Dealing with the Heat in Southern Greenhouses

Dr. J. Raymond Kessler, Jr.

Auburn University

By definition and design, the purpose of a greenhouse is to capture solar radiation and provide an optimum environment for the rapid growth of plants. However in the south, greenhouse temperatures during the summer often soar to levels that can limit plant growth. Some of the detrimental effects of high temperatures include reduced stem strength, reduced flower size, reduced leaf size, delayed flowering, flower bud abortion, and reduced growth rate. Greenhouses heat up during the day because of the greenhouse effect, and because a greenhouse is an enclosed space. How much heat builds up during high-light periods depends on how much solar radiant energy is transmitted through the glazing, how that energy is distributed, and how much is retained within the structure.

Solar energy striking the glazing material of a greenhouse can either be reflected, absorbed, or transmitted into the interior. Much of the sunlight striking a greenhouse is transmitted into the interior as shortwave radiation. When this shortwave radiation enters the greenhouse, some is reflected from various surfaces and passes out of the greenhouse, some is absorber by plants, soil, benches, walks, etc. and converted to latent heat, and the rest is absorbed and re-emitted as long wave radiation. When objects absorb radiation, some of the energy is lost as latent heat. The rest is re-emitted as longer wave radiation, mostly infrared. Water in the greenhouse atmosphere absorbs the infrared radiation and converts most of it to latent heat. In addition, glass is transparent to shortwave radiation but opaque to long wave radiation so a lot of the energy of sunlight is trapped in a greenhouse as heat.

Traditionally, excess heat was removed from the greenhouse using passive ventilation. Air in the greenhouse is generally warmer than that outside, so when top vents are opened, the warmer air rises and escapes pulling cooler air from the outside in through side vents establishing a circulating air pattern. Whether vent control is automated or manual, the circulation principle remains the same. Passive ventilation, often viewed as old fashion, has recently undergone a revival in the form of innovative greenhouse designs incorporating hinged roof panels, roll-up sides, and new vent designs. Saw-tooth style greenhouses are gaining popularity in the south because the design is applicable to natural ventilation.

Properly designed forced air ventilation increases the efficiency of excess heat removal from a greenhouse compared to passive ventilation by using mechanical (exhaust) fans to create a negative pressure within the structure. Cooler air from outside then rushes in through side vents to fill the void. Maximum cooling efficacy from forced air systems is usually achieved using one greenhouse volume, air exchange per minute to a height of eight feet. Several design criteria must be considered to achieve maximum efficiency including fan capacity and location and vent opening size.

Fan-and-pad evaporative cooling combines forced air ventilation with the ability of evaporating water to remove heat from the greenhouse. Water absorbs a relative large amount of heat when moving from a liquid state to a gaseous state. The most widely used evaporative cooling system in greenhouses consists of exhaust fans along one wall and cross-fluted cellulose pads along the other wall. Warm air from outside is drawn through the pads by the exhaust fans. The pads are kept constantly wet, and through the process of evaporation, heat is removed from the air passing through the pads into the greenhouse.

More recently, systems utilizing fog evaporative cooling have been developed that operate on the same principle as fan-and-pad systems, but is implemented in a different way. A high-pressure pump delivers water into the greenhouse through overhead water lines. Specially designed fog nozzles generate fog containing water droplets with a size of less than ten microns when the system is activated. These droplets are so small, they stay suspended in the air and evaporate quickly. A properly designed and controlled system generates fog overhead and cools the greenhouse while people and plants below remain dry. This system is especially useful for seed germination and cutting propagation areas where high rates of air exchange from exhaust fans are undesirable.

The application of liquid shading compound or shade cloth to the outside of the greenhouse structure helps reduce the greenhouse heat load in summer by decreasing the amount of solar energy entering through the glazing. A 50% reduction in solar radiation lowers the average inside temperature be at least 6F making other types of cooling systems more efficient. Crop growth is often less affected than might be expected by a 40-60% shade during the summer because even though the light intensity is reduced, the total amount of energy received throughout the day may be large due to longer day lengths. One problem with fixed shading is that light intensities are often reduced to undesirable levels during cloudy days and at the beginning and end of the day. Several greenhouse product suppliers and individual growers have developed automatic shading systems that pull shade cloth over the plants inside the greenhouse. These systems can be adjusted to provide more light during cloudy periods and reduce the light intensity to reduce heat load during bright periods.

Aside from mechanical cooling systems, several steps can be taken by the greenhouse operator on a day-by-day basis to reduce the effects of high summer temperature on plant growth. One of these is to syringe areas of the greenhouse, either manually with a hose or with installed mist nozzles, to take advantage of heat removal when water evaporates. Areas to syringe might include walks, paths, sidewalls, under benches, and empty bench tops. The foliage of many plants can also be syringed as long as disease physiological problems will not occur.

Part of the problem with high temperatures on plants is the drying effect of low humidity. Therefore, another way to diminish the effects of high temperature is to fill the greenhouse as full as possible so that plants create their own humid surrounds. If an area of a greenhouse must be open, group all plants close together and syringe the open areas.

Combining where possible the aforementioned methods for reducing high greenhouse temperatures should provide growers some tools and strategies for dealing with summer heat in the south. A conscious effort should be made to keep track off weather forecasts to anticipate when hot conditions will occur. Steps can then be taken, beginning early in the days, to reduce and prevent heat accumulation within the greenhouse.