Along with low tunnels and high tunnels, greenhouses are structures used to grow plants under protected conditions. The progression of terms shows the level (low to high) of technical sophistication in the plant-growing systems. Low tunnels, also called row covers, primarilyadvance the growing season for outdoor crops (for example, tomatoes, melons, strawberries, and sweet corn). Low tunnels are created using long, narrow strips of transparent plastic material (often polyethylene) buried in the ground along their outer edges to cover one or several adjacent rows of plants grown directly in the soil. High tunnels are large versions of low tunnels, raised sufficiently above the ground that people can walk within them. Greenhouse (or glass-houses) are relatively permanent structures (usually glass or plastic with aluminum or steel frames) equipped with several means of environmental modification.
Free-standing greenhouses are the most basic structural type. Cross-sectional shapes can be classified as arch, hoop, or gable (see illustration). Multispan greenhouses are typically connected by a series of roof gutters to create a single airspace. Large multispan greenhouses can cover several hectares under one roof, and they are the design of choice for larger commercial greenhouse operators. Floors are frequently made of concrete, although gravel floors with concrete walkways may be used to reduce cost.
Common commercial greenhouse shapes.
Light transmittance is important when selecting a covering material. Glass provides the mostlight to the plants and retains its light transmittance; however, various rigid and film plastic glazing materials are used because of initial lower costs. See also Glass.
Environment control typically encompasses air temperature, supplemental light, air movement (circulation and mixing), and carbon dioxide concentration. Some degree of relative humidity control may also be included. Integrated control by computer, found in most modern greenhouses, provides the flexibility of zoned control of each environmental parameter without conflicting control signals (for example, ventilating while supplementing carbon dioxide).
Structural insulation opportunities in greenhouses are minimal and heat requirements are high in comparison to most other types of buildings. Efforts to conserve heat, such as insulating the north wall and part of the north roof, have often shown negative benefits by reducing natural light and degrading plant growth and quality. Movable, horizontal, indoor curtain systems (which also double as movable shade systems) can save approximately one-quarter of the yearly heat in a cold climate.
Greenhouses can be heated by oil or natural gas. Heat delivery is either hydronic (hot water) or by steam. See also Heat.
Solar loads in greenhouses are so great that mechanical cooling, as by air conditioning, is prohibitively expensive. Options for greenhouse cooling are thus limited. The most typical cooling mechanism is ventilation, either natural or mechanical. The next step of cooling is to use evaporative means. Cooling can be obtained by spraying a fine mist into the ventilation air or by pulling outside air through matrices or structures that are wetted to cool the air flowing past them.
Greenhouse lighting may be used for photoperiodic reasons, or for enhancing growth. Photoperiodic lighting is a very low intensity light during the night to break the darkness period and induce plant responses representative of summer (short nights and long days). Greenhouse supplemental lighting is usually provided by high-pressure sodium (HPS) lights because of their relatively high energy efficiency. See also Light; Photoperiodism; Photosynthesis.
Optimum concentrations of carbon dioxide are often in the range 800–1000 ppm, which can lead to 25% greater growth provided that other inputs are not limited. Carbon dioxide can be added through carefully controlled flue gases from the greenhouse heating system, or from tanks of liquid carbon dioxide.
Mechanization and automation
Many greenhouse operations that were formerly done by hand are now mechanized or automated. Root medium is mixed, fertilized, and placed directly into flats and pots by machine. Seeding and transplanting can be by machine. Plant watering and fertilizing (termed “fertigation” when combined) can now be automated. Automatic material movement at harvest, coordinated by a computer, is no longer unusual.
Nutrition management and hydroponics
Plant fertilizers are composed of a mix of salts that are electrically conducting when dissolved into water. This characteristic leads to the use of electrical conductivity as a measure of fertilizer concentration. Computer programs have been developed that are suitable for balancing a nutrient mix to achieve close approximations to the desired molar ratios of elements. See also Plant mineral nutrition.
Hydroponics is defined as growing plants without using soil. However, a root medium such as sand, gravel, or rockwool may be used. Two common hydroponics systems that use no root medium are the nutrient film technique (NFT) and deep flow troughs (DFT). See also Hydroponics.
Modern greenhouse technologies have mirrored developments in most of agriculture in that increased labor efficiency, larger sizes of greenhouse operations, and mass production of a few crops, or even a single crop, have become the rule to be profitable. The current dynamic in the greenhouse industry in the United States is characterized by the entry of many growers in small,specialized operations, and consolidations and mergers of large operations. See also Floriculture; Plant growth; Plant-water relations.