VEGETABLE PRODUCTION USING PLASTICULTURE

William James Lamont, Jr. Department of Horticulture 114 Tyson Building The Pennsylvania State University University Park, Pennsylvania 16802-4200 USA

ABSTRACT

Plasticulture is a system of growing crops in a way so that a significant benefit is de- rived from using polymers. The discovery and development of polymer in the late 1930s, and its subsequent introduction in the early 1950s in the form of plastic films, mulches, and drip- tubing and tape, revolutionized the commercial production of several vegetable crops and gave rise to plasticulture. The later discovery of other polymers such as , polyproplene, and , and their use in pipes, fertigation equipment, fil- ters, fittings and connectors, and row covers further extended the use of plastic components in this production system. The plasticulture system consists of plastic and non-plastic components: Plastic mulches, , fertigation/chemigation, fumigation and solarization, windbreaks, stand establishment technology, seasonal-extending technology, pest management, cropping strate- gies, postharvest handling and marketing.

INTRODUCTION • Better management of certain insect pests; To be competitive in today’s marketplace, • Fewer weed problems; vegetable growers must strive continually for high • Reduced soil compaction and elimination quality, superior yields, and extended production of root pruning; and cycles that include spring and autumn crops. • Opportunity to double- or triple-crop Plasticulture is a management tool that enables veg- with maximum efficiency. etable producers to realize greater returns per unit of To realize these benefits, the grower needs land. to integrate the various components of a plasticulture Such a system may offer many benefits: system. The components include plastic mulches, • Earlier crop production (7 to 21 days drip irrigation, chemigation/fertigation and soil fu- earlier); migation or solarization (which may or may not be • Higher yields per hectare (two to three needed depending on the location). Other compo- times higher); nents are wind-breaks, stand establishment technol- • Cleaner and higher quality produce; ogy, season-extending technology, pest manage- • More efficient use of water resources; ment, cropping strategies, and marketing. The • Reduced leaching of fertilizers, especially plasticulture system can be used effectively by grow- on light, sandy soils; ers with a small or large land holding. The basic • More efficient use of fertilizer inputs principles and intensive management required to through fertigation technology; operate a plasticulture system successfully are simi- • Reduced soil and wind erosion; lar, regardless of the size of the operation. A • Potential decrease in the incidence of component list for a ten hectare /drip disease; irrigation system is presented in Table 1. The crops

Keywords: drip irrigation, fertigatiion, plastic mulches, season extension technology

1 that have shown significant increases in earliness, the early university research before 1960 was con- yield, and/or fruit quality with the use of plasticulture ducted on the impact of color (black or clear plastic include muskmelon, tomato, pepper, cucumber, film) on soil and air temperature, moisture retention squash, eggplant, watermelon and okra. Other crops and vegetable yields (Emmert 1957). Most plastic such as sweet corn, snap bean, pumpkin, decorative mulches used in the United States are made of linear gourds, crucifer crops and herbs have shown similar low- or high-density polyethylene. They are .012 to responses and may lend themselves to double- or .031 mm thick, 122 to 152 cm wide, and on rolls 607 triple-cropping strategies. to 1463 m long, depending on the thickness of the mulch. Linear high-density polyethylene is used to PLASTIC MULCHES reduce weight and cost, and is stronger than the same thickness of low-density polyethylene. The plastic Plastic mulches have been used commer- mulch is either slick (smooth), or embossed with a cially on vegetables since the early 1960s. Much of diamond-shaped pattern. This pattern helps reduce

Table 1. Components for a nine­hectare plasticulture vegetable production system

2 expansion and contraction, which results in the loos- surface of clear plastic mulch is usually covered with ening of the mulch from the raised bed. The raised condensed water droplets. This water is transparent bed is generally 10 - 15 cm high and 75 cm wide, and to incoming shortwave radiation but is opaque to has a slope of 3 cm from the center to the edge. Soil outgoing longwave infrared radiation, so much of under a raised bed will warm up more quickly in the the heat lost to the atmosphere from a bare soil by spring, and will also shed excess water from the infrared radiation is retained by clear plastic mulch. middle of the bed, thus keeping the crop plants drier Thus, daytime soil temperatures under clear plastic and preventing deterioration in the quality of the mulch are generally 4-8°C higher at a 5 cm depth, and product. 3-5°C higher at a 10 cm depth, compared to bare soil. Plastic mulches directly affect the microcli- Clear plastic mulches are generally used in the cooler mate around the plant by modifying the radiation regions of the United States such as the New En- budget (absorbitivity vs. reflectivity) of the surface gland states. Using clear plastic mulch will require and decreasing the soil water loss (Liakatas et al. the use of a herbicide, soil fumigant or solarization to 1986, Tanner 1974). The color of a mulch largely control weeds. determines its energy-radiating behavior and its in- White, coextruded white-on-black or silver fluence on the microclimate around a vegetable reflecting mulches can result in a slight decrease in plant. Color affects the surface temperature of the soil temperature (-1°C at 2 cm depth, or -0.4°C at a mulch and the underlying soil temperature. Ham and 10 cm depth, compared to bare soil), because they Kluitenberg (1994) found that the degree of contact reflect back into the plant canopy most of the incom- between the mulch and soil, often quantified as a ing solar radiation (Ham et al. 1993). These mulches thermal contact resistance, can affect greatly the can be used to establish a crop when soil tempera- performance of a mulch. If an air space is created tures are high and any reduction in soil temperatures between the plastic mulch and the soil by a rough soil is beneficial. Depending on the degree of opacity of surface, soil warming can be less effective than a white mulch, the use of a fumigant or herbicide may expected. be needed to control weeds. The soil temperature under a plastic mulch Another family of mulches includes the depends on the thermal properties (reflectivity, wave length-selective or photoselective mulches, absorbitivity or transmittancy) of a particular mate- which selectively transmit radiation in some regions rial in relation to incoming solar radiation (Schales of the electromagnetic spectrum but not in the pho- and Sheldrake 1963). Black plastic mulch, the tosynthetic wavelength (Loy et al. 1989). These predominate color used in vegetable production, is mulches absorb protosynthetically active radiation an opaque blackbody absorber and radiator. Black (PAR) and transmit solar infrared radiation, provid- mulch absorbs most ultraviolet (UV) visible and ing a compromise between black and clear mulches. infrared wavelengths of incoming solar radiation, These infrared-transmitting (IRT) mulches give the and re-radiates absorbed energy in the form of ther- weed control benefits of black mulch, but are inter- mal radiation or long-wavelength infrared radiation. mediate between black and clear mulch in terms of Much of the solar energy absorbed by black plastic increasing the soil temperature. The color of these mulch is lost to the atmosphere through radiation mulches can be blue-green or brown. These mulches and forced convection. The efficiency with which warm the soil in the same way as clear mulch, but black mulch increases soil temperature can be im- without the accompanying weed problems. proved by optimizing conditions for transferring Additional colors that are currently being heat from the mulch to the soil. Because thermal investigated are red, blue, yellow, gray, and orange. conductivity of the soil is high relative to that of air, All of these have distinct optical characteristics and much of the energy absorbed by black plastic can be thus reflect different radiation patterns into the canopy transferred to the soil by conduction, if contact is of a crop, thereby affecting plant growth and devel- good between the plastic mulch and the soil surface. opment (Decoteau et al. 1989, Orzolek and Murphy Soil temperatures under black plastic mulch during 1993). In a study by Loy et al. (1998) differences in the daytime are generally 2.8°C higher at a 5 cm reflectivity between a red, black, and red on black depth, and 1.7°C higher at a 10 cm depth, compared mulch were minimal at 40 cm above the mulch to those of bare soil. surface on the shaded side of the row. Loy et al. In contrast, clear plastic mulch absorbs suggest that for red mulch reflectivity to have a more little solar radiation but transmits 85-95%, the extent consistent effect on biomass accumulation and yield of transmission depending on the thickness and in tomato, the rows may need to be oriented in a degree of opacity of the polyethylene. The under north-south direction. 3 Light reflectivity may affect, not only crop mulch. growth but also insect response to the plants grown on the mulch. Examples are yellow, red, and blue DRIP IRRIGATION mulches, all of which increased green peach aphid populations (Orzolek and Murphy 1993). The yel- Drip irrigation is an important part of a low color was especially altractive to pests, and drew plasticulture production system. It should be used increased numbers of striped and spotted cucumber with plastic mulch for the greatest benefits. Drip beetles. Yellow has long been used in irrigation can save as much as 80% of the water used to monitor insects. Mulches with a printed silver by other irrigation methods (Bogle and Hartz 1986). surface, or shiny silver coextruded mulches, have You also can double- or triple-crop by fertilizing been shown to repel certain aphid species and reduce succeeding crops through the drip irrigation tape or or delay the incidence of aphid-borne viruses in tubing, using a fertilizer proportioner. This allows summer squash (Lamont et al. 1990). Like a white more production for the same investment in plastic mulch, the gray mulches may transmit enough solar mulch and drip irrigation equipment. The major radiation to require a herbicide or fumigant for components of a drip irrigation system are preventing weed growth. Some of these colored • Drip tubes or drip tapes; mulches (eg. the blue and the red) have a dramatic • Filters—media, screen or disc; impact on soil temperatures. In one field trial, they • Pressure regulators—spring or valve; raised soil temperatures to 75°C and 76°C, respec- • Valves—hand-operated, hydraulic, or tively, at a depth of 2 inches when the ambient air electric; temperature was 40°C (Lamont, unpublished data). • Controllers, which may range from A lot of research still needs to be done on the effect simple time clocks to complex computer which different colors have on the microclimate, controlled units that run many zones; crop growth and yields. and, Photodegradable plastic mulch is an alter- • Injectors, which introduce chemicals and native to the conventional plastic mulches with all fertilizers into the irrigation system. their retrieval and disposal problems (Ennis 1987). Because vegetables are planted in rows, a Although photodegradable plastic looks very much drip tube or tape is used to wet a continuous strip like other plastic mulches when it is installed, it can along the row. Drip tape is generally 8 mm thick, and be broken down by ultraviolet sunlight. The actual is used for one year and then discarded. Drip tube is rate of breakdown depends on several factors, in- heavier, 20 mm thick, and used repeatedly for several cluding temperature, the extent to which the plastic years. The outlet holes are spaced from 20 to 60 cm is shaded by the crop, and the amount of sunlight apart, although 30 cm is the most common spacing received during the growing season. When using for vegetable crops. photodegradable plastic mulch, it is important to The source of the water supply for drip keep in mind that decomposition of the buried edges irrigation is extremely important. It includes wells, (commonly referred to as the "tuck") is initiated by ponds, lakes or municipal water systems. Well- lifting them out of the soil and exposing them to water is generally fairly clean, and may require only sunlight. a simple screen or disc filter to remove particles. It Research has also been conducted on a is very important to determine if precipitates or other photodegradable mulch overlay system, in which the contaminants in the water could cause a plugging top layer of black photodegradable mulch degrades problem. Water analysis is essential before installing and increases the exposure of a white nondegradable a drip system. Municipal sources generally provide layer (Graham et al. 1995). This particular change documentation of water quality, which makes it would lower the soil temperature later in the growing easier to spot potential problems. season. The potential use for this would be in a Surface water such as streams, ponds, pits, double-cropping system, in which the same mulch is or rivers contain bacteria, algae, or other aquatic life. used for spring and fall crops (e.g. bell peppers Consequently, the use of agricultural sand media planted in the spring followed by squash in late filters with surface water is necessary. These filters summer). The concept could be pursued further with are generally more expensive than screen or disc several color changes during the season. The color filters. Assistance from an irrigation dealer or exten- changes would be accomplished by having more than sion agent familiar with drip irrigation system design one coextruded layer of differently pigmented pho- and installation is strongly recommended, and can be todegradable plastic on top of the nondegradable

4 very helpful in avoiding problems later. Other major exposed to sunlight during the hot summer months considerations are crop water management. This (Stapleton 1991). depends on soil type and the stage of crop growth. WINDBREAKS FERTIGATION The use of windbreaks, whether permanent Once a drip irrigation system has been (trees) or annual (grain crops), is an important part installed, it makes economic and environmental sense of the plasticulture system of production, but is often to fertilize the crop via irrigation water. If done overlooked. In the United States, windbreaks con- properly, this results in more efficient use of fertiliz- sisting of strips of winter wheat, rye, or barley are ers, and probably reduces fertilizer contamination of often planted to protect young vegetable seedlings groundwater. More nutrients are taken up by the from prevailing winds. A combination of permanent crop, and fewer leach down below the plant root and annual windbreaks can modify wind profiles and zone (Hochmuth 1992). influence temperatures and other microclimate fea- In its broadest sense, fertigation means tures (Hodges et al. 1994). Windbreaks may also feeding a crop by injecting soluble fertilizers into serve as a habitat for both beneficial and pest insects water in the irrigation system. There are a variety of (Dix and Leatherman 1988). For maximum effec- ways to introduce chemicals into a drip system, tiveness, the grain strips should be planted in the fall. based on different kinds of pump — small electric Each strip of grain crop should be 3 to 3.5 m wide powered pumps or those powered by irrigation (the width of a sowing drill). Enough room should water, venturies, pressure differential tanks, bladder be left between the strips for five or six mulched beds tanks, and gravity. Each drip system may utilize a of vegetables, each around 2 m wide. Topdressing different method or a combination of methods. If the strips in the spring helps to ensure a dense stand fertigation is to be successful, irrigation scheduling of grain. must be coordinated closely with the nutrient needs Another option is to plant a solid grain of the crop (Clark et al. 1991). To be a good cover crop in the fall. It is important to till the crop fertigator, a grower first needs to be a good area early in spring, so that residues from the cover irrigator. crop will not interfere with application of the plastic mulch, drip irrigation and fumigation (if required). STRIP FUMIGATION AND Once wind protection is no longer required, the grain SOIL SOLARIZATION strips are mowed, and used as drive rows for spray- ing for insect and disease control, and later for In many production areas of the United harvesting. States, especially California and the Southeast, it is necessary to sanitize the soil in which or STAND ESTABLISHMENT TECHNOLOGY mulched crops are grown. Plastic mulches are used with chemical fumigants, or as covers during soil Crop establishment in a vegetable solarization. In row or strip application of a fumi- plasticulture production system involves either trans- gant, the amount of material actually applied per planting or direct seeding. Well-grow vegetable hectare will depend on the row width, and will be a transplants in suitable containers are an integral part percentage of the broadcast rate. The temperature of this production system. For early harvests of of the soil should be at least 10°C, and soil should be pepper and tomato, large seedlings in large indi- well worked, free from undecomposed plant debris, vidual containers (cells) of 9-10 cm across are rec- and have adequate moisture for seed germination. If ommended. For other vegetable crops, a cell size the weather and soil are warm, the fumigant should around 5 cm in diameter is a good general recom- escape through the plastic mulch in 12-14 days. mendation. Transplants can be set by hand or Fumigation is used primarily for nematode control, machine. The following vegetables have been trans- but a multipurpose fumigant can also provide good planted successfully: Tomato, pepper, eggplant, control of soil-borne diseases (Scoville and Leaman watermelon, muskmelon, honeydew, summer squash, 1965). Soil solarization is another way to control cucumber, onion, and okra. In specialty or niche soil pests. Solarization describes a hydrothermal marketing situations, other crops such as sweet corn, method of cleansing soil that occurs in moist soil herbs, leaf lettuce, and cole crops can also be trans- which is covered by mulch film (usually clear) and planted. Mechanical seeders are available in single

5 or multirow models that will plant directly through ered a fallow area. The use of low-pressure sprayers, the plastic mulch. This equipment is good for direct- coupled with shielded application of herbicides, is seeded crops of sweet corn and cucumber, as well as recommended for spaces between vegetable beds. other crops. Some spacing recommendations are This approach will protect the mulched crop from shown in Table 2. herbicide, so it will not be damaged by any concen- tration of herbicide in the planting hole (Bonnano TECHNOLOGY FOR EXTENDING THE 1996). HARVEST TECHNOLOGY Reflective plastic mulches such as silver mulch have been shown to interfere with the move- Row covers, high tunnels and low tunnels ment of aphids. Aphids are common vectors of virus can make possible earlier crops of vegetables by diseases on various vegetable crops (e.g., water- creating a mini-greenhouse effect. The first row melon mosaic virus II, which causes green streaks in covers used were solid polyethylene sheeting that summer squash (Lamont et al. 1990) and mottling needed support and required venting during the day and green streaks in yellow squash, melons and (Hall and Besemer 1972). pumpkins). To eliminate the need for venting by hand, a variety of materials have been developed, including CROPPING STRATEGIES slitted polyethylene covers that require wire hoops; floating nonwoven sheets; a white point-bonded, Double- or Triple-Cropping material; a spunbonded fab- ric; and a polyethylene sheet with tiny pores (Wells Double- or triple-cropping is another im- and Loy 1985). portant component of the vegetable crop plasticulture Row covers also help keep out insect pests system. One cropping scheme that has been investi- (Natwick and Durazo 1985). gated in the United States is broccoli or cabbage High tunnels are another option for pro- followed by yellow summer squash, which is then ducing vegetables in a plasticulture system. They followed by broccoli, cabbage or Chinese cabbage can be used to extend the spring and fall growing (Marr and Lamont 1992). Another cropping scheme seasons. High tunnels are covered with a single layer is followed by muskmelon (Lamont and of polyethylene film (Wells 1991). The use of high Poling 1986). tunnels is widespread in many parts of the world, In the event that the first crop is a failure, especially in Asia and the countries of and double- or triple-cropping is a way of recovering Italy. investment into plastic mulch, drip tape or tubing, and fertilizer. Fertigating through a drip system PEST MANAGEMENT makes it relatively easy to supply sufficient nutrients for a second or third crop. Plasticulture system must have a good inte- grated program for insect, disease, and weed con- MARKETING trol. To obtain good insect and disease control, it is important to use a sprayer that generates sufficient Although plasticulture makes possible pressure so that pesticide sprays penetrate to, and higher yields of vegetables, beginning earlier in the cover, the whole crop. This means using sprayers season, growers should organize their marketing with pumps capable of generating over 200 psi, with strategies and outlets before planting any crop. A appropriately sized nozzles. It is important to use large amount of an early crop could present a mar- chemical sprays efficiently, to control the targeted keting problem if there is not good advance planning pest without damaging the environment. The use of of where to market it. an integrated pest management (IPM) approach, Plasticulture production may be an asset in which combines the use of disease-resistant varieties marketing. For example, muskmelons grown in with chemical and biological control practices, crop Texas are marketed in advertisements and on indi- rotation, and effective monitoring, is recommended. vidual melon boxes: “These melons grown using Only approved herbicides for the vegetable being plastic mulch and drip irrigation”. Growers feel that grown should be used between rows of mulched this advertisement gives buyers an impression of vegetable beds, because this should not be consid- product quality and environmental awareness.

6 Table 2. Plant spacing for plasticulture

DISPOSAL OF THE USED them difficult to recycle. One option is to incinerate FROM PLASTICULTURE them to recover their very substantial fuel value. A pound of plastic has as many BTU’s (thermal units) The main question asked by growers who as an equivalent amount of fuel oil. Used plastics are interested in using plastics for the production of have been burned in waste-to-energy plants, but one vegetables is: “What do I do with the plastics when problem is that they create “hot spots” in the waste I have finished with them?" This is certainly a stream. Another incineration option being explored worldwide problem. There have been many attempts by a team of researchers at Pennsylvania State Uni- to solve if, including the development and use of versity is to create a “fuel nugget” that can be burned photodegradable or biodegradable materials (which to supplement coal or other waste products, or used just disappear), using the plastics several times (de- alone to heat various kinds of structures. More laying the problem), reducing the weight of films research needs to be done on the proper collection, (less material to deal with), recycling (making the preparation and transportation of the plastics from material into other products) and incineration (for a the site of use to the point where it is processed, and discussion of various disposal methods, see Hemphill then to the point of incineration. Incineration may be 1993). Plastic mulches and drip irrigation tapes are the answer to the disposal problem, but much more the hardest to deal with. After a season in the field, work is needed. these materials are dirty and often wet, which makes

7 CONCLUSION New York, USA. Emmert, E.M. 1957. Black polyethylene for The production of vegetable crops using mulching vegetables. Proceedings of the plasticulture is a certainly a production system that American Society of Horticultural Sciences involves high input costs and levels of management 69: 464-469. and is subject to mismanagement and risk, just like Ennis, R.S. 1987. PlastigoneTM a new, time- any other production system. With proper planning, controlled photodegradable plastic mulch attention to details, and dedication to all aspects of film. Proceedings, 20th National Agricul- the plasticulture system, the opportunity exists to tural Plastics Congress, pp. 83-90. reduce the acreage of an existing operation and Graham, H.A., D.R. Decoteau, and D.E. possibly increase profits using efficient production Linvill. 1995. Development of a poly- techniques. Some yields of various vegetable crops ethylene mulch system that changes color using plasticulture are presented in Table 4. in the field. HortScience 30: 265-269. Hall, B.J. and S.T. Besemer. 1972. Agricul- tural plastics in California. HortScience REFERENCES 7: 373-378. Ham, J.M., G.J. Kluitenberg, and W.J. Bogle, O. and T.K. Hartz. 1986. Compari- Lamont. 1993. Optical properties of son of drip and furrow irrigation for plastic mulches affect the field temperature muskmelon production. HortScience 21: regime. Jour. American Society of Horti- 242-244. cultural Sciences 228,2: 188-193. Bonnano, R.A. 1996. Weed Management in Ham, J.M. and G.J. Kluitenberg. 1994. Plasticulture. HortTechnology, 6, 3: 186- Modeling the effect of mulch optical 189. properties and mulch-soil contact resis- Clark, G.A., C.D. Stanley, D.N. Maynard, G.J. tance on soil heating under plastic mulch Hochmuth, E.A. Hanlon, and D.Z. Haman. culture. for Meteorology 71: 1991. Water and fertilizer management 403-424. microirrigated fresh market tomatoes. Hemphill, D.D. 1993. Agricultural plastics Transactions of the American Society of as solid waste: what are the options for Agricultural Engineering 34: 429-435. disposal? HortTechnology 3, 1: 70-73. Decoteau, D.R., M.J. Kasperbauer, and P.G. Hochmuth, G.J. 1992. Fertilizer management Hunt. 1989. Mulch surface color affects for drip irrigated vegetable in Florida. yield of fresh-market tomatoes. Jour. Hort Technology 2: 27-32. American Society of Horticultural Sciences Hodges, L., M.E. Dix, J. Brandle, R. Wright, 114: 216-219. and M. Harrell. 1994. Effects of Dix, M.E. and D. Leatherman. 1988. Insect shelterbelts on insect pests in muskmelon. management in windbreaks, p. 501-502. Proc. Nebraska Fruit and Vegetable In: J.R. Brandle, D.L. Hintz, and J.W. Growers Conference, pp. 19-18. Sturrock (Eds.). Windbreak Technology: Lamont, W.J., K.A. Sorensen, and C.W. Proceedings of an International Sympo- Averre. 1990. Painting aluminum strips sium on Windbreak Technology. Elsevier, on black plastic mulch reduces mosaic

Table 3. Yields of selected vegetables using plasticulture

8 symptoms on summer squash. The effect of colored polyethylene mulch HortScience 25: 1305. on the yield of squash and pepper. Pro- Lamont, W.J. and E.B. Poling. 1986. A ceedings 24th National Agricultural Plas- fresh way of looking for profits: double- tics Congress, pp. 157-161. cropping , muskmelons. Fruit Schales, F.D. and R. Sheldrake. 1963. South 7, 4: 8-11. Mulch effects on soil conditions and to- Liakatas, A., J.A. Clark, and J.L. Monteith. mato plant esponse. Proceedings 4th Na- 1986. Measurements of the heat balance tional Agricultural Plastic Congress, pp. under plastic mulches. Part I. Radiation 78-90. balance and soil heat flux. Agriculture for Scoville, R.H. and D. Leaman. 1965. Poly- Meteorology 36: 227-239. ethylene uses in soil fumigation. Proceed- Loy, B., J. Lindstrom, S. Gordon, D. Rudd, ings 6th National Agricultural Plastics and O. Wells. 1989. Theory and devel- Congress, pp. 98-99. opment of wavelength selective mulches. Stapleton, J.J. 1991. Use of soil solarization Proceedings, 21st National Agricultural for the control of soil pests. Proceedings Plastics Congress, pp. 193-197. 23rd National Agricultural Plastics Con- Loy, B., O.S. Wells, N. Karakoudas, and K. gress, pp. 266-271. Mibert. 1998. Comparative effects of Tanner, C.B. 1974. Microclimate modifica- red and black polyethylene mulch on tion: Basic concepts. HortScience 9: 555- growth, assimilate partitioning, and yield 560. in trellised tomato. Proceedings, 27th Wells, O.S. and J.B. Loy. 1985. Intensive National Agricultural Plastics Congress, vegetable production with rowcovers. pp. 188-197. HortScience 20: 822-826. Marr, C.W. and W.J. Lamont. 1992. Prof- Wells, O.S. 1991. High tunnels shelter early its, profits, profits—three good reasons to crops. American Vegetable Grower 39, try triple cropping. American Vegetable 2: 44,46-47. Grower 40: 18, 20. For additional information on plasticulture, Natwick, E.T. and A. Durazo, III. 1985. contact the following websites: Polyester covers protect vegetables from American Society for Plasticulture: http:// whiteflied and virus disease. California www.plasticulture.org Agriculture 39(7+8): 21-22. Center for Plasticulture, Pennsylvania State Orzolek, M. D. and J.H. Murphy. 1993. University: http://hortweb.cas.psu.edu/plastic

9