For saving space and soil, this method also has several
other benefits, including no soil-borne diseases, no
weeds to pull and no soil to till, run-of-the-mill side
benefits of soil-less gardening.

Structures and Environmental Control

The European glass structures that today are commonly being built for vegetable production in the southwestern part of the United States are very different from the polyethylene/fiberglass houses used in hydroponic production between 1965 and 1990. The European structures are much higher.

To achieve a more uniform growing environment, without rapid temperature fluctuations, more total volume of space is being allotted within a greenhouse; today the gutters of greenhouse structures are commonly more than 5 m above ground level.

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The types of polyethylene sheet films are much the same except those introduced over a decade ago that retard the loss of infrared heat These films are reported to reduce 20% of the heat loss from a greenhouse and have become common in today's industry, especially in Europe. Other glazing materials, such as fiberglass, polyvinyl chloride, Mylar and Tedlar, have proven either less appropriate, inconvenient, or in most cases, much more expensive than polyethylene, even though the latter may have to be replaced more frequently. Newer materials, such as polycarbonates and acrylics have become much more common, but their popularity has been offset by high costs.

Greenhouses are expensive, however, and controlling the environment within a greenhouse requires considerable energy. Starting 20 years ago, there was major research emphasis on the use of solar energy and reject heat from large industrial units. Although solar energy as a greenhouse heat source is technically feasible, it has not proven economical because of collection and storage costs. The economics of using waste heat from generating plants favors incorporating the heat-use system into the overall plans for new plants, rather than modifying existing ones.

In the last 10 years, there has been interest in the development of cogeneration plants; small electrical plants receive government assistance if designed to use the waste heat from the electrical generators. Several such facilities have been established that use the waste heat either to heat greenhouse vegetables or water for fish production. While such opportunities are inviting, excess government regulation and red tape have discouraged many investors from taking advantage of such opportunities.

Whatever the source of energy, it should be conserved once it is in the greenhouse. In regions of cold winter weather, thermal curtains of porous polyester or an aluminum foil fabric are installed to reduce night heat loss by as much as 57%. In the deserts of the southwest, winter temperatures are not severe enough to warrant curtains. While curtains will provide energy savings, they are not sufficiently effective to warrant their high cost. Furthermore, the shade from the curtains, even when rolled up and stored during the day, can reduce yields.

Computers can operate hundreds of devices within a greenhouse (vents, heaters, fans, hot water mixing valves, irrigation valves, curtains. lights. etc.) by utilizing dozens of input parameters, such as outside and inside temperatures, humidity, outside wind direction and velocity, carbon dioxide levels and even the time of day or night. Unlike early control systems, computers are used today to collect and log data provided by greenhouse production managers. A computer can keep track of all relevant information. such as temperature, humidity, C02, and light levels. It dates and time tags the information and stores it for current or later use. Such a data acquisition system enables the grower to gain a comprehensive understanding of all factors affecting the quality and timeliness of the product.

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