Environmental Control

Temperature: Heating and Cooling
I. Maintaining Optimal Temperatures
Minimal air temperatures, with and without CO2 enrichment are shown in Table 2. Energy conservation measures are widely used in northern latitude greenhouses to reduce heating costs. Pulling thermal curtains of porous polyester or an aluminum foil fabric over the plants at night reduces heat loss by as much as 57%. Light-weight retractable curtains are sometimes used for energy conservation on cold nights and to shade plants on hot, bright days. In most southern growing regions, however, curtains will not provide enough energy savings to justify their high cost and even rolled to the side, curtains shade plants during the day.

Table 2. Recommeded air temperatures (°C) for tomato cropping [39].

Maximal temperatures for greenhouse tomato production are less well established, but are generally considered to limit summertime production in the south, especially in humid areas where evaporative cooling is less effective. There are three basic methods of greenhouse cooling: natural ventilation, mechanical fan and pad cooling and fog cooling. Natural ventilation from ridge or side vents is popular in areas with relatively low heat loads and few problems with pest entry and is utilized in most new glass and rigid plastic greenhouses. For natural ventilation, some part of the greenhouse, usually at the peak or ridge, is opened and air movement created by wind pressures or by gradients in air temperatures draws cooler air into the house. In the past, roof venting has been restricted to glass and rigid plastic greenhouses, but new designs for polyethylene film greenhouses with roof openings are being tested [26]. Some polyethylene houses have retractable curtains on the side which can be rolled up for ventilation. With any type of natural ventilation system, however, screening side or roof vents to prevent pest entry and escape of pollinators or beneficials greatly reduces its effectiveness.

While natural ventilation is used in some warm climate greenhouses, hot conditions outside and lack of wind reduce its effectiveness. In the southeastern US, pest pressures, high humidity and high temperatures force most growers to invest in active, mechanical cooling, usually with a combination of fans and pads. With mechanical cooling, low-pressure propeller blade fans are placed opposite the air intake, which is covered by cellulose evaporative cooling pads. Louvers are placed outside the cooling pads and are closed when the greenhouse is not venting. Ventilation fans are normally sized to allow one air exchange per minute, although researchers in North Carolina have documented increased cooling at higher rates, especially when combined with evaporative cooling [27].

Evaporative cooling equipment and the electricity to operate them are expensive, so in some warm climates, sites are selected at high enough elevations that evaporative cooling is unnecessary during the summer, but low enough elevations that heating costs are still reasonably low in winter. In southern Arizona, such an elevation for tomato production ranges from 1250 to 1675 m [28]. High-pressure fog systems can also be used for cooling in dry climates if the water supply of high enough quality to prevent clogging of the fog nozzles.