be used extensively for food production in greenhouses. The most popular soilless medium for hydroponic vegetable production is rockwool. The nutrient film technique is also often used, but to a much lesser extend than rockwool. In some regions, the availability of low-cost materials may provide alternative substrates. For example, in British Columbia, sawdust, a residue of the large forestry industry, is commonly used as a substrate. Both aeroponics and DFT remain in little use today.

Insect and Disease Control in Greenhouses

One objective of hydroponics is to avoid insects and diseases that may occur in soil. In a soilless culture system, such as rockwool, it is easy to remove infected plants. However, spread of diseases can occur very quickly in systems where nutrient solution is recirculated. Methods such as filtration of the nutrient solution, and disinfection with ozone or ultraviolet light, have been developed to eliminate pathogens that may be present in the nutrient solution. However, these methods are often expensive and not completely effective.

Greenhouses are used to create and maintain an environment ideal for plants. However, this environment is often favorable for insects and pathogens too. In the past, the control of insects and diseases in greenhouses was accomplished with the use of pesticides, but over time both insects and diseases have developed resistance to such pesticides, while consumers have begun to demand pesticide-free produce. Biological agents are now used to control whitefly, thrips, aphids, and two-spotted spider mite in greenhouses; few reliable biological agents are currently available for the control of diseases.

Research on Greenhouse Food Crops

In countries or regions where greenhouse production is an important industry, government and universities are generally involved in research on greenhouse production. The general objective of the research is to improve yield and quality of produce and profitability of production, by investigating all aspects of greenhouse production: greenhouse design and covering materials, growing methods, environment controls, substrates, plant nutrition, plant pathology, and insect control. Grower associations may also be involved in the development of research priorities, and may contribute financially to the expenses of research.

Due to the presence of a large and technologically advanced greenhouse industry in the Netherlands, the most notable research institutions are found there. The Research Station for Floriculture and Glasshouse Vegetables (under the Ministry of Agriculture, Nature Conservancy and Fisheries) has five sites. The other important Dutch institution is the University of Wageningen.

In the United Kingdom, Horticulture Research International (HRI), the largest horticultural research establishment in the world, maintains an active research program on greenhouse crops and provides its services (from fundamental research to technology transfer) to research councils, government departments, growers, and commercial industries, in the European Community (EC) and other countries.

In the Americas, the Greenhouse and Processing Crops Research Centre (GPCRC; Agriculture and Agri-Food Canada) is the largest research facility specializing in greenhouse vegetables. The GPCRC is a leading member of the Canadian Network for Greenhouse Vegetable Research.

Japan, Spain, and Israel are some of the other countries with important research programs in horticulture, including greenhouse food production.

The International Society for Horticultural Science (ISHS) is an international organization of horticultural scientists, which aims at promoting research in all branches of horticulture, including greenhouse food production. Within the ISHS, there are various commissions and working groups related to greenhouse production.

Future of Greenhouse Food Production

As the world population continues to increase, and more agricultural land is lost to urban development, intensive food production in greenhouses may play a more important role in food production. Furthermore, improving economic conditions in developing countries and an increasing preoccupation with health and nutrition will increase demand for high-quality food products. Through controlled climate and reduced pesticide use, greenhouses can meet this consumer demand. Foods with improved health characteristics or containing nutraceuticals (substances with pharmaceutical or health-beneficial properties that can be extracted or purified from plants) can be grown pesticide-free in greenhouses.

See also Chili Peppers; Crop Improvement; Cucumbers, Melons, and Other Cucurbits; High-Technology Farming; Horticulture; Tomato .

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Athanasios P. PapadopoulosDominique-André Demers