High tunnels are not greenhouses, although it is sometimes hard to distinguish between the two. High tunnels, although resembling traditional plastic-covered greenhouses, are a completely different technology. In their purest form, high tunnels have a pipe or other framework covered by a single layer of greenhouse-grade 4- to 6-mil plastic and they have no electrical service, automated ventilation, or heating system (Fig. 1) (Lamont et al., 2002; Wells, 1996). Although there is no permanent heating system, a standby portable heater or other method of heating is sometimes employed to protect crops against unexpected low temperature events during the spring or fall. Most high tunnels do have a source of water for irrigation, and in most instances, they use drip irrigation. On the other hand, a greenhouse has an air-inflated double layer of plastic, with fully automated ventilation and heating systems. A greenhouse provides much more environmental control of the cropping environment.
Wittwer (1993) discussed the impact that the utilization of plastics had on the production of horticultural crops worldwide. One area that he documented was the use of high tunnels and plastic greenhouses. At that time, there was a large number of high tunnels/greenhouses being used for the production of horticultural crops in Asia, Italy, Spain, and the Middle East. Since 1993, the area devoted to the production of horticultural crops in high tunnels has continued to grow, and the variety of crops grown in high tunnels has also expanded (Baudoin, 1999; Wittwer and Castilla, 1995; Zhang, 1999). The estimated area of greenhouses/high tunnels by country or region and their listing in order of importance based on the number of hectares of crops grown are presented in Table 1 (Papadopoulos and Demers, 2003). China has a long history of and by far the largest area devoted to protected cultivation of horticultural crops (Jiang et al., 2004). Again, plastic-covered high tunnels are included under protected cultivation in China, as elsewhere.
Estimated area of greenhouse and high tunnels for major food crops in different countries/regions.
In the temperate parts of the world, high tunnels are used to extend the growing season by creating a warmer environment for crop growth (Jiang et al., 2004; Wells, 1998), while in the tropical regions of the world, high tunnels also extend the growing season by permitting crop production during the rainy or monsoon seasons (Jensen and Malter, 1995). Greenhouse/high tunnel usage by regions of the world is presented in Table 2 (Papadopoulos and Demers, 2003). The different structures, cropping systems, and crops produced in high tunnels will be discussed in the remainder of this article.
Estimated area (ha) of protected crops (plastic greenhouse and high tunnels) by region worldwide.
Jiang, W.J., Qu, D., Mu, D. & Wang, L. 2004 Protected cultivation of horticultural crops in China Hort. Rev. (Amer. Soc. Hort. Sci.) 30 115 162
Lamont, W.J., McGann, M., Orzolek, M., Mbugua, N., Dye, B. & Reese, D. 2002 Design and construction of the Penn State high tunnel HortTechnology 12 447 453
Papadopoulos, A. & Demers, D. 2003 Greenhouse horticulture. Encyclopedia of food and culture 27 June 2008 <http://www.answers.com/topic/greenhouse-horticulture>.
Wells, O.S. 1998 Rowcovers and high tunnels-growth-enhancing technology Proc. Vegetable Production Using Plasticulture Seminar Amer. Soc. Hort. Sci. and Amer. Soc Plasticulture, Charlotte, NC 49 54