Protected Cultivation ofHorticultural Crops WorldwideSylvan H. Wittwerl and Nicolas Castilla2

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rotected cultivation, which enables some control of wind veloc-ity, moisture, temperature, mineral nutrients, light intensity, andatmospheric composition, has contributed and will continue to

contribute much to a better understanding of growth factor require-ments and inputs for improving crop productivity in open fields. Pro-tected cultivation is a unique and specialized form of agriculture. De-vices or technologies for protection (windbreaks, irrigation, soilmulches) or structures (greenhouses, tunnels, rowcovers) may be usedwith or without heat. The intent is to grow crops where otherwisethey could not survive by modifying the natural environment to pro-long the harvest period, often with earlier maturity, to increase yields,improve quality, enhance the stability of production, and make com-modities available when there is no outdoor production. The primaryemphasis is on producing high-value horticultural crops (vegetables,fruit, flowers, woody ornamental, and bedding plants).

Production areas, structures, and crops have been expanding rap-idly during the past century (see Sidebar). From the beginning, agri-cultural production has been primarily outdoors. It is a major industrythat is primarily climate- and weather-dependent. In fact, the mostdeterminate factor in horticultural crop production is the climate.

lProfessor Emeritus, Department of Horticulture, Michigan State University, East Lansing, MI 48824-1325.‘Head, Dept. of Horticulture, Agricultural Research and Development Center, P. O. Box 2027, 18080, Granada, Spain.Other sources of information for the tables: Castilla (1992), El-Aidy (1992), Garnaud (1994), Germing (1985), Nishi(1986), Olympios (1991), Schales (1990), Tognoni (1991), Wittwer ( 1993), Wittwer and Robb (1964), Wittwer andHonma, (1979). We are grateful to the following for providing the data on protected cultivation in the various countries:L. Reguieg (Algeria); J.C. Zambo and J.C. Favaro (Argentina); F. Benoit and M. Sirjacobs (Belgium); Y. Solakov(Bulgaria); A. Aljaro (Chile); A.F. Abou-Hadid (Egypt); Lea Kurki (Finland); J.C. Garnaud and P. Printz (France);C. von Zabeltiz (Germany); C. Olympios (Greece); C. van Winden, M.v.d. Linden, and Ad. N. M. de Koning (Holland);K. Gerundas (Hungary); A. Braverman and A.H. Halevy ( Israel); R. D’Amore ( Italy); A. Ahdel-Nour (Jordan); H.Reyes (Mexico); R. Choukr-Allah (Morocco); P. Sivek (Poland); A. Monteiro (Portugal); E.J. Johansson and AsgerKlougart (Scandinavia and Denmark); J.I. Montero (Spain); A. Mougou (Tunisia); K. Abak (Turkey); G.L. Bondarenko,(Ukrainia); J.T. Ashdown (United Kingdom); W.H. Carlson, Carl Hoefor, J.Kelly, W.J. Lamont, Jr., H. Grey, and O.S.Wells (United States).

The magnitude of the impact of climateand weather agriculture productivity andquality of the product even now is appreciatedonly by farmers and a small segment of thescientific community, namely horticulturists,and hardly at all by the general population.

For example, the perceptive documentAgriculture: Toward the Year 2001 (FAO,1979) makes no mention of climate. Farmersappreciate and fear the impacts of weather.Climate remains the primary factor as towhere food and fiber are produced and whichcrops are grown (Dalrymple, 1973).

Among the greatest constraints in horti-cultural crop production are a lack of sun-light, temperatures that are either too hot ortoo cold, moisture deficiencies or excesses,weed growth, deficiencies in soil nutrients,excessive wind velocity, and atmospheric car-bon dioxide. Most of these are climatic fac-tors or directly related to them. Many of theseconstraints have been alleviated or lessenedby protected cultivation or controlled envi-ronments.

The lack of water is the single mostimportant environmental impediment to plantgrowth and global food production. Water isour most precious and most wasted resource.The greatest crop losses in the United Statesfrom 1930-78 were caused by drought. Lossesfrom drought were almost equal to all otherclimate-induced losses, including excess wa-ter, floods, cold, hail, and wind (Boyer, 1982).

By far, the most intensive and ancientmeans of protected cultivation of crops isirrigation. Windbreaks, the magnitude ofwhich is not known, provide a second means.By irrigation, crop production has been ex-tended to deserts and semi-arid lands thatotherwise would be nonproductive. Many ofthe hazards of drought have been overcome.

The recent rate of increase of irrigatedland has been precipitous. From 1950 to theearly 1980s, the crop area irrigated in theworld increased from 94 to 250 million ha(Postel, 1989). This increase accounted for40% to 50% of the increase in agriculturaloutput. While the expansion of irrigation inthe late 1980s and early 1990s has sloweddramatically in the United States and else-where, some nations, such as China, antici-pate further expansion. Today, 17% of theworld’s cropland that is irrigated produces athird of the agricultural output. Four coun-tries-China, India, the United States, andPakistan-contain half the world’s irrigatedland. More than two-thirds of all the food inChina is produced on land that is irrigated,and more than half of the land in India,Indonesia, and Japan is irrigated. Irrigationtechnologies for China date back more than2200 years to the construction of the DuJiang Weir in what is now Sichuan Province.

World-wide agriculture claims two-thirds ofall the fresh water removed from rivers, lakes,streams, and aquifers (Postel, 1993). Furtherexpansion of irrigation is now limited byshort water supplies in much of the UnitedStates, Russia, China, India, Pakistan, Spain,Italy, Mexico, Chile, and Australia and al-most all Mediterranean, Arabian Gulf, andAfrican countries. The high agricultural useof water is increasingly under challenge, fromthe standpoint of rising food costs and othercompetitive options such as recreation, mu-nicipalities, industry, energy generators, andthe current low efficiency (35% to 40%) foragricultural use. Coupled with irrigation ofhigh-value crops has been the rise in the useof the highly efficient drip or trickle systems,which, at the global level, now comprisebetween 1.5 and 2.0 million ha. Drip irriga-tion systems are frequently an important com-ponent of other structures designed for pro-tected cultivation of horticultural crops.

Controlled-environment agriculture orprotected cultivation has now extended farbeyond the realms of crop irrigation andwater management. The focus of this paper ismostly on structures and technologies, otherthan irrigation, that have emerged largelyduring the past century.

Types of plant protectionAll plant species have an optimal range

for each environmental factor. Installing ascreen or shelter alters the energy and otherexchanges between the whole plant (or apartof it) and the environment. The position ofthe screen or shelter, relative to the plant,determines the kind of protection. In mulches,the screen is located below the above-groundparts of the plant, over the soil. Greenhouses,tunnels, and direct covers are other types ofprotection, where the screen is placed overthe plants as a cover. Windbreaks are placedlaterally to the crop (CPA, 1992).

Windbreaks. Windbreaks are used formechanical protection, particularly for vinecrops, as protection against the whipping ofthe wind, which reduces evapotranspiration.Windbreaks enhance early maturity, improvegrowth, increase production, and result in abetter-quality product. The action of windon crops is direct and indirect. It inducesmorphological and anatomical changes, simi-lar to those produced by drought; reducesphotosynthetic activity, and increases evapo-transpiration (FAO, 1986). On bare soils,wind erosion can be very destructive, espe-cially in arid regions and deserts. The mainadvantage of windbreaks is to reduce cropdamage either from breaking and drying,particularly of the stems and leaves of vinecrops, or blowing particles that affect the

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Historical Perspective of Protected Cultivation

Fig. 1. Thousands of small glass greenhouses were

Fig. 3. Rows of newly transplanted celeryplants covered with tunnels or tubes ofparchment paper for protection againstspring frosts, wind, snow, and rain inMichigan.

Attempts to adapt crop production to theenvironment with protective devices orpractices dates back to ancient times. Struc-tures for crop protection began in the earlypart of the Roman Empire to accommodatethe Emperor Tiberius Caesar (14-37 AD).They consisted of movable beds of cu-cumbers, and perhaps other crops, placedoutside on favorable days and inside dur-ing inclement weather. Transparent slate-Iike plates or sheets of mica or alabasterwere used as covers. Such cultural meth-ods ceased with the decline of the RomanEmpire (Dalrymple, 1973).

It was not until the late 15th to 18thcenturies that the precursors of green-houses appeared, primarily in England,Holland, France, Japan, and China. Theywere crude square or rectangular woodenor bamboo frames or structures coveredwith panes of glass, oiled paper, or glassbells to cover hotbeds. A wide variety ofout-of-season vegetables and small fruitwere grown. During the late 1600s to 1700s,stoves and heating flues began to be usedwith the first glass greenhouses. They hadglass on one side only in the form of asloping roof. Later in the 18th century,glass was used for the front and on bothends of lean-to houses (Dalrymple, 1973).The two-sided greenhouse emerged in the1800s. Leading in those early develop-ments were England, Holland, France, andthe Scandinavian countries. Forcing cropsbecame especially popular in Englandduring the late 1700s to 1800s. Producewas no Ionger confined just to the wealthy.By the middle of the 1800s, greenhouse-grown grapes, melons, peaches, and straw-berries appeared on English markets,and then tomatoes from the 1870s to 1880s.By the end of the 19th century, commercialgreenhouse crop production was well-established.

The development of forcing framesand greenhouses soon spread from Eu-rope to America and elsewhere. In China,Japan, and Korea, near the major cities,glass greenhouses were built low, withglass only on the roof and southern wall,The northern and side walls were con-structed of either concrete or adobeembanked with bales of rice straw for insu-lation, These houses were devoted to pro-ducing cucumbers, tomatoes, eggplant, and

8

beans. Many were not heated, but werecovered with straw mats at night(Romanowski, 1981).

In the United States, several kinds ofstructures were built between 1700 and1800 with varying degrees of protection

houses. Lettuce and radishes werethe first crops to appear in the 1860s,followed by cucumbers and then to-matoes. Growth in the Boston areawas followed by expansion aroundGrand Rapids, Mich., and later nearCleveland and Toledo, Ohio, By theturn of the century, there were anestimated 1000 greenhouses inthe United States devoted primarily toforcing winter vegetables, with anannual retail value of about $4.5 mil-

total, followed by cucumbers with 33%,lettuce with 18%, and all others with 6%. Apeak in glass greenhouse construction forthe United States and Canada, with primaryconcentration in the Cleveland area and theLeamington area of Essex County, Ontario,

Canada. was reached in the late 1950sand early 1960s. Simultaneously, thiswas also occurring in northwesternEurope, for vegetables, primarily to-matoes, and an emerging cut-flowerand ornamental plant industry, par-ticularly in England, including theIsle of Guernsey, Germany, Holland,and Scandinavia. By 1960, Hollandhad the world’s most concentrated production of glass greenhouse-grown crops, estimated at 5000-6000ha, 75% of which consisted of toma-

Simultaneously, in Michigan, a $2million dollar industry emerged: fore-ing rhubarb (Fig. 2) in heated housesduring winter and covering rows ofearly transplanted field celery withparchment paper (Fig. 3). Now, how-

Fig. 4. Entire valleys are covered with plasticgreenhouses west of Almeria, Spain.

Fig. 6. A.J. Cooper (left) and G. W. Winsor (right),originators of the nutrient-film technique of theGlasshouse Crops Research Institute in LittleHampton, U.K.

ever, these two types of vegetable forcingor protected cultivation are almost nonex-istent.

In the United States, dramatic changeshave occurred since the 1960s in cropsgrown in greenhouses. Hydroponic cul-ture, especially for tomatoes, became anattractive and expensive venture. Such unitsbecame widely distributed across the na-tion (Jensen and Collins, 1985). As late asthe 1960s and early 1970s, there was morethan 400 ha devoted to vegetable produc-tion (primarily tomato, cucumber, and let-tuce). That hectarage has now diminishedto less than 100 ha. Of total greenhouseproduction, scarcely more than 5% is veg-etables, the balance being flowers, pottedplants, ornamental, and bedding plants.

By 1980, there were an estimated150,000 ha of greenhouses (glass, fiber-glass, plastic) globally for high-value foodcrops (Wittwer, 1981). Annual gross pro-ductivity ranged from $20 to $40 billion. Inaddition, about 50,000 ha was devoted to

Fig. 5. Double protection for melons—mini-tunnels inside a plastic greenhouse in Almeria,Spain.

the production of flowers, ornamen-tal, and bedding plants.

Meanwhile, other areas, par-ticularly in Asian and Mediterra-nean countries, witnessed a phe-nomenal increase in greenhouseculture of high-value vegetablecrops, originating with the use ofplastic for nonheated greenhouseconstruction beginning in the 1950s.This is reflected in current data,which show that the production of

vegetables in Europe has been shiftingsouth to the Mediterranean area, wherethey are grown in plastic tunnels and green-houses (Figs. 4 and 5). Meanwhile, therehas been a shift in northern Europeanglass greenhouses to added-value cropsof ornamental, flowers, and potted plants.The expansion in plasticulture in the Medi-

terranean is still gaining momentum, againwith a gradual transition from the produc-tion of food crops to flowers, potted andbedding plants, and ornamental. Nowherewere these expansions and transitionsgreater than in China and the Mediterra-nean countries, with Spain and Italy asleaders. They were accompanied by a like-wise remarkable increase in drip irriga-tion, originating in Israel during the 1960sand traveling to the United States in 1970,when there was a worldwide hectarage ofabout 2000. By 1974, this had grown to65,000 ha, with about 200,000 in 1978and 500,000 hectares by 1980 (Wittwer,1981).

Concurrently with all these develop-ments of plastic greenhouses and expan-sion of areas covered and protected bywindbreaks and with drip irrigation hasbeen an ever-expanding use of plastic row

tunnels, covers, and plastic soil mulches,with an enormous increase in China (Wittweret al., 1987), where currently an area ofmore than 2.8 million ha of crops is coveredwith plastic soil mulch. Meanwhile, a re-markable set of automated plant-growingtechnologies, including the nutrient-filmtechnique developed at the GlasshouseCrops Research Institute in Littlehampton,U.K. (Fig. 6), was followed by the construc-tion of the first phytotrons and plant growthchambers. Finally, free air CO field experi-

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ments are in progress for evaluating theimpact of rising levels of atmospheric CO2

for the enhancement of plant growth, ini-tially observed over a century ago for cropsgrown in some of the first glass green-houses constructed in western Europe(Wittwer,1986).

In the Westlands and elsewhere in

Holland today, where much of theexploratory work on protected culti-vation had its origin more than 2centuries ago, there is more than10,000 ha covered by glass green-houses, with the area increasing eachyear, devoted to the year-round pro-duction of high-quality vegetables(tomato, pepper, eggplant, cucum-ber, lettuce), cut flowers (rose, chry-santhemum, carnation, tulip, lily),and potted plants (ficus, dracaena,

kalanchoe, begonia, azalea). Carbon diox-ide enrichment, along with other high-tech-nology inputs (differences in day and nighttemperature), artificial substrates (media),bottom heat, and precise control of thegrowth inputs of water, temperature, andnutrients to maximize use of sunlight, isused throughout the entire year. The atmo-spheric level of CO2 is maintained at 750–1000 ppm even during summer. Duringsummer, natural gas is burned just for CO2.The heat produced is stored in water tanksto elevate night temperatures. The benefitsof CO2 enrichment result in higher yields,earlier harvests, and improved quality for allcrops. Research is now focused on storingthe heat released from the combustion ofnatural gas to achieve high daytime Ievels ofatmospheric CO2 in the greenhouse envi-ronment and computer tracking of neededinputs (Nederhoff, 1994). A counterpart isfound in the latest technological inputs forbedding plant production in the United States(Erwin et al., 1993).

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Fig. 7. Windbreaks madefrom dry cane (Arundodonax) are popular in theMediterranean.

qualityof the produce.For greenhouse-growncrops, there is a reduction in use of fuel andheating costs. Crop yields are also improvedwith early maturity less disease incidence.improved pollination from increased bee ac-tivity, and less drying of the pollen in somecrops, such as kiwi and vine crops (CPA,1992.)

A windbreak is any device designed toobstruct wind flow and protect against any illeffects of wind (Van Eimern et al., 1964).The main effect is to reduce surface windspeed, which changes the microclimate, re-sulting in an increase in air temperature dur-ing the day, and a reduction at night. Con-comitantly, loss of soil moisture and evapo-transpiration are reduced (Rosenberg, 1979;Van Eimern et al., 1964), especially in aridclimates.Windbreakshading must be consid-ered when planning installations near green-houses.

Windbreaks can be natural or artificial.TreesandDerennial bushes are natural wind-breaks. Eucalyptus, cypress, casuarina, pop-lar, oak, maple, acacia, and some conifers areused frequently. Wind protection may also beprovided by companion cropping with a tallcrop, such as corn, alternating with sweetpo-tatoesasagroundcover, or watermelon plantsinterspersed with grain crops. Successiveplantings with established plants protectingthe emerging younger ones are effective, as isalley cropping practiced in the tropics. Somesuch practices date back to ancient times.Artificial windbreaks may be made from theresidues of vegetation or from manufacturedmaterials.Deadcane,wooden stakes, or sticksand bamboo have been used widely (Fig. 7).Plastics have been increasingly used during

the past 2 decades. Polyethylene (PE) andpolypropylene (PP) nets and screens, with apreference for nets (Fig. 8), are effective. Thestructural framework for a windbreak mayconsist of wood, bamboo, iron, or concreteposts reinforced with wire. The species se-lectedfor natural windbreaks is based on rateof growth in height, foliage density, anduniformity; adaptation to climatic and soilconditions; depth of rooting; resistance towind; and continuous protection for all sea-sons.

Themost-efficientwindbreaksmusthaveanoptimal permeability of40% to 50%. Densewindbreaks greatly reduce the wind directly

behind, but generate eddies and strong tur-bulences in the nearby vicinity. The horizon-tal range of wind reduction by a windbreak,expressed in percentage, is proportional to itsheight,andis practically independent ofwindspeed. The optimum distance between wind-breaks of the permeable type is 15 and 25times the height (FAO, 1986). Rectangularnetworks, found extensively in northernChina, account for variations in wind direc-tion and usually give the best wind protec-tion. Aside from mechanical plant protec-tion, the main effects ofwindbreaks on yieldsresult from an increase in the air temperatureduringthedayandimprovedmoisture condi-tions. A combinationof natural and artificialwindbreaks may be advised (CPA, 1992).

Soil mulches. Soil mulches of straw orcrop or plant residues, including other livingcrops, sand, gravel, and rocks, are used on awidevarietyof crops, including strawberries,tomatoes, peppers, asparagus, melons, cu-cumbers, squash, banana, and sweetpotato.Mulching is the application of a soil cover that

is a barrier to the transfer of heat orvapor. Mulches reduce the energyused in evaporation of water fromthe soil surface and competitionfromweedgrowth.They effectivelyblock the transport of vapor out ofthe soil, altering net radiation atthe soil surface (Rosenberg, 1974).Different kinds of mulches havebeen used from ancient times. Insitu materials, such as weeds andotherresidual plant or crop materi-als, may be dried and spread as alayer over the soil. Straw mulcheshave been used worldwide. Livingmulches of grass, although oftenreferred to as such, are not realmulches, for they do not constitutea barrier to the transfer of heat orvapor,butimprovesoil characteris-tics and fertility and alter the mi-croclimate. Water consumptionmay be increased.

Manufactured or processed

Fig. 8. Plastic nets arereplacing the conventionaldry cane windbreaks of theMediterranean.

mulching materials include paper of varioustextures and colors, aluminum foil, gradedgravel, sand, and many kinds of plastic films.Small stones constitute a natural mulch ingravel-type soil when they rise to the surfacefrom tillage. Gravel of volcanic origin also hasbeen used for mulching. It is often moreeffective than natural organic mulches in thatwater is stored in its internal pores. Suchgravel is used for mulching in the CanaryIslands and in north-central China. Sand isused in the Mediterranean as a mulch (Castillaet al., 1986; Castilla and Fereres, 1990a).This allows use of low-quality irrigation wa-ters.

Mulching crops during the past 3 de-cades has been prompted by the use of plastics(Wittwer, 1993). Low-density PE film is useduniversally. The use of linear low-density PEhas allowed a decrease in thickness because ithas greater mechanical resistance. A recentreview (Lament, 1993 ) emphasizes that theadvantages of plastic mulches are increasedearlier and higher-quality yields, better weedcontrol and insect management, and increasedefficiency of the use of irrigation water andfertilizer. There is reduced surface evapora-tion, fertilzer leaching, and soil compaction.Disadvantages of plastic mulching are thenecessity of removal, disposal after use, andthe high initial cost.

Black, transparent, and white colors pre-vail in commercial mulching, but co-extrudedwhite and black PE film is increasingly beingused in developed countries. Transparent filmacts in a similar manner to a greenhouse. Thecondensate on the underside of the film isopaque to long-wave infrared radiation andthus limits heat loss. Soil temperatures duringthe day, compared to bare soil, are higher

under transparent and black filmsand slightly lower under the white(Lament, 1993). The crop canopy,however (shading of the film), canalter these patterns. Reflection intothe plant Canopy of incoming solarradiation with white or co-extrudedwhite-on-black and silver mulchesat high latitudes increases availableradiation for greenhouse crops.During cold seasons, transparentfilm is preferred to black, if weedscan be controlled efficiently in non-tropical climates. When weeds arethe prevailing problem, black-on-white should be used rather thanblack. Colored PE mulches reflectdifferent radiation patterns of redand far-red light into a crop canopy.These consequent effects on pho-tosynthetic efficiency and plantmorphogenesis may increase yields(Decoteau,1989).

Recent developments include mulchesthat selectively absorb photosynthetically ac-tive radiation (PAR) and transmit solar infra-red radiation, thus combining the herbicideeffect of black film with the thermal effect oftransparent film (CPA, 1992 ). However, theherbicide effectiveness may not be total. A PEfilm impregnated with herbicide has beendeveloped in Japan. Mulches with aluminumor silver surface colors repel some aphids andreduce the incidence of aphid-borne viruses(Lament et al., 1990). Yellow plastic film isuseful in reducing whitefly populations(Bemisia tabaci) in young tomato plants. Theflies are attracted by the yellow color of theheated plastic as it is exposed to the sun, whenthe plants are small and the crop canopy doesnot shade it (Geisenberg and Stewart, 1986).

Soil solar disinfection or polarization,using transparent plastic mulch in high solarradiation areas (Katan, 1980), has developedin certain latitudinal regions of California,Israel, Australia, and Spain. The soil tempera-ture increase has been effective in reducingthe incidence of Fusarium, Verticilium, andSclerotinia and significantly limiting the pres-ence of some weeds. Several weeks near thesummer solstice are necessary for proper po-larization. Deep thermal conduction of heatinto the soil is improved if the soil profile isproperly moistened. Solarization, combinedwith soil fumigation at low levels of fumigant,has proved effective for controlling soil-bornediseases and is environmentally friendly(Melero et al., 1989).

Photodegradable and biodegradablefilms are available commercially, but theyhave not been adopted widely because oftheir variable rates of degradation.

Plant covers. The most visible compo-

Fig. 9. A combination ofhigh and low technologymay be seen in plasticgreenhouses in Korea.

. . . .

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nents in protected cultivation are light-trans-mitting covers that protect plants from theenvironmental hazards of adverse tempera-tures, wind, hail, rain, and sun. Covers thattransmit sunlight are essential for growingplants. It is not economically feasible to growplants with artificial lights, except under spe-cial circumstances or for experimental pur-poses. Artificial lighting, however, may beused as a supplement to sunlight and toextend the photoperiod. A cover over andaround plants affects the enclosed microcli-mate as it alters the light and temperature,wind speed, air and soil humidity, and theatmospheric composition. The different typesof protection involving placing a screen overthe plants include direct covers, tunnels, andgreenhouses.

Direct covers. Also known as floatingmulches, direct covers consist of a screen ofperforated plastic or nonwoven film (CPA,1992 ), and offer simple, cheap, and effectivesemi-protection. Plants and soil are directlycovered and there is no framework. Plantsunder direct covers benefit from the green-house effect of higher daytime temperaturesand a windbreak effect of reducing evapo-transpiration. The cover protects plants and

andCaustermans,1985;Wellsand Loy, 1985 ).Theprotectionperiodwithdirect covers can beduring a small ‘part of or the entire growingcycle. Direct covers. also provide protectionagainstinsectsandbirdsand are environmen-tally friendly. They reduce the need for pesti-cide sprays and also reduce the incidence ofinsect-borne viruses. Plant uniformity is im-proved, fruit quality enhanced, the harvestseason extended, and total yields increased(CPA, 1992).

Perforated PE is preferredalong with occasional use of ethyl-ene-vinylacetate (EVA) film. Theperforation number (1 cm in diam-eter) lies between 500 and 800 persquare meter consisting of 4% to6.4% of “cover area (CPA, 1992).PE film with small fissures andplastic nets are used occasionally.Nonwovenmaterials are normallymade of PP or polyester (PET).

Low tunnels. Low tunnelsare small structures that providetemporary crop protection. Theyarealsoknown as rowcovers. Theirheight is generally 1 m or less, withnoaislesforwalking.Cultural prac-tices must be performed from theoutside. Their use enhances earlyand total yields and they offer pro-tectionagainstunfavorableclimaticvariables (CPA, 1992 ). Thermalfilms of infrared PE, EVA, copoly-. .

mer, polyvinylchloride (PVC), and conven-tional PE are being used worldwide (CPA,1992).

High tunnels. Also known as walk-intunnels, high tunnels are protective struc-tures tall enough that the crop can be ma-nipulated from within. Moderately tall cropscan be cultivated in high tunnels, althoughplanting near sidewalks is limited. Statisticsfor high tunnels are often included in thesamecategoryaslow-costplasticgreenhouses,because the cladding or covers are similar.Claddingmaterialsarepracticallythe same asthose used in low tunnels.

Greenhouses. Greenhouses differ fromother protection structures in that they aresufficiently high and large to permit a persontoconvenientlystanduprightandworkwithin(Nelson, 1985). Although there were manyearly attempts, greenhouses did not appearuntilglasswasavailableforcladding.This wasfollowed by the introduction of plastic films,which resulted in a worldwide greenhouseindustry. Greenhouses protect crops againstcold, rain, hail, and wind and provide plantswith improved environments over thosegrown in open fields. (CPA, 1992). Further-more, in greenhouses, crops can be producedout-of-seasonyear-roundandtheir yields andquality are higher than those grown in openfields. Greenhouses also have allowed theintroduction of crops normally foreign in thatregion (Germing, 1985 ).

One type seeks maximum climatic control inan environment that optimizes productivity.The other type provides minimal climaticcontrol, enabling the plants to survive andproduce an economical yield (Enoch, 1986).These two types are characterized by the

..-

Fig. 10. Plastic mulching ofsweetpotato in Nigeria.

contrasts in the northern and southern Euro-pean greenhouse industries (Castilla, 1994;Castilla and Fereres, 1990 b). In the north,optimal conditions for year-round produc-tion are provided in the glass greenhouses ofHolland, the United Kingdom, and Scan-dinavia. In the Mediterranean region, theunheated plastic houses adapt the plant tosuboptimal conditions (Castilla et al., 1992;Tognoni and Serra, 1989). The technologiesfor environmental control in the greenhousesof northern Europe, the United States, Ja-pan, and elsewhere have been characterizedby many new developments during the past 3decades. The variables of light, temperature,air and soil humidity, and CO2 content of theatmosphere are computer-programmed 24 ha day to achieve maximum crop yields, all asa part of dynamic climate control systems(Nederhoff, 1994; Seginer, 1993). Furtherrefinements in adjusting the greenhouse cli-mate to optimal crop productivity and re-source inputs can be expected. Choice of agreenhouse type depends on the cultivatedcrop, its location, and financial resources.There is a strong relationship between localconditions, greenhouse design, cladding ma-terials, and insulation needs (Germing, 1985 ).The original greenhouse function has beenchanged for some climatic zones from a green-house effect to either that of an umbrella oroasis effect. The umbrella effect is found inthe rain shelter used in humid tropical andsubtropical climate areas. The roof is coveredwith film but the sides are left open ( Garnaud,1987). The oasis effect is found in green-houses in arid tropical or subtropical regions.Light intensity and evapotranspiration arereduced and minimum temperature is in-creased (Nissen et al., 1990).

The structure of a greenhousedepends on the climate and thecladding used. There are variousroof, space, and height geometrieswith single-span materials such aswood and bamboo, which are usedfrequently in low-cost structures,particularly in China and in semi-tropical and tropical areas. Clad-dingmaterials were limited to glassuntil the middle of the 20th cen-tury. Beginning in the 1950s, plas-tic films, because of their low cost,light weight, and adaptability todifferent frame designs, generateda great increase in structures, espe-cially in the semi-tropics and inmore benign climates than those ofnorthern Europe (Nelson, 1985).

Conventional and modified(infrared, UVlong-life anticonden-sate PE) films, together with PVC,prevail worldwide. New co-ex-

truded films combine in one film the advan-tages of different layers of materials. Rigidlystructuredplasticcoveringsinsingle or doublelayers have had limited acceptance. They arecostly, but offer interesting possibilities. Fi-berglass-reinforced polyester (FRP),polymethylmethacrylate (PMMA), polycar-bonate (PC), and PVC are the usual materialsin rigid plastic panels. The diversity of green-house coverings has been reviewed recently(Giacomelli and Roberts, 1993). All cover-ings can perform well, depending on thedesired use and location. Single plastic filmprevails in warm climates; inflated doubleplasticfilmorrigid single plastic panels wouldbe adequate for cool areas. For cold areas,glass or twin-wall rigid plastic panels wouldbe best (Germing, 1985 ). A combination ofhigh and low technology may be seen inKorea (Fig. 9). Plastic covers for greenhousesin northern Europe are not acceptable be-cause of low light transmission compared toglass. Selection should also take into accountcrop requirements, cost, and how to achievethe highest financial returns (Castilla, 1994).Covering greenhouses with nets is commonin tropical areas or during hot weather intemperate zones. The nets reduce pest dam-age and the extremes of temperature and airhumidity.Theyalsoprovide a windbreak andreduce the damage from heavy rain and hail(Castilla, 1994).

Objectives and advantagesThe overall objective of protected culti-

vation is to modify the natural environmentby practices or structures to achieve optimalproductivity of crops by enhancing yields,

Fig.11. Living windbreaksfor protecting watermelonsin China.

imprving quality, extending the effectiveharvest period, and expanding productionareas. There are also specific objectives andadvantages in selected geographical areas forlimiting rainfall and hail damage and reduc-inghighsun radiation by shading.The overallintent is the most effective use of land, water,energy, mineral nutrients, and space, and theclimatic resources of sunlight, temperature,relative humidity and atmospheric CO2.

Protecting crops from drought. Wa-termanagementthroughirrigationanddrain-age systems is nearly global in practice andimpact. Water shortages for high-value horti-cultural crops prevail in some areas in almostall countries and during part of the growingseason in others. Irrigation may supplementthe natural precipitation in the eastern andMidwestern United States and in many partsof Europe or provide the entire water re-quirement in the southwestern United Statesand Egypt.

Reducing water requirements. Plas-tic soil mulches, organic residue mulches,rock, gravel, and sand mulches are used ex- tensively in areas with limited soil moisture toreduce evaporative water losses from soil sur-faces and competitive weed growth. Rockand sand mulches have been used for centu-ries in the semi-arid lands of northwesternChina. Plastic mulches are being used in suchdiverse areas as Nigeria to control weeds andimprove yields of sweetpotatoes and cassava(Fig. 10) to European Mediterranean coun-tries to reduce water requirements.

frost and freezing. Crops have been pro-tected in restricted areas through use of over-head sprinkle irrigation for strawberries andvegetables (tomatoes, peppers, melons) and

forseveralflower and ornamental crops.Wind

pots have also been used for limited protectionof fruit orchards in the United States andelsewhere.Plantandrowcoversinvolvingsoil,parchmentpaper, glass, and plastics (floatingand rigid), have been, and are still, used forfrost-susceptible crops, involving continuouslow tunnels over the crop row. Specific ex-amples are strawberries, melons, cucumbers,tomatoes, peppers, and celery.

Reducing wind damage. Windbreaksare especially important for all vine crops—melons,cucumbers,squash—andhelpfulwith

either from living crops (Fig. 11) or struc-tures, are used widely in the Mediterraneancountries, almost all of China, Japan, andKorea, and in many parts of the United Statesand Canada.

Lessening the impact of hot aridlands. Cooling is achieved by evaporationandforcedair circulation within greenhouses.It has been found especially important in thesouthwestern United States, Mexico (Fig.12), Arabian Gulf, and Mediterranean coun-triesthat produce small fruit, vegetables, andflowers. Cooling also substantially reducesthe requirement for flesh water.

Reducing damage from insects, dis-eases, nematodes, weeds, birds, and otherpredators. Within the environment of agreenhouse, fumigation of the soil and atmo-sphere is feasible. Soils may also be removedand replaced. Artificial root-growing media(bag culture, rock wool, nutrient film tech-nique, or other forms of hydroponics) havereplaced soil culture in many growing areas,including Holland, the United Kingdom, theUnited States. and most of Scandinavia, espe-

cially Denmark. Thus, many soil-borne diseases and pests have beencontrolled or eliminated. Plasticnets over fields of strawberries andother high-value crops, often un-der plastic covers and grown withsoil mulches, coupled with dripirrigation, are used in Israel (Fig.13) and the Mediterranean coun-tries of southern Europe and north-ern Mica. Soil solar disinfection(polarization)usingplasticmulch isan effective and environmentallyfriendly technique, used widely inhigh light intensity.

Extendinq production ar-eas and growing seasons. Pro-duction has been extended by us-ingglassandplastic structures,withor without heating (greenhouses,lowtunnels, and high tunnels), andsoil mulches throughout Europe,theMediterranean,NorthAmerica,

China, Korea, Japan, and manycountries in the southern hemi-sphere,specificallywatermelons innorthern Chinese provinces, toma-toes and cucumbers in Alaska, andthe local availability of tomatoes,peppers, cucumbers, and manyflower and ornamental crops in allnorthern European countries.

Increasing crop yields, im- proving quality, and conserv-ing resources. Protected cultiva-tion,in its various forms, efficientlyuses the natural resources of land,water, sunlight, energy, and atmo-

maybe increased several-fold overthoseinopen-fieldagriculture. Fur-thermore, production per unit oflandareaisstillincreasing.Twenty-one years ago (1974), yields oftomatoes grown in the glass green-house of the Westlands of Holland

Fig. 12. Plastic greenhousesin desert areas give windprotection, reduce therequirement for freshwater, and provideevaporative cooling.

was 15 kg·m-2, 10 years ago it was 40-45kg·m-2, today it is more than 60 kg·m-2, witha projected potential increase to 100 kg·m-2.Indeterminate crops (tomatoes, cucumbers,peppers) in greenhouses or high tunnels maybe grown for extended periods beyond thatof open-field agriculture in all temperatezones, or several crops maybe grown duringthe same year. Improved quality is a result ofvine-ripe harvests, and improved appearanceis a result of protection against wind, rain,hail, and damage from insects, birds, andother predators.

Controlling day and nigbt tempera-tures and photoperiod to optimize prod-uct maturity, productivity, and quality.Elevated root temperatures are important inthe production of greenhouse-grown toma-toes and cucumbers and are widely used innorthern Europe, resulting in the placementof heating units in or near the rooting media.Adjusting day and night temperatures tocontrol plant growth is revolutionizing pro-duction practices for many flower and veg-etable crops (Erwin et al., 1989). The recentconcept of DIF (the difference between dayand night temperatures) is now being used by

and may be useful for tomatoes, beans, andmelons to control plant growth, height, andproductivity (Heins and Erwin, 1990). Suchcontrol is only possible with some degree ofenclosure with protected cultivation.

Controlling atmospheric CO2 level.Greenhouses and tunnels (plastic or glass)provide unique structures for the CO2 en-richment of atmospheres for enhancing plantgrowth. The beneficial effects of elevatedlevels of atmospheric CO2 were observedmore than 100 years ago in western Europe

and in the United States. Commercial enrich-ment was first produced in the Westlands ofHolland in the early 1950s (Wittwer, 1986).Widely used in commercial protection ofwinter and spring flower and vegetable cropsand for bedding plants in the United States,its most extensive use is in Holland’s 10,316ha of glass greenhouses. Enrichment to 750–1000 ppm is practiced widely to increaseproduction and plant density.

Stabilizing the market for high-qual-ity horticultural products. Protected cul-tivation circumvents many of the hazards ofclimate and weather and greatly increasesdependability and chances for availability.The Dutch have achieved this for manyflowerand vegetable commodities, not only fordomestic use but for many foreign outlets.Gourmet melons and fruit are produced inJapan for elite local and foreign outlets. TheMediterranean area is now a dependablesource of winter and early spring vegetablesand is becoming such a source of flowers andornamental for northern Europe. In China,where consumption is high, there is a con-tinuous supply of watermelon from earlyspring to late fall, thanks to windbreaks andplastic mulches and covers. There is now adependable source of high-quality beddingplants in the United States, beginning in earlyspring and continuing through mid-summer.

Finally,controlled-environment agricu-lture, in contrast to open fields, offers a con-tinuing technological challenge to the inno-vative home gardener. Millions of amateurand some professional gardeners use minia-ture windbreaks, plant protectors, plasticmulches (floating and rigid), and tunnels.These are often coupled with drip irrigation

15

Fig. 13. Strawberries grownin Israel under plastic netsfor bird protectioncombined with plasticcovers and soil mulches.

systems,small greenhouses, and sometimes adegree of hydroponics to hasten maturity,increase yields, and extend the growing sea-sonsforhigh-quality home-grown fruit, flow-ers, and vegetables. This is especially typicalofgardens in Europe, Korea, China, and partsof the United States (Fig. 14).

Present status and outlook—major areas of production

Historically,using windbreaks predatedby many centuries (Table 1) those of protec-tive structures such as tunnels and green-houses. There is no accurate estimate of areasinvolved, and no distinction is made betweeneither the natural or artificial windbreaks orsoil mulches. Other crops than those pro-tected are found particularly in China, wheregrain crops, before harvest, are often used toshelteryoungwatermelonplants.Plasticwind-breaks are rapidly replacing wood or bamboocanes in the Mediterranean countries to shel-ter tender vegetable and flower crops andeven entire ranges of plastic greenhouses ortunnels. The most extensive use of wind-breaks,artificialandnatural,isfoundin China,Russia, and Ukraine. Windbreaks are simi-larly used for large volumes of high-valuecrops in all southern countries of Europe andnorthern countries. of Mica bordering theMediterranean and Canary Islands. Duringthe past 2 decades, a tremendous transitionand expansion in the use ofplastic soil mulcheshas occurred, especially in some of the Medi-terranean countries and in China, Japan, andKorea. The world’s most extensive uses ofplastic soil mulches (not listed in Table 2) arenow in China for cotton, peanuts, rice seed-beds, and corn.

The generalized use ofphotodegradablemulch film is limited to the corn crop inFrance. Mulching is increasingly being com-binedwithdrip irrigation. Clear or black filmsare preferred. Greenhouses, glass and plastic,and plastic tunnels high and low, constitutethe structures most commonly associatedwithprotected cultivation of crops. They are highlyvisible,haveadegree of permanence, and posea continuing attraction and enchantment bythe enclosed environments created, the cropsgrown, and the facilities for automated irriga-tion, nutrient feeding, and root media andplant support systems. Worldwide, there aretwo vast areas of greenhouses-one of glass in the Westlands of Holland near the village ofNaaldwijk between Rotterdam and DenHague, the other ofplastic along the seacoastvalleys near Almeria, on the MediterraneanSea in southern Spain (see Sidebar). Otherprominent expanses of glass or plastic or bothcan be seen in China, Japan, Italy (Sicily),Greece (Crete), Essex County, Ontario, westof Cleveland, San Diego County, and the Isleof Guernsey, U.K. Table 2 summarizes theglobal distribution ofhorticuhural crops mostcommonly grown under protective struc-tures. The principal greenhouse-grown veg-etable crops produced world wide tradition-ally have been, and still remain, tomato andcucumber. Cucumber is by far the most im-portant greenhouse crop in Russia.

Cucumbers,muskmelons,watermelons,and squash within, the past 2 decades, havecome into prominence in many countries,particularly in China, Japan, Italy, Spain,Greece,Turkey,Tunisia, Morocco, and Egypt,with the expansion ofplastic greenhouses andtunnels (Wittwer, 1993). Further expansionsin glass greenhouses during the past decade

have been only in northern Eu-rope. Tomato, with its many culti-vars adapted to greenhouse cul-ture, remains the most importantof the solanaceous crops producedin the greenhouses of Holland, theCanary islands, and Morocco. Thered bell sweet pepper is running aclose second. This is particularlytrue in Holland, which is still theworld’s leader in glasshouse veg-etablesandflower production, pro-viding a current annual return ofabout $3 billion. Peppers requirelessspaceinheight and less trainingand pruning of the stems than to-matoes. They produce abundantlyover an equally long season. Har-vests can be spread further apart,thus requiring less labor. Finally,the bright red product has an ex-pandingdomesticandforeign mar-ket. During the 1994 season in

Fig. 14. Protected cultiva-tion in a home garden inSwitzerland.

Holland, the value of the pepper crop antici-. . .pated to equal that of the tomato (Nederhoff,1994).

Of all the fruit crops, the strawberry ispreeminent for greenhouse and tunnel cul-ture. It is widely grown in greenhouses and inhigh and low tunnels in Japan, China, and inalmost all Mediterranean countries, includ-ing Israel. It can be seen under protectivecovers in most all small gardens of northernand central Europe. Grape production, evi-dent 25 years ago in hundreds of small green-houses north of Brussels in Belgium, largelyhas disappeared. Significant areas withingreenhouses and rain shelters devoted tograpes and tree fruit (banana, citrus, pear,peach, plum, pineapple) are still found inJapan, Italy, Morocco, Spain (Canary Is-lands), and Portugal (Azores Islands). Saladcrops, principally lettuce of various types, aregrown in the greenhouses of northern Eu-rope and Japan and in the eastern Europeancountries of Russia, Poland, and Hungry.There is still a remnant of lettuce productionin the greenhouses in the United States.Greenhouse culture of Chinese and ordinarycabbage may be seen in China, Japan, andKorea. Very minor quantities of celery, rad-ish, and asparagus are found in the green-houses of Germany, France, and Belgium.Some green beans are produced in the green-houses of countries bordering the Mediterra-nean Sea and in China.

Production of flowers, bedding plants,potted plants, and ornamental constitute thelargest and most recent takeover in green-

house and tun-nel culture. Forthe UnitedStates, 95% ofgreenhouse cul-ture is now de-voted to pro-ducing beddingplants, pottedplants, cut flow-ers, foliageplants, and or-namental (Fig.15). Of the to-tal productiono f beddingplants, only 10%to 20% is veg-etables, the bal-ance being flow-ers. A similar,but less striking,transition is oc-curring in sou-thern countiesof the provinceof Ontario. The

transition from vegetables to flower crops inthe greenhouses of Holland, while formi-dable, is less dramatic than in the UnitedStates. In 1960, greenhouse vegetable pro-duction in Holland was about 90%, but nowit has declined to about 40%, with the remain-ing percentages 45% for cut flowers and 15%for potted plants. Similar trends have oc-curred in Denmark and the United King-dom, including the isle of Guernsey, and isnow evident in the plastic greenhouses of theMediterranean area and in China, Japan, andKorea.

Table 3 assesses the global status ofprotectedcultivation.Whilenodata are given,the most ancient widely used and those ofgreatest impact are windbreaks and irriga-tion. While millions of hectares of cropland,perhaps hundreds of million, have a degree ofnatural or artificial protection, the exact fig-ures are not available. Their ancient andpresent use in China and the countries of theMiddle East are recognized. Coupled closelywith the rise of structures and materials forprotective coverings of high-value crops hasbeen the rise of drip or trickle irrigation.Although still only a small component of theapproximate 250 million ha of the cultivatedcropland of the earth, drip or trickle irrigationhas become an integral part of most green-house and tunnel cultures as well as that ofplastic mulches for high-value crops. The cur-

from 1.5 to 2.0 million ha. This system ofirrigation originated during 1950s in En-gland, and was introduced in Denmark as a

17

Table 1. Global assessment of protectpurpose).

Windbreaks (trees, canes, bamVegetables (tomato, strawb

China, Japan, Korea,TaBelgium, Tunisia, Egyp

Mechanical protectiFruit orchards (citrus, grap

China, Japan, Korea, TaiIsrael, United States, C

Mechanical protectionSoil mulches

Artificial (PE or other plaStrawberry

China,Japan,UnitedStaC a n a d a

Weed control, eaNatural (straw, sand, grav

TomatoUnited States, China

Weed control, eaMelon

China, Japan, TaiwanWeed control, ear

Artificial or naturalWatermelon

China, Spain, GreeceWeed control, ea

Cucumber -

China, Japan, KoreaWeed control, ea

SquashChina, Japan, Taiwa

Weed control, earPepper

Italy, Morocco, SpWeed control, ear

AsparagusSpain, Greece, Belgi

Weed control, eaBanana

Morocco, Spain (CaWeed control, ea

SweetpotatoNigeria

Weed control, ea

patentedspaghetti system for greenhouse cropculture in 1959. It was further developed foropen field applications by Israel during 1960sand began to have worldwide distribution by1970, with about 50,000 ha in 1974, ex-

panded to about 400,000 ha by 1980(Wittwer, 1981).

Second only to the use of windbreaksand irrigation is that of soil mulching, specifi-cally the increased use of plastics as a soilmulch. Globally, the most significant devel-opment has been in China, where 1.4 millionha of cotton is mulched annually. Second inimportance is peanuts, and more than 75% ofthe watermelons produced receive a plasticmulch plus wind protection wherever pos-

ed cultivation using windbreaks and soil mulches and main horticul

boo, crop residues, PE, PP, or other plastics)erry, all vine crops, cucumber, melon, squash)iwan, Israel, Portugal, Spain, Chile, Argentina, Rus

t, Jordan, United States, Canadaon, reduced evapotranspiration, increased yieldes, pear, kiwi)

wan, Spain, Portugal, Italy, Argentina, Russia, Ukrainanada, reduced evapotranspiration, increased yield

stics)

tes,Taiwan,Spain,Italy,France,Portugal,Israel, Greece,

rliness, water savings, increased yield andqualityel, living)

, Japan, Taiwan, Italy, Spain, Korea, Morocco, Argerliness, water savings, increased yield and quality

, United States, Spain, Italy, Greece, France, Israel, Arliness, water savings, increased yield and quality

, Japan, Korea, Taiwan, Portugal, Italy, Israel, Nigeriarliness, water savings, increased yield and quaky

, Taiwan, Egypt, Morocco, Poland, United States, Carliness, water savings, increased yield and quaky

n, Greece, Italy, Algeria, Jordan, United States, Canadliness, water savings, increased yield and quality

ainliness, water savings, increased yield and quality

um, Germany, Holland, China, Taiwanrliness, water savings, increased yield and quaky

nary Islands)rliness, water savings, increased yield and quaky

rliness, water savings, increased yield and quality

sible. An explosive area for plastic mulchingof many high-value crops is the Mediterra-nean region, particularly Spain, Italy, andFrance.

Floating rowcovers, requiring no wiresupport and consisting of spunbonded PP,have gained favor for plant protection, par-ticularly in France and in northern Europe.

Low tunnels or wire-supported row cov-ershave emerged during the past 2 decades ashighly significant for the protection of rowcrops in China, Japan, and Korea. They canbe seen for plant protection throughout theMediterranean countries during winter andspring, as seas of plastic cover squash, musk-melons, and watermelons. Limited areas are

tural crops involved (structure, crop, predominant country, and

sia, Ukraine, Morocco, France, Greece, Algeria,

e, Chile, France, Greece, Tunisia, Jordan, Algeria,

Belgium, Argentina, Egypt, Turkey, Poland,

ntina,Egypt, Jordan,Israel, Canada

gentina, Egypt, Portugal, Hungary, Jordan, Canada

, United States, Canada

nada

a

Fig. 15. Bedding plant product

will increase in importance in providing astable, dependable, source of high-qualityfood products in a market that is becomingincreasingly competitive.

One can view the changing patterns ofprotected cultivation worldwide by severaldimensions. There has been, since ancienttimes, a continuing expansion in areas cov-ered, going from a few small holdings 2centuries ago in western Europe, the FarEast, and North America, to commercialgreenhouse food production. Several thou-sand acres are now under glass in the UnitedStates and equally large areas in England andHolland, where horticulture under glass waspracticed over a century ago, with the pro-duction and marketing of tomatoes, grapes,cucumbers, and salad crops. By 1980, therewere an estimated 150,000 ha of green-houses (glass, plastic, fiberglass) in use glo-bally for the production of high-value foodcrops (Wittwer 1981), and an almost equalarea for flowers, potted plants, bedding plants,and other ornamental. Today, if one consid-ers the vast expanses that have occurred inplasticulture during the past decade in China,Japan, Korea, and in all countries borderingthe Mediterranean Sea and those in the south-ern hemisphere, the area comprising plantsgrown under protective covers (greenhousesand tunnels) exceeds 500,000 ha.

The changing pattern of protected culti-vation is reflected in the crops produced. Themost remarkable pattern is that of food cropsconsisting of vegetables and fruit, and on toflowers, potted plants, bedding plants, andother ornamental. All producing areas haveseen an impact, with the most dramatic oc-curring in the past 2 decades in the UnitedStates, followed by Holland, the United King-dom, and Denmark. That transition has beenin progress since 1970 in Israel, the Isle ofGuernsey, and in Japan. It is now occurring inFrance, Italy, Spain, and Greece, althoughthe market demand can not allow a quick

ion in the Frank Smith greenhouses, Carleton, Mich.

transition. Only recently have flowers andornamental begun to replace greenhouse-grown vegetable crops in China. No longer islettuce an important greenhouse-grown cropin the United States. Its value has diminishedin northern European countries, particularlyHolland and the United Kingdom. There isonly a remnant left of rhubarb forcing and thecovering of celery plant rows with parchmentpaper in Michigan. A once-thriving industryof grape production in the glasshouses ofBelgium is essentially nonexistent. Only Withinthe last decade has the sweet pepper emergedas a leading greenhouse-grown vegetable.Within the past 2 decades, major food andfiber crops have been subjected to the advan-tages of plasticulture. The world’s most ex-tensive use of plastic for protected cultivationis for soil mulching. In China, almost 2million ha is devoted to cotton, peanuts, andcovered rice seedbeds. Plastic soil mulcheshave also been used extensively for promot-ing earlier maturity and improved yields ofmaize in China and France. Plastic soil mulcheshave also proved useful for sweetpotato, pine-apple, and cassava production in the tropics.

While production of crops in green-houses and tunnels may be the most intensivein the valleys and hillsides of countries bor-dering the Mediterranean Sea and adjacent tothe major cities in China, the United Statesand Russia, protective production is nowspread over widely dispersed areas. Nowhereis this more evident than in the home gardensand commercial establishments of northernEurope. Greenhouse culture is now found inall the provinces, autonomous regions, andmunicipalities of China, in every state in theUnited States, almost all the provinces ofCanada, and throughout Japan and Korea. Itmay be seen in such distance places as Kenya,Nigeria, Alaska, and all the major countries ofthe southern hemisphere (South Africa, Ar-gentine, Chile, Colombia, Ecuador, Austra-lia, and New Zealand).

Table 3. Global distribution of protected cultivation using biological and other structures or barriers.

Geographical areaStructure Asia Mediterranean North/South America Northern Europe Total

Plastic soil mulches 3,080,000’ 191,000 85,000’ 15,000 3,371,000

Direct cover (floating types) 5,500 10,300 1,500’ 27 ,000 44 ,300

Low tunnels (rowcovers) 143,400 90 ,500 2 0 , 0 0 0 3,300 257 ,200

High tunnels 27, 6 0 0 x 27,600

Plastic greenhouses 138,200 67 ,700 15,600 16,700 238,200

Glasshouses 3,000 7,900 4,000 25 ,800 40 ,700

Windbreaks Millions of hectares, globally. Exact figures not available. China andMediterranean countries predominate.

zIncludes cotton, rice, and peanuts in China; corn in France; and rice seed beds in the Orient.yCrude estimate only.xHigh tunnels are often computed together with plastic greenhouses in countries other than Mediterranean. France is included in theMediterranean group.

A look to the futureThe inability of people to predict the

weather and control the climate persists. Thisis accompanied by a rising population withincreasing numbers of affluent and demand-ing people who insist on improved diets.Coupled with the above are limitations ofland, water, energy, mineral nutrients, andthe need for improved management of allresources. Thus, we will likely see a continu-ing increase in the extent and dispersion ofprotected cultivation throughout the world.Only within the past 2 decades have green-houses, rowcovers, and plastic soil mulchesattracted the attention of agriculturally devel-oping countries in Asia, South America, andAfrica. In the United States and Europe,protected cultivation commands the atten-tion and interest of the professional and ama-teur gardener. For horticulturists, foresters,crop, and other scientists, it provides a site fortesting and verification of new technologiesfor open field culture. The explosive use ofplastic soil mulches in China for major fieldcrops, including cotton, peanuts, and cornonly within the past decade, is indicative of anexpansion beyond the use only for high-valuecrops.

There will be new developments in theuse of plastic and glass as covers for green-houses. The Dutch are successfully develop-ing new glass structures for improved trans-mission of sunlight. Innovations in the use ofplastic for nonheated structures in the tropicsand semi-tropics will appear. Biodegradableand light-degradable plastics will make theiruse more environmentally compatible.

The development of intelligent films willopen new perspectives to protected cultiva-tion (Garnaud, 1994). There will be filmsthat limit the transmission of infrared solarradiation (700–900 mm), thereby reducingthe greenhouse effect in tropical areas, wherelimiting the air temperature may be desirable,or films that reconvert the radiation from thephotosynthetically inactive wave lengths of

the solar energy spectrum into PAR to im-prove crop productivity.

New labor-saving technologies, evenbeyond those now in the plant-growing andbedding plant industries in the United States,will become commonplace and even morespecialized. The thermomorphogenic andphotoperiodic responses of crops grown ingreenhouses and other protective structures,and the interaction of these responses toother environmental variables (sunlight, hu-midity, mineral nutrient status, elevated lev-els of atmospheric CO2), are as yet littleknown (Bakker, 1991; Erwin et al., 1993).

Unique developments will appear in thehandling of conventional and innovative com-modities. New high-value crops may replacethe conventional, as is now occurring, withred sweet peppers now replacing tomatoes.Alternatively, tomatoes may find new life inthe extended shelf life of Calgene’s “FlavorSaver” and the Pioneer Vegetable GeneticsSuper Life (Derrer et al., 1994; Oeller et al.,1991).

There will be a continuing expansion inflower and ornamental crop production tomeet the needs and desires of people forhome and community beautification. Theexamples set by the Dutch in maximizingproduction of vegetable crops and those inthe United States in the regulation of day andnight temperatures and in mechanization ofseeding and transplanting of seedlings,coupled with the other components of thegreenhouse environment for bedding plantsand flower crops, will keep protected cultiva-tion a viable option to open field agriculture.

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