World-wide Use of Plastics in Horticultural...
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World-wide Use of Plasticsin Horticultural Production
Sylvan H. Wittwer1
N o technological development has modified so much thecourse of controlled environment agriculture in the produc-tion of fruits, vegetables, flowers, and ornamentals as the recent widespread use of plastic films and related materials.
Plastic films are used extensively for the design and covering of new green-houses; the covering of old conventional glass greenhouses; for air-inflated,self-contained, or supported bubble houses; as interior curtains and exter-nal barriers to reduce heat loss in conventional greenhouses; and for thecontainment of CO2-enriched atmospheres. Plastic films also are used forconstructing high and low tunnels or covers for many row crops, protectivecovering of seed beds, and for soil mulches. Plastic tubes, laterals, and driplines are important components of drip irrigation systems for high-valuehorticultural crops, which now cover 300,000 ha world-wide. Drip irriga-tion is often a component with plastic soil mulches, crop rowcovers, andplastic greenhouses. Recent introductions using plastic films include peatmodules, bag cultures, rockwool, the nutrient-film technique, andnonvariable root environments for field-grown tomatoes (Geraldson, 1982;Wittwer, 1981). Flats, pots, and even labels for growing plants in green-houses involve plastics. Plastic components contribute to the precise controlof top and root environments of plants. In many modern greenhouses, allplant growing structures, other than the root media itself, are plastic (Fig.
Department of Horticulture, Plant and Soil Science Bldg., Michigan State University, East Lansing MI 48824-1324.1Professor Emeritus.
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Fig. 1. A modern greenhousewith plastic throughout forgrowing bedding andpottedplants.
The modern-day plastic revolution forhorticulture is manifest in the precipitous riseand expansion of cultivated areas during thepast 2 decades under plastic greenhouses,row tunnels, rowcovers, and soil mulches.
The most intensive crop-producing sys-tems are those in greenhouses or other pro-tective structures, and the most widely usedprotective structures world-wide are now pre-dominately plastic. The end product is stableand extended crop productivity-at high lev-els. Plastic films ofvarious composition, types,and structural designs help circumvent mostof the hazards ofclimate and weather. Amongthe greatest constraints in crop productionare adverse temperatures; moisture and lightdeficiencies; destructive winds, rains, snowand hail; the growth of weeds; deficiencies insoil nutrients; and atmospheric CO,. Withplastic film covers or mulches, near-optimalconditions of temperature, moisture, nutri-ents, air movement, and atmospheric compo-sition are possible. One of the most sophisti-cated is the nutrient film technique (NFT),which employs a variety of plastic structures,films, and materials (Fig. 2). NFT is promo-tive of plant growth to near-optimal levels ifcombined with or enclosed in a plastic green-house. It offers the potential of maximizingcontrol of both the top and root environ-ments and optimizing both water and nutri-ent uptake (Cooper, 1976; Rudd-Jones, 1977;Spensley et al., 1978).
On the negative side, plasticulture needsfinancial capital, intensive management, andextensive resources, including human capital.It is a labor-intensive production system. Dis-posal of used plastic in ways that are environ-mentally friendly is also a challenge.
Significant developments in the produc-tion of horticultural crops linked with newtechnologies and the most efficient uses of re-sources are occurring world-wide in the cur-rent plastic revolution. Plastics provide an in-tensified expansion in both the vertical andhorizontal dimensions for high-value crops(vegetables, cut flowers, fruit, bedding plants,potted plants, nursery stock, ornamentals).The technology is especially appealing inChina, Japan, the Netherlands, Israel, andEgypt, where population densities are high,constraints of land and water resources areserious, and where the climatic endowmentsare most favorable for plasticulture, such as insouthern and northwestern Europe, the Medi-terranean countries, and the Near East. Thepotential is equally good for advanced as wellas agriculturally developing countries. Plasticgreenhouses, high tunnels, low tunnels, rowcovers, and plastic soil mulches now can beobserved from the equator to the Arctic andAntarctic regions. One may well ask how it allbegan.
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Historical and CurrentPerspectives
The early post-World War II period wit-nessed the first introduction of plastic sheet-ing for greenhouses, rowcovers, and soilmulches in the production ofhigh-value hor-ticultural crops. Until then, the basic glassgreenhouse structure had not changed greatlyin almost a century (Dalrymple, 1973).
Plastic greenhouses and hightunnels
The la te Emory M. Emmert , of theUniv. of Kentucky, played a key role in the de-velopment of the plastic greenhouse in theUnited States. Because he could not afford aglass greenhouse, he designed crude woodenst ructures over which he p laced woodenframes covered with stretched cellophane sup-ported by wires. Emmerts plastic houses re-sembled the early glass greenhouses built outof sashes. He grew commercial crops of let-tuce , tomatoes , cucumbers , and beddingplants in such structures for several years.When polyethylene became available, he usedit for a small, privately owned range. The firstplastic greenhouse was constructed at theKentucky Agricultural Experiment Station inWinter 1953-54. It was still functional in1959 (Fig. 3). The drive for the plastic green-house was its low cost (Emmert, 1955). Green-houses in 1992 in the United States occupiedslightly more than 5000 ha, four-fifths ofwhich were either plastic or fiberglass, usedprimarily for bedding plants, potted plants,cut flowers, ornamentals, and a few vegetablecrops (tomatoes, cucumbers, and leaf let-tuce).
In Europe, the first experiments involv-ing plastic for horticultural crops were re-ported in 1945 (Dalrymple, 1973). Polyethy-
Fig. 2. The nutrient filmtechnique (NFT). (CourtesyD. Rudd-Jones, GlasshouseCrops Research Inst.,Littlehampton, England.)
Fig. 3. The late Dr. EmoryM. Emmert, father ofagricultural plastics inAmerica. (photographed atthe Univ. of Kentucky, May1959.)
lene was used in the early 1950s in Scandinaviato line the inside of glass greenhouses or toform overhead curtains to reduce heat loss(for which it is still used). The first polyethy-lene-covered greenhouse in the United King-dom was erected in Nov. 1955. Growth in theuse of plastic in Europe and elsewhere wasslow throughout the 1950s and early 1960s. Itwas not until the mid- to late-1960s and dur-ing the 1970s that widespread use of plasticsfor greenhouse construction began in Russia,southern Europe, the Mediterranean area,and the Orient. Italy (Tesi, 1989; F. Tognoni,personal communication) and Spain had theirfirst plastic houses by the early 1960s (Castilla,1991), and Greece by 1966 (Olympios, 1991).Meanwhile, in Israel, plastic covers for cropswere introduced in the late 1950s. Israel hadextensive areas of high-value horticulturalcrops under plastic tunnels, rowcovers, andplastic soil mulches by 1971, paralleling thedevelopment and use of drip or trickle irriga-
tion (Wittwer, 1971). China began using plas-tic film covers to protect rice seedbeds fromthe adversities of cold, windy spring weatherin the central and southern provinces in 1958.There was widespread use in both China andJapan by 1965, and rapid expansion throughthe 1970s in China, Japan, and Korea. By1980, there were 8600 ha of plastic green-houses in Japan (Lawson, 1981); and, by1984, 16,000 ha in China (Wittwer et al.,1987).
Beginning in the 1970s, the Mediterra-nean and Middle Eastern countries of Spain,Italy, Greece, Algeria, Portugal, Turkey, Jor-dan, and those countries bordering the Ara-bian gulf (Fontes, 1973; Hodges, 1981; Jensen,1977) became prominent in the constructionof plastic greenhouses and in the productionand marketing of high-value greenhouse-grown crops. It was not until 1980, however,that the first plastic greenhouses were con-structed in Egypt, following a research projectfinanced by the World Bank (Wittwer, 1986).The use of plastic greenhouses for horticul-tural crops in Australia, New Zealand, Chile,Argentina, and South Africa paralleled thosein the United States.
Many problems arose with the early com-mercial adoption of plastic greenhouses inthe United States and elsewhere, which slowedprogress in their use (Cotter and Chaplin,1967; Dalrymple, 1973; Wittwer, 1963). Therewere problems ofventilation and heating. Toretain the early advantage of low constructioncost, inexpensive methods had to be found.Single layers of plastic covering were not aseffective as glass in reducing heat loss. Therewas also excessive condensation underneaththe plastic, which led to reduced and diffusedlight transmission and often greater diseaseproblems. Two layers of plastic, with an airspace in between, partly solved this problemand resulted in a 30% to 40% savings in fuel.Plastic used in the early houses also had to bereplaced every year because of poor durabilityand deterioration induced by the suns ultra-violet rays (Dhillon and Kirschling; 1971;Moore, 1972; Shales and Massey, 1968).
Plastic rowcovers or low tunnelsand soil mulches
Rowcovers for high-value field-grownvegetable crops did not originate with the in-troduction of plastic. The use of parchmentpaper for covering early spring plantings ofcelery for protection against wind, cold rains,and frost in the Grand Rapids and Kalamazooareas of Michigan preceded plastic use bymore than 50 years (Wittwer and Lucas,1956).
Much of the early history on the rise ofagricultural plastic in the United States isgiven by Wells and Loy (1985a, 1985b) and
Fig. 4. Plastic covers for riceseed-beds in Japan, one of themost extensive uses for plasticin the Orient.
elsewhere by Dubois (1978) and Spice (1959).A g a i n , t h e p i o n e e r i n g w o r k o n p l a s t i crowcovers and soil mulches dates back toEmmett (1957). He simultaneously researchedand promoted plastic greenhouses, rowcovers,and soil mulches. His work was followed byShadbolt and McCoy (1960), who estab-lished the merits of rowcovers over hot tents,and Hall and Besemer (1972), who commer-cialized the use ofplastic rowcovers for toma-toes, cucumbers, and peppers in San DiegoCounty, Calif., where they became a standardpractice. Plastic soil mulches were first usedfor strawberries in California in 1958 (Vothand Bringhurst, 1959). Muskmelons also werefound to be very responsive to both blackpolyethylene soil mulches and rowcovers inNew Hampshi re (Wel ls and Loy, 1985a ,1985b) and Michigan (Carolus and Downes,1958). These uses soon became standard prac-tice on muskmelons (Bonanno and Lamont,1980; Motsenbocker and Bonanno, 1989).This crop is sensitive to low soil and airtemperatures and to wind damage, but toler-ant to high temperatures.
These early introductions were followedby covers that were perforated, or slitted, andfloating rowcovers of spunbonded polypro-pylene (Dubois, 1978; Wells and Loy, 1985b).The latter are very light in weight, appliedloosely over the row, and require no wirehoops for support (Fig. 4). In the UnitedStates and Canada, and in many other coun-tries, particularly in the Mediterranean area,rows ofhigh-value horticultural crops (straw-berries, muskmelons, watermelons, cucum-bers, tomatoes) are covered with wire-sup-ported tunnels of plastic, in combinationwith black polyethylene mulch. The two arefrequently combined with drip irrigation.Plastic rowcovers and soil mulches have evena wider geographical distribution, cover moreextensive land areas, and have a greater spec-trum of uses in the production of high-valuecrops than do plastic greenhouses (Hopen,1965; Makus and Gonzalez, 1991; Marr et al.,1991; Nerd et al., 1989; Pritts et al., 1992;Stevenson et al., 1986).
Current Global Status ofPlasticulture for
Horticultural CropsG e o g r a p h i c a l l y , p l a s t i c c o v e r s a n d
mulches may be found in every state in theUnited States, including Hawaii and Alaska;and in all Canadian provinces, including themaritimes. They are found in all countries ofwestern , nor thern , eas tern , and southernEurope, as well as North Africa. Included arethe British Isles, and especially the Isle ofGuernsey; Scandinavia; the Baltic States;
Russia; all nations bordering the Mediterra-nean, including Spain, with the Canary Is-lands, Algeria, Agadir of Morocco, Malta,and especially Italy and Greece; and with theIs le of Cre te , and Egypt . Moving to theOrient, plasticulture can be seen throughoutall the provinces of China, Japan, and Korea,and in some of the higher elevations in thePhilippines and near Chiang Mai in Thailand.In the southern hemisphere, it may be seennear metropolitan areas in Australia, NewZealand, Chile, Argentina, and South Africa.
Plasticulture is often very localized. Atpresent, the heaviest concentrations in theuse ofplastic as plant-growing structures andas films in the United States are in southernCalifornia, especially San Diego County; theGrand Rapids, Kalamazoo, and Monroe ar-eas of Michigan; Cleveland, Ohio; and south-ern New Jersey. Rather extensive, but local,use may also be seen in New Hampshire, NewYork, Arizona, Texas, Pennsylvania, and NorthCarolina. The Leamington, Vineland, andToronto areas of Ontario, Canada are mostconcentrated. In Europe, there are plantingsusing plasticulture in Denmark and Holland,and vast areas in the Almeria Valley andCanary Islands of Spain, and in Portugal,Italy, and Greece, including the Island ofCrete. There are also extensive areas through-out the United Kingdom, especially the Isleof Guernsey; around Moscow and St. Peters-burg in Russia; throughout Israel, in Jordan,the Adana and Tarsus regions of Turkey, and,more recently, the desert bordering newlands areas of Egypt, especially in the vicinityof Cairo. In the Far East, plasticulture may beseen throughout Japan, which has the high-est intensity of arable land covered with plas-tic of any major nation on earth. Japans areaof plasticulture is exceeded only in China,
which is now the worlds number-one user ofplastic for crop production. Areas of specialconcentration are found in the municipalitiesof Beijing, Tianjin, and Shanghai, and nearthe major cities of Harbin, Nanjing, andHangzhou .
Table 1 gives summaries of the approxi-mate world-wide distribution ofplastic green-houses and high tunnels by country for theprimary crops produced. Table 2 lists somecountries where low tunnels or rowcoversand plastic film soil mulches are now used ex-tensively, and the major crops produced. Therowcovers and soil mulches may be usedsingly or in combination. They often arecombined with drip irrigation systems.
Plasticulture in ChinaIn China, plastic covers were first used in
1958 on rice seed-beds in early spring toaccelerate growth and speed up the dates fortransplanting by 10 days to 2 weeks. By themid 1960s and early 1970s, this was a standardpractice, as it also was in Japan (Fig. 4). InJune 1977, it was observed that, adjacent tosome of the major cities (Beijing, Shanghai,Nanjing, Xian, and Yangzhou), there were>1000 ha ofgreenhouses of single-layer poly-ethylene plastic. This compared with 60 haof glass greenhouse structures. Plastic tentswere used to protect high-value crops from
rain in southern China (Plucknett and Beemer,1981).
Plastic-film mulching of crops began in1977 with a mere 50 ha. This was followed byan exponential growth in the use of plasticfilms for mulching during the next 4 years, asseen below (Wittwer et al., 1987).
Year Hectacres1979 501980 2,0001981 20,0001982 200,0001983 2,000,000
Stabilization occurred at slightly morethan 2 million ha from 1983 through 1992.The crops to which plastic mulches are ap-plied are cotton, watermelon, peanuts, riceseed beds, corn(including some sweet corn),tobacco, some 25 different vegetable crops,and strawberries.
As of Sept. 1992, there were two kinds ofplastic-covered greenhouses in China. Theso-called solar greenhouse, or lean to, hasa brick wall on the north side and a plasticcover extending from the top of the northwall to a somewhat lower plastic-covered orbrickwall on the south side. About 12,000 haare covered with such structures.
The second type of plastic greenhouse isthe plastic-film shed or tunnel. These com-prise a total of 50,000 ha and are seen overextensive land areas around all major citiessuch as Beijing, Shanghai, Tianjin, Nanjing,Harbin, and Yangzhou, especially in centraland northern China (Fig. 5).
Many plastic greenhouses are continu-ous spans covering several hectares, othersare separated by 3 to 6 feet with rows ofvegetable crops in between. Individual housescover an area of 600 m2. They may be heatedor unheated. The plastic usually is removedduring the summer or the crops simply area l lowed to grow through and ou t o f theplastic. Plastic covers are replaced annually,with the growing season usually extendingfrom October to June. Soil enriched withanimal manures or compost is the usual grow-ing medium, but the nutrient-film techniqueand other artificial growing media or soilmixes are being introduced. Many vegetablecrops are grownwithin these structures, alongwith strawberries, and sometimes grapes. Theyinclude cucumber, tomato, eggplant, sweetpepper , ho t pepper , melon , green bean-both bush and pole, Chinese cabbage, gourdsof many kinds, green onion, the Chinesechive, leek, stem lettuce, broad bean, celery,Swiss chard, and ordinary cabbage. Produc-tion is under the individual contract respon-sibility system and marketing and distribu-tion through the market economy system.
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Fig. 5. A typical modernplastic greenhouse in theShanghai Municipality ofChina, Sept. 1992.
Both production and marketing are privateoperations. (Wittwer et al., 1987).
Plasticulture in GreeceIn Greece, low tunnels (rowcovers) were
first used in 1967. At that time, there were300 ha. The first plastic greenhouses were
constructed in Winter 1955-56, with an areaof only 0.2 ha. By 1973, there were 600 ha oftunnels and 1300 ha of greenhouses. The1990 producing areas covered 4000 ha withplastic greenhouses, and about 7500 ha withrowcovers and soil mulching. Most of thegreenhouses are in areas where crops can beproduced without or with minimum heatingrequirements.
The principal crops for greenhouses arethe tomato and cucumber. There are alsosome plantings of pepper, eggplant, greenbeans, and cut flowers. Watermelon, musk-melon, strawberry, squash, and tomato arethe principal crops under low tunnels orrowcovers and mulching. Watermelons andmuskmelons comprise 90% of the total. Only19% of the vegetables are grown in heatedgreenhouses, while 57% of the ornamentalsand flowers are grown with heat. Vegetablesare grown in >90% of the greenhouses, with
over the soil surface as soon as possible aftertransplanting. Associated with high soil tem-perature is a 50% increase in total yield anddouble the early yield. Drip irrigation, intro-duced in 1957, is used with the plastic (Vothand Bringhurst, 1990).
Plasticulture for strawberries with suit-able varieties is used for winter production,either in the open or in greenhouses through-out the nor thern hemisphere . Rowcoversalong with the plastic mulches were nextintroduced in the Mediterranean countriesand in China and Japan.
Cucumbers. The cucumber also is well-adapted to growth in plastic greenhouses andtunnels. Cucumbers rank first for culture inplastic greenhouses in Japan, China (Fig. 8),Korea, and Russia, and is second only totomatoes in Greece. Different varieties aregrown throughout all European countries,especially those in the eastern and northernparts. Production is high, the crop matures
earlier and is less exacting than the tomato incultural requirements, and production maybe carried through winter months. Two cropsa year may be grown in many countries bor-dering the Mediterranean. Aside from growthin greenhouses, cucumbers are highly pro-ductive under rowcovers and with plastic soilmulches.
Muskmelons and watermelons. Melonsare among the most responsive of all crops toplasticulture. They are sensitive to both lowsoil and air temperatures and to wind, but arevery tolerant to high temperatures. With per-forated or slitted rowcovers and spunbondedfloating rowcovers, there is some frost pro-tection (Wells and Loy, 1985b). Both water-melons (5000 ha) and muskmelons (1500ha) are grown under rowcovers in Greeceand, to a lesser extent, in greenhouses. Water-melons and muskmelons are two of the lead-ing crops for row tunnels in Spain and Portu-gal. Melons are grown in the greenhouses ofJapan, China, and in most northwestern Eu-ropean countries, as well as those near theMediterranean. Rowcovers and black plasticsoil mulches are seen in Egypt and Israel andin dispersed areas of the northern UnitedStates and Canada. Early, as well as total,yields may be increased for muskmelons. Theuse of black plastic mulches, often coupledwith transparent or slightly opaque rowcovers,is common in California, Arizona, Michigan,New York, New Jersey, and Essex County,Ont., Canada.
Watermelons in China are especially re-sponsive to clear plastic soil mulching (Wittweret al., 1987). They are grown extensively in allprovinces, autonomous regions, and munici-palities. The progressive use of plastic soilmulches for watermelons in China is givenbelow.
At present, 360,000 ha of watermelonsare mulched with plastic annually in China.This accounts for 18% of the 2 million ha ofmulched crops, being exceeded only by cot-ton and equal to that for peanuts. In thenor thern , nor theas tern , and nor thwesternregions of China, 75% of the total water-melon area is mulched with plastic. There isalso an increase in the use ofplastic rowcoverscombined with the soil mulches. Transparentsoil mulches increase soil temperatures andprevent surface evaporation of soil moisture(Fig. 9). With plastic, the melons in northernChina mature 20 days earlier, and are themain source of supply for all of China during
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Fig.10. A medium-sizedplastic house in China with flexible bamboo frames.
late August and through September. In south-ern China, the plastic mulch reduces soilerosion and prevents compaction from heavyrains. The mulch promotes the growth andfruiting of watermelons and increases totalyields by 29% to 30% (Runfang, 1990).
Plastic tunnels often are combined withplastic mulch. This is the most popular methodof forcing culture in watermelon production.Such a double layer of film cover is usedwidely in northern China and Korea to takeadvantage of sunny spring weather for earlymaturity and to give wind and frost protec-tion. In the lower reaches of the Yangxi rivervalley and the suburbs of Shanghai, the grow-ing of watermelons in plastic tunnels pro-motes early fruit set, early maturity, and re-sults in enormously high yields.
Watermelon production in small to me-dium-sized plastic houses has progressed rap-idly in central China. The structures are simple,with flexible bamboo frames (Fig. 10). Some-times a tunnel with mulch is prepared withinthe plastic house (Fig. 11) to increase warmthThis results in a three-layer film cover. Thearea oftunnels and plastic houses in TongshanCounty of Jiangsu Province now totals 1800ha (Runfang, 1990).
Bedding plants, potted plants,cut flowers, and vegetable cropsin the United States
There are 12,500 acres (5100 ha) ofgreenhouses in the United States, two-thirdsof which are now plastic and four-fifths eitherplastic or fiberglass (USDA, 1988). As late asthe 1960s and early 1970s (Wittwer andHomna, 1979), there were 1000 acres (405ha) devoted to vegetables (primarily, tomato,cucumber, and lettuce). That acreage hasdiminished to 250 acres or less, althoughthere are still large plantings in Ohio andCalifornia, with small plantings in Pennsylva-nia, New York, Michigan, Florida, and a fewother states, a part of which are relativelysmall hydroponic units. Bedding plants pre-dominate as crops grown in plastic structures.Of total production, 10% to 20% are veg-etables, and the balance flowers. Beddingplants are, in turn, followed in importance bypotted plants (see Fig. l), cut flowers, foliageplants, nursery crops, and vegetables. Veg-etables make up scarcely more than 5% of thetotal greenhouse production in the UnitedStates (USDA, 1988). Cut flowers and pottedplants, with about two-thirds grown in plasticgreenhouses, are making inroads into whathas been almost exclusively traditional green-house vegetable production in Canada, north-ern Europe, Japan, the Mediterranean coun-tries, and, more recently, in China and Korea.
In no country, however, has the transi-
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Fig. 11. A low plastic tunnelinside a high plastic tunnelto give added protection fortender crops in early spring.
Fig. 12. A polyethylene soilmulch replaces staking forseed yam production inNigeria.
tion from vegetables to flowers been so dra-matic as in the United States. Southern Cali-fornia, along with New York, Pennsylvania,Ohio, and the Grand Rapids and Kalamazooareas of Michigan have the most-concen-trated areas for plasticulture of bedding andpotted plants. In Michigan, it is an industrythat rose from celery fields. The small glassgreenhouses where celery transplants weregrown formerly have been replaced by largerandmore-efficient plastic structures designedspecifically for bedding plants. In Michigan,Ohio, New York, and Pennsylvania there arenearly 500 plastic greenhouse farms in eachstate. These states, along with California,probably represent the largest concentrationsin the world of bedding plants grown inplastic greenhouses.
Some novel uses of plasticmulches and covers
Seed yam production in Nigeria is en-hanced greatly by the use of polyethylene
mulch. It makes staking unnecessary, con-trols weeds, reduces soil erosion, preserves soil moisture, maintains a desirable soil temperatureperature for good plant growth and develop-ment, ensures a good stand, and shortens theperiod during which the land is occupied inseed production. The mulch covers the plant-ing ridges and a part of the furrow betweenridges (Fig. 12). Soil on the edges holdsdown the plastic and holes are cut in the plas-tic for setting the plants (Asnani et al., 1985).
Plantings for sweetpotatoes are mulchedin a similar manner for weed control, reduc-tion in soil erosion, and preservation of soilmoisture. Related to the sweetpotato in thebeneficial responses to plastic soil mulches iscassava. Plastic soil mulches ensure goodplant establishment (up to 95%) and vigorousplant growth. Weeding is greatly reduced,soil erosion held to a minimum, and soilmoisture is conserved. There is less mealy bugand spider mite infestation because of an im-proved moisture regime. Plastic mulches arelow in cost, easy to use, and economical(IITA, 1984).
Sweetpotato plant beds, covered withclear plastic produce a greater number andweight of transplants. Early harvest of trans-plants is accelerated by 8 to 12 days (Porter,1991).
The benefits of earlier maturity fromplastic mulching of sweet corn, demonstratedby Hopen (1965), has since been adoptedelsewhere. It was reported in 1979 that, inFrance, up to 5000 ha of field corn weremulched wi th c lear p las t ic . Yie lds weredoubled and harvest advanced by 3 to 4weeks (Wittwer, 1981). It appears that, withclear plastic, corn production may be moved400 miles further north or into mountainareas with greater elevations. This is preciselythe case where sweet corn is produced as farnorth as Alaska, and production has beenmoved to high mountain areas in southernChina.
Opaque plastic rowcovers (Makus andGonzales, 1991) allow the production of whiteasparagus without mounding of beds. Theplastic also protects against short durations offrosts and produces higher trimmed weights.The asparagus requires less-frequent cutting.
Plastic covers are being introduced fortropical fruit production, both in the easternand western hemispheres. Bunches of ba-nanas are encased in colored plastic films asthey approach field maturity (Fig. 13). Theplastic offers protection from insects, birds,and other pests. The result is a product that ismore attractive and of uniform maturity.Frames of clear plastic encircling young pa-paya plants give wind and insect protectionand hasten early growth and fruit production(Fig. 14). High-quality, year-round produc-
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Fig. 13. Plastic covers are used in the tropics to encase bananas as they approach fieldmaturity offering protection from pests and encouraging uniform ripening
Fig. 15. Plastic greenhouse tomato production near Nakuru Kenya, July 1988.
tion of tomatoes and cucumbers in plasticgreenhouses constructed from native materi-als just a few kilometers from the equator nearNakuru, Kenya, provide fresh salads for anever-increasing tourist trade in nearby Nairobi(Fig. 15).
Advantages of Plast iculture
The initial driving forces for the use ofplastics in horticulture were, first, to increaseearliness and total crop productivity of high-value horticultural crops (Abdul-Baki andSpence, 1992); second, to take advantage ofout-of-season production; and third, to usesomething as effective, but much less expen-sive, than glass as a protective covering. Thislatter advantage was the primary stimulus forEmmerts (1955) pioneering studies in Ken-tucky.
The promotion ofearliness, coupledwithmore-productive and predictable total yields,holds for most all field-grown horticulturalcrops where plastics are used. The effective-ness of plastic covers in compensating for lowsoil temperatures and the reductions in winddamage has been phenomenal for vine crops,especially watermelon, muskmelon, squash,cucumber, sweet corn, strawberry, yam, andsweetpotato and cassava in the tropics. Har-vest may be hastened by a few days, a week, ormonths, depending on the structures and thecombinations of covers used. Earliness is es-pecially important in the response to plasticcovers for tomato, cucumber, pepper, egg-plant, melon, sweet corn, and cut flowers.The improvement of cut flower quality isoften a compelling factor for using plasticcovers.
Extending the geographical areas forsuccessful production applies to a wide rangeof vegetables, flowers, and fruit crops fromthe equator (Figs. 12 and 15) to the arctics(Fig. 9). Earlier maturity is promoted. Mostsuccessful has been the plastic greenhouse forcucumber, tomato, pepper, eggplant, straw-berry, melon, green bean, and green onion;and for tomato, cucumber, melon, and sweetcorn in the field. Modern greenhouses, con-structed with double-layer plastic with an airspace between, are more retentive of heat andC O2 than are glass greenhouses. Protectionagainst short-term frosts and cold waves withtemperature depressions ranging from 1 to3C is possible in the open field with sometypes of spunbonded and floating rowcoversof polypropylene (Wells and Loy, 1985b; Fig.16).
Other beneficial effects of plasticulturewith row crop mulching and/or covers in thefield are the conservation of water throughreductions in surface evaporation; a reduc-tion in the loss of soil nutrients by leaching
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(Wein and Minotti, 1987); weed control (es-pecially important for vine crops); alleviationof soil compaction by the beating action ofrain; protection of plants against wind, rain,hail, and insects; and an increase in atmo-spheric CO2. One, several, or all of the aboveadvantages of plasticulture may accrue, de-pending on the crop, where it is grown, andthe plastic structures employed.
In summary, plasticulture provides oneof the few options for further expansion ofboth the horizontal and vertical dimensionsof horticultural crop production and for themost-effective management and use of land,water, fertilizer, sunlight, and atmosphericCO, as essential crop inputs. There are alsonew vistas for marketing the improved qualityvirtues of fresh fruits, vegetables, and cutflowers.
D i s a d v a n t a g e s o fP l a s t i c u l t u r e
Cost is a chief constraint in the use ofplastics for horticultural crops. While row-covers are intermediate in cost between plas-tic soil mulches and plastic greenhouses, theyare seldom used alone. A usual combinationwith rowcovers is a plastic soil mulch and dripirrigation, as is used for strawberries in Israel(Fig. 17). In fact, drip irrigation often is com-bined with plastic soil mulching and plasticbag culture inside a plastic greenhouse, some-times with the addition of rowcovers as well(Fig. 18). Each input requires an additionalexpenditure. A further cost, which must befactored in, is the annual or periodic replace-ment of plastic coverings for greenhouses.
Plastic culture demands high-produc-tion technology and significant human re-source inputs. Those engaged in growingcrops with the aid of plastic mulches, covers,or greenhouses should receive technical train-ing as to factors controlling plant growth andthe art of plant growing. Heating, ventilat-ing, cooling, and condensation difficultiesstill exist with crops grown inside plasticenclosures. Programming inputs of mineralnutrients, and meeting optimal requirementsfor temperature, light, and atmospheric CO,levels are desirable and should be considered.Plasticulture involves a series of productiontechnologies, beginning with plant growingthrough to harvest, that relate to one of themost intensive types of agricultural produc-tion coupled with the most efficient use ofhuman and natural resources. It is also one ofthe most labor- and management-intensiveagricultural production systems. The cost oflabor along with the cost and availability ofother inputs could be prohibitive.
A final problem is disposal of used plas-tic. Plastic pollution is unsightly and is recog-
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Fig. 19. Photo-degradableplastic at the HorticulturalResearch Inst., HeilongjiangProvince, China, Sept. 1992.
Fig. 20. A self-inflatedplastic bubble house in north-central Ohio used for wintervegetable production.
nized as an environmental hazard and a con-straint wherever plasticulture has flourished.A possible approach for the future is the useof photo- or biodegradable plastic (Fig. 19).Several products of biological origins havebeen introduced that show some promise.Degradability, however, has not been soughtafter for plastic greenhouse covers until re-cently. Durability has rather been the primaryconcern. Recycling agricultural plastics isanother option for disposal. The presence offoreign particles is, however, an obstacle forreprocessing.
F u t u r e P r o s p e c t s
There will be many new developmentswith plastics for controlled-environment ag-riculture. Plastics offer an option for bothincreased vertical and horizontal expansion inhorticultural crop production and neededimprovements in qua l i ty of the productsgrown. Use of plastic films will be extendedduring the next decade to many additionalcrops and in developing countries. Case his-tories for the past decade exist in China andEgypt. Plastic films with colored stripes andother color patterns may be developed thatwill help repel insects (Brown and Brown,1992; Scott et al., 1989). Light- and biode-gradable plastics ultimately will be fabricatedto avoid undesirable residues, alleviate dis-posal problems, avoid plastic pollution,and be more environmentally acceptable. Nu-trient film technologies, first developed in theUnited Kingdom, for control of root as wellas shoot environment will expand world-wide, as is occurring in China, Japan, andEgypt for both greenhouse and open-fieldproduction of fruits, vegetables, flowers, pot-ted plants, and ornamentals.
Plastic tubing networks and emitters,combined with soil and plant covers of plas-tic, will facilitate further drip irrigation andrefinements in controlled environment agri-culture in China, Japan, Mexico, Australia,New Zealand, the Mediterranean area, andmany developing countries world-wide. Newdevelopments also can be projected for bubblehouses (Fig. 20) and solar still technologiesfor desert areas, and in the design of low-energy solar and conventional greenhouses.
The expanded use of plastic film coversfor greenhouses and row crops and as soilmulches will continue and will push the pro-ductivity of high-value fruits, vegetables, flow-ers, and ornamentals into new geographicalfrontiers, including the tropics, in both agri-culturally developed and less-developed na-tions, as has been witnessed recently for China,Korea, Egypt, and Nigeria. Plasticulture as atechnology for enhancement of plant growthis not confined to the Western world.
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The phenomenal world-wide rise in-theuse ofplastics during the past 2 decades, withparticular focus on high-value horticulturalcrops, with the goals of earliness, increasedproductivity, and improved product quality,will result in farmers learning new skills forenhancement of crop productivity and more-effective management of resources. Manysuch skills and technologies can be trans-ferred to the production of crops in openfields. This will relate to improved fertilizerand water-use efficiency. Land areas not nowsuitable for traditional farming can be put touse. There will be new developments forintegrated pest management, control of soilerosion, sustainable systems of soil manage-ment, the arts of successful modeling of plantgrowth, and unique methods of plant grow-ing and propagation. There will be program-ming of harvests for meeting market de-mands of increasingly discerning consumers.Postharvest handling and new marketing and
packaging skills will be needed. The results ofr e s e a r c h o n c r o p s g r o w n u n d e r p r o t e c t i o nc a n b e q u a n t i f i e d e a s i l y a s t o r e t u r n s o ni n v e s t m e n t s f o r . i m p r o v i n g c r o p y i e l d s a n dincreased earnings.
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