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GREENHOUSE HORTICULTURE

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 con- trolled 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, exces- sive rain, or hail) that would negatively affect them. For the purpose of this article, the term “greenhouse” is de- fined as a structure covered with a transparent or translu- cid material, in which environmental conditions can be modified or controlled, for the cultivation of plants. Tun- nels are also used to modify environmental conditions for plant production, but are not usually considered green- houses. Since the distinction between greenhouses and tunnels is not always clear in the literature, both struc- tures, 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 nine- teenth century, the largest growth and expansion of the greenhouse industry occurred throughout the world fol- lowing World War II. Today, food production in green- houses can be found in all continents. Most popular food crops grown in greenhouses are tomato (beefsteak, clus- ter, 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 as- paragus. 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 introduc- tion 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 ma- terial in the world. Glass-covered greenhouses are con- centrated 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 green- houses built now in Canada are covered with PE. Stan-

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dard 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, ra- diation (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 pan- els, but their use is generally limited because of their high cost, compared to PE. Beside glass and PE, polycarbon- ate is often used on the sidewalls of polyethylene green- houses 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 min- imal 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, CO2

enrichment, evaporative cooling

pads, exhaust fans, roof vents, thermal/shade curtain, computer controls, light sensors). Most sophisticated greenhouses are generally found in the developed, north- ern countries. Phytotrons are highly sophisticated struc- tures that allow for accurate control of environmental conditions including light, and are generally used for sci- entific 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 re- gions 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 en- vironment 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 ex- tend the production season (e.g., one early crop in spring), then a less sophisticated greenhouse could be sat- isfactory.

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

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