Merle H. Jensen
University of Arizona,
Tucson, Arizona 85721, USA

Keywords: Plasticulture, protected agriculture, controlled environment agriculture, greenhouses, row covers, mulches, drip irrigation.

Introduction

No technology has modified the course of horticultural crop production as the use of agricultural plastics (1). As the Green Revolution expanded the production of agronomic crops, plasticulture has provided yet another revolution. While more silent, plasticulture has enabled countries throughout the world to greatly extend their food production capability. Plastic films and related materials are used extensively to cover greenhouses, high and low tunnels (row covers) and for soil mulches. High tunnels are walk-in hoop structures that are normally unheated and naturally ventilated. Plastic tubes and accessories are important components of drip irrigation which today covers more than 400,000 hectares worldwide. Pots, labels, and flats, major components in transplant and ornamental plant production, even involve plastics. Plastic films for ground covers and bag cultures are common in hydroponics/controlled environment agriculture. Plasticulture consists of many components, not only plastic but a complete management system that may include pest control, marketing, etc. Plasticulture is a whole system approach to modifying microclimates in producing high quality, high yielding horticultural products.

Economic Relationships

Crop production, using systems of plasticulture, is usually more expensive per unit of product than production without such systems during comparable periods of the year. These additional costs are usually justified if the monetary return per unit of product is higher. This occurs when the product is of better quality if overall production costs are compensated through better yields or if crop production occurs when local cropping is impossible. Calculating the economic advantage is more complicated when imports from other producing areas are considered. For example, the cost of heating fuel required to produce a kilo of greenhouse tomatoes in New York State during the winter is far greater than the cost of fuel to transport to New York a kilo of tomatoes grown in the open field in Mexico at that time.

Economic factors are key determinants of the method and system of plant culture most applicable for crop production in a given situation. If early harvest of one to two weeks is desired to take advantage of high market prices, then mulches alone may provide that margin of time and profit. If an advantage of several weeks is desired, a combination of mulches and row covers may be required. Naturally, the cost of using both methods in combination is higher than using mulches alone; however higher market prices might more than compensate for the additional cost.

While nearly all crops can be grown successfully in any system of protected agriculture, only those crops that bring a high yield per unit area of land, as well as high market prices, are economically viable. Usually these are perishable crops that cannot be transported long distances without expensive packaging and shipping costs.

In addition, each method demands specialized knowledge and presents its own economic risks. For instance, greenhouse crop production, in combination with hydroponic culture, is possibly the most intensive method of crop production in today’s agricultural industry. It requires high technology and is very capital intensive. Every detail of crop production requires close attention. But excellent management skills in plasticulture are not enough: a thorough knowledge of markets and the ability to produce high quality products on schedule are also essential.

Economic problems come from many causes: underestimating the cost of production, including capital and operating cost requirements; poor management and marketing skills; and lack of diversification in response to competition from less expensive imported products. Economic success is highly dependent on making plant culture methods of production competitive with systems of open field agriculture not using any method of crop protection, whether produced locally or imported.

Early Development and Status of Plasticulture

Plasticulture was first used in Southern Europe, Japan and the United States. The first use of polyethylene as a greenhouse cover was 1948, when Professor Emery Myers Emmert at the University of Kentucky used the less expensive material in place of more expensive glass (2). Often considered the father of agricultural plastics in the U.S., Dr. Emmert developed many principles of plastic technology for agricultural purposes through his research on greenhouses, row covers and mulches.

Mulches.

Natural mulches such as leaves, straw, sawdust, peat moss and compost have been used for centuries to control weeds and hold moisture in the soil. None of these materials have been employed to any great extent in commercial vegetable production (3).
It is only in the last fifty years that synthetic materials have altered the methods and benefits of mulching. Their potential for mulching was established through early research projects with polyethylene, foil, and paper.

Paper mulches attracted a good deal of attention in the early 1920's. They were not adapted for commercial vegetable production because of their short life, as well as the cost of material and labor, which was not mechanized (4). In the late 1950's and early 1960's, improved formulations of paper - including combinations of paper and polyethylene, foils and waxes - stimulated research and the use of mulching materials. Petroleum and resin mulches for arid climates were developed at the same time. Of these mulches, only those made of polyethylene are still used today in the agricultural industry. The preferred colors are clear and black, although a wide variety of shades and colors are used for specific reasons in the production of food crops. Currently, red, blue, yellow, gray and orange are being investigated. Each has distinct optical characteristics and appear to affect plant growth and development (5, 6). Mulches with a silver surface color have shown to repel certain insects, insects that are often vectors of various viruses (7). Significant advances in the use of mulches occurred during the early 1960's with mechanization, the invention of mulch applicators, and transplanters which would plant directly through the mulch.

Infrared transmitting (IRT) mulches, which transmit most of the solar heat portion of light radiation but absorb most of the visible portion, were introduced to the market in the last decade (8). IRT mulches provide weed control as does black mulch but increase the soil temperature, as with clear plastic mulch. Unfortunately, labor requirements to remove plastic mulch from the field after the growing season can be high. New bio- and photo-degradable polyethylene and combinations of polyethylene-paper and polyethylene-starch show promise in eliminating the need for mulch removal.

Today, millions of hectares are planted to plastic mulch. In the People’s Republic of China alone, over 2,867,000 ha. of mulch was used in 1989, a phenomenal increase over the 44 ha. in 1979. (9).

Row Covers.

Row covers, or plastic low tunnels, protect crops from frost and create favorable conditions for plants to achieve early production. Before the introduction of polyethylene, early spring crops such as cucumbers were started and grown in muslin-covered wooden box frames measuring approximately 17 meters square at 0.3 meters in height. This was a costly but effective method of producing early fruit from 1935 to 1945 (10). In the mid-forties a method using two separate paper caps replaced the wooden box frame. A small cap, 28 cm. in diameter and 14 cm. in height, was used to start the plants. A second, larger, tent-type paper cover was installed when the plant filled the smaller cap. This second cap measured 35 cm x 28 cm x 21 cm in height. This tent cap was constructed so that one or both ends could be opened. Usually it was the leeward side which was opened and the plants were trained in that direction. The paper tent thus acted as a wind break. The early fruit could develop while the plant had partial protection during adverse weather. The double cap produced fruit as early as the wooden frame method but was less costly. Paper covers are still used today in some parts of the world. Paper covers have one serious liability: while they help protect plants from early spring frost and wind, they also reduce the amount of light reaching the plant, with the result that plants may be succulent and weak. In Japan, more translucent materials, such as vinyl or polyethylene film are replacing paper as plant covers or hotcaps. Such hotcaps not only protect against light frost but also provide extra heat and protection against chilling winds, blowing sand and soil particles.

Plastic row covers were initially used in Europe and the United States, and especially in Japan. In fact, in 1959, France and the U.S.A. totaled less than 400 ha. under plastic while Japan had more than 8,000 ha. (11). Since then, this method of protected agriculture has become common throughout the world.

Today, as in 1959, Japan uses mostly polyvinylchloride (PVC) film for row covers. In other countries, polyethylene predominates. There are historical as well as economic reasons for the selection of different materials for the same task. PVC films have a better heat-retaining (infrared radiation) capacity than polyethylene but they are also more expensive. Early in the development of row covers, it was not possible to produce PVC sheets wider than 1.6 meters while polyethylene films of 2-12 meters in width were available. With government financial support, Japan was the first country to develop wide PVC sheets (2-3 meters); as a result, Japan selected this material as the predominant type of film. France and Italy found the equipment for the extrusion-blowing process of making polyethylene much less capital intensive than the PVC equipment, and therefore selected polyethylene for use as row covers.

In the United States, the first use of polyethylene row covers for early crop production was for a cucumber planting in California in 1958. With careful venting adjustments for weather changes, plastic row covers produced a margin for marketing of four to five weeks over that of the two paper cover methods and produced good yields as well (10).

For 25 years there was steady growth in the use of plastic row covers. However, no significant increase has occurred in recent years with the exception of the People’s Republic of China where there are over 100,000 ha. under cover at the present time and expectations of greatly expanded use in the near future.

In 1994 a total of some 70,000 ha. were covered with PVC film worldwide: 60-62,000 ha. in Japan, 4,000 ha. in France and 1,500 ha. in Italy. Low density polyethylene film, on the other hand, is used on about 195,000 ha., of which over 80,000 ha. are located in the People’s Republic of China (12). In 1988, the hectareage increased over 30,000 ha. in China alone.

The simplest and most economical form of row covers is the direct, or floating, covers with no sustaining wire or cane hoops. First introduced in Germany in 1970, floating covers then were adopted by neighboring countries. Perforated polyethylene film 50 mm thick, generally with 500 holes per m2 (i.e., 4% ventilation, 46 g/m2) now competes with non-woven/spunbonded fabric materials (PP, PA, polyester), which are porous and much lighter (10-25 g/m2). In the mid 1990's these latter materials were particularly successful in France (2,800 ha. out of 4,500 ha.), and in Japan (100% of 4,000 ha.).

Today non-woven covers are common throughout the world. The light weight and the permeability of these films allow gas exchange and penetration of rain, controls insects, enhances growth and freeze protection and eliminates hand ventilation (13). Loy and Wells (8) reported that the harvest of cantaloupes was initiated one week earlier with the use of floating covers compared with the controls with no covers. Prior research by Mansour (15) demonstrated that floating covers offered protection against viruses and feeding damage from insects such as aphids, loopers and beetles. In research at the University of Arizona, floating covers were placed over summer squash to exclude whitefly, Bemisia tabaci, a vector of gemini viruses (16). The results were phenomenal with yields increasing 60% if the plants were protected by covers. Those treatments having plastic mulch along with the floating covers produced yields 160% greater than those plants planted to bare soil without plastic mulch and floating covers.

Greenhouses.

The total world area of glasshouses is over the 40,000 ha.(17); with most of these found in northwestern Europe. In contrast to glasshouses, plastic greenhouses have been readily adopted on all five continents, especially in the Mediterranean region, China and Japan. Most plastic greenhouses operate on a seasonal basis, rather than year round, as is the case with most glasshouses. PVC film for greenhouses is still dominant in Asia, especially Japan.

In Japan, the area covered by plastic film greenhouses increased 35,000 ha. in just 20 years (1965-85). In Korea, these greenhouses increased 6.3 times, from 3,099 ha. in 1975 to 21,061 ha, in 1986. The People’s Republic of China showed equally dramatic growth: 5,300 ha. in 1978 to 34,000 ha, in 1988. The combined growth for both greenhouses and row covers in China exceeded 96,000 ha. in just ten years. Most plastic greenhouses in Asia are high tunnels, while in Europe and the United States most greenhouses are multispan or gutter connected structures. Undoubtedly, China is one of the largest users of agricultural plastics in the world, where over one billion people - 20 percent of the world’s population - are being fed from only 5 percent of the earth’s cultivated land.

Since 1960, the greenhouse has evolved into more than a plant protector. It is now better understood as a system of Controlled Environment Agriculture (CEA), with precise control over air and root temperature, water, humidity, plant nutrition, carbon dioxide and even light. The greenhouses of today can best be seen as plant or vegetable factories. Almost every aspect of the production system is automated, with the artificial environment and growing system under nearly total computer control. In a research setting, such a totally enclosed system, with artificial light, is called a growth chamber or a phytotron. In the United States and Japan, such systems may cover large areas.

Controlled environment agriculture has gained in horticultural importance not only in vegetable and ornamental crop production but also in the production of plant seedlings, either from seed or through tissue culture procedures. Prior to 1960, there was commercial interest in hydroponics but this cultural system was not widely accepted because of the high cost in construction of the concrete growing beds. Interest in hydroponics was renewed with the advent of plastics. Plastics were used not only in the glazing of greenhouses, but also in place of concrete in lining the growing beds or plastic bags were filled with soilless growing media. Plastics were also important in the introduction of drip irrigation. While hydroponics and CEA are not synonymous, CEA usually accompanies hydroponics.

Today, the technology of hydroponic systems is changing rapidly with systems producing yields never before realized. Due to plastics and better environmental control systems, including new cultivars and biological control practices, the future of hydroponics appears more positive than anytime over the last 50 years.

Energy Conservation in Greenhouses.

Control of the environment within a greenhouse may require large amounts of energy, making energy a prime factor in computing profitability. Since 1972, the cost of greenhouse heating with natural gas in the United States has risen three to four times. The rapid increase in energy cost posed a major threat to the continuance and expansion of the greenhouse industry in the United States.

Starting in 1973, in response to the rapid rise in energy cost, extensive research occurred throughout the United States on energy conservation and alternatives. While greenhouses are inherently good solar collectors when sunshine is available, they also have a high thermal loss at night, when over 75% of all supplemental greenhouse heating is required. A number of projects were initiated in the development of solar heating greenhouses (18). After extensive research, solar heating showed little promise as a substitute for greenhouse energy needs.

What did show promise was the development of ways to conserve energy. Covering a greenhouse with a double layer of polyethylene to reduce the loss of heat energy was reported by Sheldrake and Langhans (19). Inflating two layers of polyethylene with air will reduce the loss of heat energy from a greenhouse by 40% (20).

In Japan, growers place a removable sheet of polyethylene over the crop and a polyethylene row cover over each plant row in order to reduce heat loss from the greenhouse during the night. The plastic row cover and inner polyethylene covers are pulled to the side during the day to maximize incoming light. According to Takakura (21), more than 90% of the heated greenhouses have at least one layer of movable thermal screen.

Various types of plastic/aluminum materials are used as interior curtains. Often termed thermal blankets, curtains are one of the most practical and economical methods of energy conservation. It is common for the films to double as shade curtains in summer or during periods of excess light.

Drip Irrigation/Fertigation.

Drip irrigation dramatically increased the water use efficiency over methods used in the past - sprinkler and furrow irrigation. Drip irrigation is the best means of water conservation along with control over increasing costs of water, fertilizer, labor and machinery. A major advantage is that up to 50% less water is used to grow a crop as compared to other methods of irrigation. This is especially true in soils having a high sand content. Generally speaking, drip irrigation will have an application efficiency of 90-95% compared with sprinkler at 70% and furrow at 60-80%, depending on soil type, level of field, and how the water is applied to the furrows.

In irrigation trials in North Africa, the author found that drip irrigation produced twice the tomato yield as the same amount of water used in sprinkler irrigation. In Southern California, a comparison between the effect of furrow irrigation and drip irrigation on tomato yields indicated that drip irrigation could provide a 26.8% increase in total yield and a 13.7% increase in fruit size (22).

When plastic mulch is used, it is advisable to install drip irrigation under the mulch. Using drip irrigation in combination with mulch will normally increase yields significantly through the application of water and fertilizer directly to the plant roots growing beneath the mulch. Injecting soluble fertilizers into a drip irrigation system is termed fertigation. In the U.S., this technology was used as early as 1964, when the author was confronted with adding fertilizer to tomato, cucumber and melon crops on different mulches under row covers (23).

Trials in New Jersey, showed that much higher yields of eggplant can be achieved if drip irrigation is used in combination with plastic mulch (Table1).

Today, this technology is commonly used throughout the world. Along with fertilizer, drip irrigation facilitates the application of numerous other materials, whether it be fumigants, chemicals to prevent plugging of emitters, beneficial bacteria and various soil conditioners such as humic acid.

Other Uses of Plastics.

In transplant and ornamental plant production, flats, pots, saucers and even labels involve plastics. Even plastic flowers and foliage plants are common. Recent statistics on the value of plastic flowers are not readily available. In 1961, more than $112 million worth of plastic flowers and plants were sold in the United States (24). Plastic is used to produce foam used in making floral arrangements as well as propagation blocks and ingredients for growing media (25).

In 1967, the author first used white plastic film as a ground cover to reflect light back into a greenhouse tomato crop to aid in plant photosynthesis (26). Light reflection can be over four times greater with the white ground cover versus bare soil. The ground cover also serves as a barrier to weed growth, separates any disease infested ground from the growing area and decreases evaporation of moisture from the soil to the greenhouse environment which in turn helps to control humidity in the greenhouse. Today, white ground covers are common throughout the world, especially for greenhouse vegetable production. An entire field may be covered with polyethylene or with strips of mulch for fumigation with chemical fumigants. Soil solarization is a method of soil disinfection that occurs in moist soil covered by clear plastic and exposed to sunlight during the hot summer months.

Plastics are used throughout the world as windbreaks. Windbreaks increase soil and air temperature and can extend the growing season, resulting in increased crop development, earlier crop maturity and market advantage. Plant-water relations and irrigation efficiency are improved by shelter. Overall, modifications to the microclimate in sheltered areas contribute to higher crop yields of 5% to 50% (27).

Plastic nets are common for bird protection. Various types of plastic cloth/fabric are used to shade greenhouses or as shade houses. Plastics are benefiting the harvesting and packaging of nearly all horticultural products. It has contributed greatly to improving the ability of the horticultural industry to deliver a high-quality product to the consumer. The impact of plastic on the horticultural industry can be realized by just looking through a horticultural supplies catalog. The number of products is overwhelming.

For a complete review of plasticulture worldwide, the following publications give an excellent review:

• Jensen, M. H. and Alan J. Malter. 1994. Protected Agriculture, A Global Review. World Bank Technical Paper. No. 253, Wash. D.C.
• Hort Technology (three quarterly publications)
  • Plasticulture - Jan./Mar. 1993. 3(1)
  • Protected Cultivation of Horticultural Crops Worldwide - Jan./Mar. 1995. 5(1)
  • Special Compendia: Using Plasticulture to Produce Vegetables - July/Sept. 1996. 6(3)

Geographic Considerations

When considering the use of any system of plasticulture, the world may be divided into three geographic regions: (1) temperate, (2) semi-arid/arid and (3) tropical. In the temperate regions, all methods of protected agriculture are often used for early crop production and to produce summer crops out-of-season, during the winter. In the temperate regions, mulches add warmth to the root area; in tropical regions, mulches protect fruits from the disease or discoloration that might occur from contact with the soil.

Row covers are commonly used during early spring in both the temperate and arid regions, but are seldomly used in the tropics. One exception to use in the tropics might be the introduction of non-woven materials as protection against chewing insects or insects which are vectors of plant viruses.

Greenhouse structures are enclosed to provide temperature control and opened only to provide ventilation in both temperate and arid regions. In arid regions, during both summer and winter, evaporative cooling systems are commonly used to lower greenhouse temperatures. Closure also provides valuable protection from disease and pest infestations, and weather damage. Because of this, greenhouses are especially effective in tropical regions. In the tropics, the sides of a greenhouse structure are often left open for natural ventilation but if pest infestation is threatening the sides are covered with screens.

Soilless/hydroponics culture is commonly used in combination with greenhouses, especially where no suitable soil exists and for more efficient use of water and fertilizer. Hydroponics is also used for optimum control over disease and insects.

The Future: Challenges and Opportunities

The future presents some real challenges but also great opportunities. A major challenge is the disposal of waste plastic as landfills have become overburdened with waste polyethylene. In Japan, the handling of waste plastic is one of the biggest problems yet to be solved. Plastic consumption has increased dramatically in Japan in recent years (21). In 1985, the total amount of waste exceeded 165,892 tons, which included waste materials from greenhouses and row covers, as well as plastic mulches. Since 1970, Japan has treated plastic waste under the law of industrial wastes. Growers are themselves responsible for handling the wastes and, in the process, must not produce any air or water pollution. It is illegal to carelessly discard the waste plastic in a manner that might create obstacles in rivers and other public places.

The three methods used in Japan to discard plastic waste are: (1) recycling, (2) burial and (3) incineration (21). Recycling of plastics in Finland (28) is a major business for a private company producing heavy-duty plastic sacks, agricultural films, and construction grade films. The company collects used films from the community and returns to the plant to process them. The film is separated by type, whether clear, colored, or printed; it is then washed, dried, and repelletized for feedback into the cycle. Since reprocessed resin is not of the quality of virgin resin, only 15 percent, or less, reprocessed plastic is used with virgin raw material. Except for medical or food packaging, injection molded plastic processors use half reprocessed plastic and half virgin material for products such as furniture and toys.

Degradable plastic mulches are receiving a great deal of attention, especially the photodegradable mulch. These plastic mulches have many attributes of standard polyethylene mulch: they are easy to lay and provide the usual benefits associated with mulch. The major difference is that photodegradable mulches decompose after the film has received a predetermined amount of UV light. Once it has received sufficient light the mulch becomes brittle and develops cracks, tears, and holes. Like regular mulch, pieces of mulch are often blown away by the wind to adjoining fields or communities, except the photodegradable pieces are usually less than 5-6 square cm. The photodegradable film will finally disintegrate into small flakes and disappear into the soil. Like regular mulch, the edges of the photodegradable mulch covered by soil will retain their strength and interfere with future tillage.

Biodegradable mulches, while still in the experimental stage, will provide a huge breakthrough in reducing the cost of plastic removal from the field and eliminating the problem of plastic disposal. It will be important that the end products of biodegradable plastics be void of any undesirable residues, and that they be environmentally acceptable.

The challenge in developing biodegradable mulch is how to control the degrading process. In the future, certain bacteria, enzymes or catalysts might be sprayed onto the mulch to trigger the degrading process. The challenge is whether these products will interfere with new plastic the following year and will such treatments degrade the buried mulch as well. Liquid plastic mulches may one day be sprayed onto the ground, solidify and be biodegradable on demand. Until now, there has not been enough scientific research and development dedicated to this opportunity.

In the future, there will be a greater understanding of the role of various colored mulches in crop production. Each color has distinct optical characteristics reflecting different radiation patterns into the crop canopy. How this reflectivity affects plant growth and development as well as the response of insects to plants remains of vital interest to researchers.

The application of products through drip irrigation will undoubtedly increase in the future. Today, chemical fertilizers, nematacides, insecticides and fumigants are commonly put through drip irrigation. In the future, beneficial bacteria, bio-inoculates, root stimulates, and soil conditioners will be increasingly used.

Greenhouse covers that limit the transmission of infrared solar radiation or films that reconvert the radiation from the photosynthetically inactive wave lengths of the solar energy into PAR will gain increased attention (17). Films that might reduce night heat loss will continue to be sought through research and development.

Greenhouses with retractable roofs will gain in popularity, especially for transplant and bedding plant production. More attention will be given to conditioning the plants for open field production rather than hardening the plants. Environment control will focus on measuring plant temperature rather than air temperature. Remote sensing devices to measure various occurrences within a plant will dictate the growing environment both in the air and the rhizosphere. Hydroponic/soilless systems will continue to gain popularity. In the future, little or no solid growing media will be used in hydroponics in order to allow rapid response in the control of electrical conductivity, pH and nutrient balance. Forty years ago, greenhouse tomato yield per year averaged 10 kg/m2. Today the yields are approaching 70 kg. In the future, it is conceivable that tomato yields will reach 100 kg/m2/year.

For many years, plasticulture systems of agriculture have been concentrated in developed countries but today research developments have made it possible to extend the benefits of the technology to less affluent regions of the world.

There seems to be a kind of technological imperative driving development of plasticulture. Like manufacturing, it generally moves toward higher-technology more capital-intensive solutions to problems. Plasticulture is highly productive, suitable for automation, conservative of water and land, protective of the environment and yet, for most employees, requires only basic agricultural skills. It can be argued (and has been) that protected agriculture is “the next logical step” after traditional OFA. Given present circumstances, however, there seem to be no rational basis for anticipating a much wider and faster diffusion of technology than is presently occurring. The future growth of plasticulture is greatly dependent on the development of production systems that are cost-competitive with open field agriculture.

Continuing research and development may lead to more cost-efficient structures and materials, reduced energy requirements, new cultivars more appropriate to controlled environments and mechanized systems and better control (including improved plant resistance) of diseases and pests. To the extent that these improvements increase crop yield and reduce unit costs of production, plasticulture will become more competitive.

For some, the economic prospects for plasticulture may change if governmental bodies determine that, in some circumstances, politically desirable effects of plasticulture merit subsidy for the public good. Such beneficial effects may include the conservation of water in regions of scarcity or food production in hostile environments; governmental support for these reasons has occurred in the Middle East. Another desirable societal effect can be the provision of income-producing employment for chronically disadvantaged segments of the population entrapped in economically depressed regions. Such employment produces tax revenues as well as personal incomes, reducing the impact on welfare rolls and improving the quality of life.

Plasticulture is a technical reality. Such production systems are extending the growing seasons in many regions of the world and producing horticultural crops where field-grown fresh vegetables and ornamentals are unavailable for much of the year. The economic well-being of many communities throughout the world has been enhanced by the development and use of plasticulture. Such systems offer many new alternatives and opportunities for tomorrow’s population, new systems that encourage conservation and preservation of the environment rather than the exploitation of the land and water.

Literature Cited

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6. Orzolek, M. D. and J. H. Murphy. 1993. The effect of colored polyethylene mulch on the yield of squash and pepper. Proc. Nat. Agric. Plastics Cong. 24:157-161.

7. Lamont, W. J., K. A. Sorensen and C. W. Averre. 1990. Painting aluminum strips on black plastic mulch reduces mosaic symptoms on summer squash. HortScience. 25:1305.

8. Loy, B., J. Lindstrom, S. Gordon, D. Rudd and O. Wells. 1989. Theory and development of wavelength selective mulches. Proc. Nat. Agric. Plastics Cong. 21:193-197.

9. Jensen, M. H. and A. J. Malter. 1994. Protected agriculture - A global review. World Bank tech. paper no. 253. The World Bank, Wash, D.C.

10. Hall, B. J. 1963. Continuous polyethylene tube covers for cucumbers. Proc. Nat. Agric.Plastics Cong. 4:112- 132.

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13. Morrison, C.T., W. T. Green and P. Hadley. 1989. Energy exchange by plastic row covers. Proc. Nat. Agric. Plastics. Cong. 21:269-275.

14. Loy, J. B. and O. S. Wells. 1982. A comparison of slitted polyethylene and spunbonded polyester for plant row covers. HortScience 17:405-407.

15. Mansour, N.S. 1991. The use of field covers in vegetable production. Proc. Intl. Workshops on Imp. Veg. Prod. Through the Use of Fert., Mulching and Irrigation, Chaing Mai Univ., Thailand.

16. Jensen, M. H., M. Valenzuela and D. D. Fangmeier. 1998. Using non-woven floating covers on summer squash for exclusion of whitefly - transmitted gemini viruses. Proc. Nat. Agric. Plastics Cong. 27:159-164.

17. Wittwer, S. H. and N. Castilla. 1995. Protected cultivation of horticultural crops worldwide. HortTechnology. 5:6-23.

18. Roberts, W. J., J. C. Simpkins and P. Kendall. 1976. Using solar energy to heat plastic film greenhouses. Proc. Solar Energy Fuel-Food Workshop. Univ. of Arizona, Tucson. p. 142-159.

19. Sheldrake, R. and R. Langhans. 1961. Heating study with plastic greenhouses. Proc. Nat. Hort. Plastics Cong. 2:16-17.

20. Roberts, W. J. and D. R. Mears. 1969. Double covering a film greenhouse using air to separate film layers. Trans. Amer. Soc. Agric. Eng. 12:32, 33, 38.

21. Takakura, T. 1988. Protected cultivation in Japan. Symp. on High Tech. in Protected Cult. Acta. Hort. 230:29-37.

22. Hall, B. J. 1971. Comparison of drip and furrow irrigation for market tomatoes. Proc. Nat. Agric. Plastics Cong. 10:19-27.

23. Jensen, M. H. 1965. Concluding results of air-supported row covers for early vegetable production. Proc. Nat. Agric. Plastics Cong. 6:100-112.

24. Bresbin Publications, Inc. 1962. The fantastic business of plastic plants and flowers. Modern Plastics. 39:94-97, 205-206.

25. Larson, R. A. 1993. Impact of plastics in the floriculture industry. HortTechnology. 3:28-34.

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27. Hodges, L. and J. R. Brandle. 1996. Windbreak: An important component in a plasticulture system. HortTechnology. 6:177-181.

28. Cornwell, J. T. 1989. The recycling of plastics in agriculture. Proc. Nat Agric. Plastics Cong. 21:60-64.

Table 1. Effect of plastic mulch and drip irrigation on eggplant yield in New Jersey

Source: Unpub. data, J. W. Patterson and N. Smith. New Jersey. Agric. Exp. Station, Rutgers Univ. New Brunswick