Greenhouse vegetable production has traditionally been located near population centers, primarily in the northeastern United States. Improved transportation and high energy costs have forced the industry south. With light being one of the most important factors in greenhouse vegetable production, the Southwest has become an ideal area for future development of this industry, particularly in the winter months when tomato and cucumber prices are at a premium.
Location of Greenhouse
The ideal location for a greenhouse has high winter light intensity, moderate winter temperatures, low humidity, and easy access to markets. The easy availability of existing utilities will help reduce establishment costs and will affect ongoing fuel costs. Avoid trees or buildings that may shade the greenhouse, although windbreaks will help reduce heating costs if properly located. Be sure to leave sufficient room for future expansion and parking.
While superhighways have made transporting greenhouse-grown vegetables easier, locating greenhouse-grown tomatoes near large population areas is still important. High quality tomatoes should be vine ripened; as ripe tomatoes are hard to transport, the closer they are to the retail market the better.
Greenhouses using native soil for vegetable production should be constructed on level sites with deep, well-drained soils. Sandy loams are best. A source of good quality water also is important. High salt concentrations in either the soil or water can significantly reduce yields. Where soils are not suitable, growers may consider bringing in a better soil or using a soil-less production system.
When considering greenhouse designs, three major factors should be considered: load limitations, light penetration, and cost. The primary load considerations include snow and wind. Roof slopes of at least 28° and heated air in the greenhouse should prevent snow accumulation on the roof. Bracing along sides of the greenhouse and roof should be sufficient to withstand wind, particularly in the spring. Bracing along the roof also should be sufficient to withstand crop loads if tomato or cucumber vines are to be supported by twine attached to the bracing. A concrete footing is preferred for a permanent greenhouse. A wide door at one end of the greenhouse will ensure easy access for equipment.
Without sacrificing strength, support structures should be kept to a minimum to maximize light penetration. Glazing materials should be highly transparent. Overhead electrical lines, irrigation systems, and heating ducts should be kept to a minimum. Support structures should be painted with a reflective, light-colored material for maximum light reflection.
Most greenhouse crops grow best in light whose wavelengths range from 400 to 700 nanometers. This range of wavelengths is called photosynthetically active radiation (PAR). Most greenhouse coverings will accommodate these short waves of visible light. Polyethylene and fiberglass tend to scatter light, while acrylic and polycarbonate tend to allow radiation to pass through directly. Scattered or diffused light tends to benefit plants by reducing excess light on upper leaves and increasing reflected light to lower leaves.
Plastic glazed greenhouses have several advantages over glass greenhouses, the main advantage being cost. Plastic also is adapted to various greenhouse designs, generally resistant to breakage, lightweight, and relatively easy to apply.
Types of Plastic Coverings
Acrylic is resistant to weathering and breakage and is very transparent. Its ultra-violet radiation absorption rate is higher than glass. Double-layer acrylic transmits about 83 percent of light and reduces heat loss 20-40 percent over single-layer. This material does not yellow. Its disadvantages are that it is flammable, very expensive, and easily scratched.
Polycarbonate resists impact better and is more flexible, thinner, and less expensive than acrylic. Double-layer polycarbonate transmits about 75-80 percent of light and reduces heat loss 40 percent over single-layer. This material scratches easily, has a high expansion/contraction rate, and starts turning yellow and losing transparency within a year (although new varieties with UV inhibitors don't yellow as quickly).
Fiberglass reinforced polyester (FRP) panels are durable, attractive, and moderately priced. Compared to glass, FRP panels are more resistant to impact, transmit slightly less light, and weathering over time reduces light transmission. This plastic is easy to cut and comes in corrugated or flat panels. It provides superior weatherability only when coated with Tedlar. Fiberglass has a high expansion/contraction rate.
Polyethylene film is inexpensive but temporary, less attractive, and requires more maintenance than other plastics. It is easily destroyed by ultraviolet radiation (UV) from the sun, although film treated with UV inhibitors will last 12-24 months longer than untreated. Because it comes in wider sheets it requires fewer structural framing members for support, resulting in greater light transmission. Using a double layer of 6 mil polyethylene on the outside and 2 mil as an inner barrier will help conserve heat; this inner layer also will help reduce water condensation. The inner layer should be 1-4 inches from the outside layer with layers kept separated by a small fan (creating an insulating dead air space) or wood spacers. Two layers reduce heat loss 30-40 percent and transmit 75-87 percent of available light when new.
Polyvinyl chloride film has very high emissivity for long-wave radiation, which creates slightly higher air temperatures in the greenhouse at night. UV inhibitors can increase the life of the film. It is more expensive than polyethylene film and tends to accumulate dirt, which must be washed off in winter for better light transmission.