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.
Regulating air temperature in the greenhouse is important for both vegetative growth and fruiting. To determine heating requirements, it is essential to know the minimum temperature requirements for the crop, the lowest outdoor temperature that might be expected, and the surface area of the greenhouse. Heat loss also will be affected by wind and site exposure.
Most vegetable production greenhouses in New Mexico are heated by natural gas. Forced-air heaters installed in the ridge area of the greenhouse will take up a minimum of space. It is important to install a heater with sufficient BTU output to heat the greenhouse on the coldest day. Natural and compressed gas heaters should be vented for safety and to prevent crop damage. Thermostats should be located at plant level where plants are growing and where drafts or sunlight do not directly affect them.
Greenhouse cooling also is important. Evaporative cooling is the most efficient and economical way to reduce greenhouse temperatures in New Mexico. Proper ventilation also is important not only for temperature control, but also to replenish carbon dioxide and control relative humidity. Relative humidities above 90 percent will encourage disease problems. Roof ventilators are seldom used on plastic houses, which instead use side vents to provide both ventilation and cooling. Vents should be installed as high on the wall as possible. Shading may be required in the late spring or early fall if daytime temperatures become too high. Various shading materials that can be sprayed or brushed on are available from greenhouse supply companies. However, shade compounds must be removed when cool weather sets in. Shade cloths with various degrees of shading also are available.
Heating, cooling, and ventilation should be automated to save labor and to ensure proper temperature control. Polyethylene ventilation tubes with 3-inch perforation holes along the tube suspended in the peak of the house from one end to the other help evenly mix cooler air with warmer air, preventing drafts.
The easiest way to get started in greenhouse vegetable production is to use the existing soil on site, but soils must be well drained. Soils can be improved by applying ample quantities of well-rotted livestock manure, compost, or composted livestock manure. Apply all manures before fumigation. Soils should be fumigated or sterilized with steam at least two weeks before planting. If the soil is steamed, maintain a temperature of 180°F for at least four hours. Avoid deep tillage after sterilization to prevent reintroduction of weed seed and disease organisms from below the sterilization or fumigation zone.
A soil test should be taken before planting to determine the amount of fertilizer to apply for each crop. All phosphorous and potassium fertilizers should be applied before planting and incorporated directly into the soil. Nitrogen fertilizers should be applied in split applications, part before planting and the rest as needed during the growing season. Nitrogen fertilizers can be applied as sidedressings or through a drip irrigation system. Secondary and minor fertilizer elements should be applied as needed.
Hydroponic culture of greenhouse vegetables involves the production of crops in sand, gravel, or artificial soil-less mixes in bags, tubes, tubs, tanks, or troughs designed to allow the circulation of nutrient media needed for crop growth. Unlike conventional soil culture, hydroponic culture of greenhouse vegetables is less forgiving and requires intense management. Although present automation systems can minimize fertilization and irrigation labor inputs, continuous monitoring of the system is important. Growers must be highly knowledgeable about plant growth, nutrient balances, cultural media characteristics, and plant physiology. Nevertheless, hydroponics allows the culture of greenhouse vegetables in areas where soils are not suitable for vegetable production.
Crops and Culture
Tomatoes are the most common vegetables grown in greenhouses, followed by cucumbers. Both are grown in the off-season (fall, winter, and spring) to take advantage of premium prices. Leaf and Bibb lettuces often are grown in the winter in cooler areas of the state. Other popular crops include bell peppers, eggplant, and herbs like basil.
Carbon Dioxide Enhancement
The introduction of supplementary carbon dioxide into the greenhouse has been found to significantly increase the yields of greenhouse tomatoes and other vegetables. Supplementary carbon dioxide is most effective on days when the greenhouse has been shut up for several days with no ventilation. Maximum results can be achieved by injecting 1000-1500 ppm CO2 into the greenhouse using propane burners or other CO2 generators.
Integrated Pest Management
Integrated pest management (IPM) is a holistic approach to the management of pests. IPM does not exclude the use of pesticides in the greenhouse. Rather, pesticides are used in combination with cultural, natural, mechanical, and biological control as well as insect monitoring to maximize the effectiveness of control methods. Reduced use of pesticides under more effective timing schedules reduces not only the adverse effects of these chemicals on the environment and people, but also reduces the chance of pests developing resistance. For more information on pest control techniques, contact your local county Extension agent.