Abstract
High tunnels have been used for many years worldwide, but in the United States, the utilization of high tunnel technology for the production of horticultural crops is a relatively recent phenomenon. Single and multibay high tunnels are used throughout the world to extend the production season. One big advantage of high tunnels in the temperate and tropical regions of the world is the exclusion of rain, thus reducing the amount of disease pressure and crop loss while improving crop quality and shelf life. In temperate regions of the world, high tunnels are used to increase temperatures for crop production in spring, fall, and sometimes winter seasons. The use of high tunnels in their many forms continues to increase worldwide, and many different kinds of vegetables, small fruit, tree fruit, and flowers are being cultivated. One impediment in determining high tunnel usage worldwide is the failure of many authors and agricultural census takers to distinguish between high tunnels and plastic-covered greenhouses. In many instances, they are presented together under the heading “protected cultivation.”
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.
Single-bay high tunnel in State College, PA.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
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.
Structures/covers
In many parts of the world, high tunnel frames are made of pipe or galvanized tubing of varying thickness. The thickness depends on the permanence of the structure and also on the amount of wind experienced in the region. In South Korea, single-bay high tunnel units are erected in the field for the production of chinese cabbage (Brassica rapa var. pekinensis), and when the production season is over, they are removed and stored at the edge of the field. They are covered with a single layer of plastic, or in some instances, a double layer of plastic is sandwiched between roping material, but is not inflated (Fig. 2). In Spain, high tunnels cover large areas and have flat, sloped roofs to allow rainwater to runoff (Fig. 3). They also have two layers of plastic film with a wire framework in-between that runs between the metal or wood posts. The main concern for Spanish growers is wind damage. In Italy, high tunnels are single (Fig. 4) or multibay structures that cover many hectares. The frames are not constructed of very heavy pipe, but seem to serve the growers' purpose. In tropical regions such as Taiwan (Fig. 5), Thailand, the Philippines, and Indonesia, the structures are mostly pipe frames covered first with a screen material and then with plastic film on top. This permits growers to raise the plastic and expose the screening material for critical ventilation. These high tunnels are used primarily to exclude the seasonal rains. In India (Fig. 6), high tunnels constructed of sturdy bamboo frames are covered with a single layer of plastic film on the top and jute on the sides, which excludes insects but permits some ventilation. In other high tunnels, pipe framing is used and is covered with a single layer of plastic and even a ridge venting system (Fig. 7). The main personal observation I can make on the structure/frames worldwide is that when climatic conditions allow outdoor cultivation, high tunnels can be used to protect crops against weather phenomena such as wind, excessive rain, or hail that would negatively impact a crop. The more permanent structures have a more substantial framework. The degree of environmental control needed depends on local climatic conditions such as the colder northern United States versus mild winter climates such as Spain.
Single-bay high tunnels in South Korea.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
High tunnel in Spain showing a flat roof that slopes toward the ocean.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
High tunnels in Taiwan showing plastic covering over screening material.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
High tunnel in India using bamboo for framing and jute for sidewalls.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
High tunnel in India using tubular pipe for framing and ridge vent.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
Cropping systems
Most crops grown in high tunnels use plastic mulch on the ground and drip irrigation to help control disease pressure and also warm up the soil (Fig. 8). Tractors and other forms of tillage equipment are used in larger tunnels, while walk-behind rototillers are used in smaller tunnels. There are individual rows of plastic mulch applied or the entire floor of the tunnel is covered with plastic film. In addition, rowcovers are routinely used over the crops inside high tunnels to enhance crop growth (Fig. 9). The use of drip irrigation facilitates the use of fertigation to supply crops with needed nutrients. A wide variety of organic fertilizers are being used such as composts, manures, etc., in high tunnels worldwide.
High tunnel in Italy using plastic mulch and drip irrigation for crop production.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
Row covers used inside a high tunnel in Taiwan.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
Crops
The following crops are being grown successfully using high tunnels in many parts of the world.
Vegetables.
The high tunnel allows growers to produce crops over a longer period of time—perhaps year-round in some climates. Many times, the plastic mulch is double-cropped, with the first crop being removed and the second crop being planted on the same plastic.
The primary vegetables, in order of importance, grown in high tunnels are tomato (Solanum lycopersicum), pepper (Capsicum annuum Grossum group), cucumber (Cucumis sativus), muskmelon (Cucumis melo), lettuce (Lactuca sativa) (Fig. 10), summer squash (Cucurbita pepo), and eggplant (Solanum melongena). Other vegetable crops grown in high tunnels are spinach (Spinacia oleracea), swiss chard (Beta vulgaris var. cicla), broccoli (Brassica oleracea var. italica), cabbage (B. oleracea var. capitata), chinese cabbage (Brassica rapa var. pekinensis), cauliflower (B. oleracea var. botrytis), kale (B. oleracea var. acephala), kohlrabi (B. oleracea var. gongyloides), okra (Abelmoschus esculentus), onion (Allium cepa), leek (Allium ampeloprasum var. porrum), and garlic (Allium sativum). In addition, a wide variety of herbs are grown in high tunnels.
Lettuce production in a high tunnel in Italy.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
Small fruit.
The extended production season and improved shelf life (K. Demchak, personal communication) of small fruit make high tunnel production of these products a very viable option. Worldwide, strawberry (Fragaria ×ananassa) (Fig. 11) is the most widely grown small fruit crop. Also, primocane-bearing red raspberry (Rubus idaeus) and thornless blackberry (Rubus subgenus Eubatus) are produced in high tunnels.
Strawberry production in a high tunnel in Italy.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
Cut flowers.
Worldwide, there are many different types of cut flowers grown in high tunnels, including herbaceous perennials, overwintered for spring production, to summer annuals, to natural season, to forced chrysanthemum (Dendranthema ×grandiflora) in Israel (Fig. 12). The high tunnel production system allows cut flowers to be harvested earlier in the spring and later in the fall in more temperate regions of the world compared with cut flowers grown in the field. They also provide excellent flower quality (W.J. Lamont, personal observation).
Chrysanthemums in production in high tunnels in Israel.
Citation: HortTechnology 19, 1; 10.21273/HORTTECH.19.1.25
Tree fruit.
There are many multibay high tunnels used worldwide for the production of tree fruit crops such as sweet cherry (Prunus avium). The use of high tunnels continues to grow because they enhance earliness and prevent stress and loss of fruit quality from rain and severe weather events such as hail.
Summary
The use of high tunnels worldwide for the production of horticultural crops described above provides benefits such as increased warming of the soil in the spring; continued warming into the fall in more temperate regions of the world; the use of drip irrigation systems, which facilitates more accurate application of water and fertilizer, as is the case in outdoor field production while also reducing diseases; protection from rain and severe weather events such as hail; and crop production during the rainy season in many tropical areas of the world. High tunnels are an integral component of the protected cultivation of crops practiced worldwide. The use of high tunnels for the production of horticultural crops will continue to grow worldwide as the demand for locally produced fresh vegetables, fruit, and flowers continues to expand.
Literature cited
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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
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