The ability to regulate temperature is fundamental to a successful greenhouse environment, as plant health and growth are directly tied to thermal stability. Greenhouses capture solar energy, but this effect can quickly lead to overheating or rapid heat loss, making the internal climate susceptible to external weather fluctuations. Maintaining a temperature range suited to the specific crops inside is paramount, often requiring both active and passive methods to counteract daily and seasonal swings. Consistent temperature management optimizes biological processes like photosynthesis, maximizing yield and quality.
Strategies for Managing Excess Heat
When solar radiation causes temperatures to climb too high, cooling strategies must be employed to prevent heat stress and plant damage. The most effective method is ventilation, which replaces hot, humid indoor air with cooler, drier air from outside. Passive ventilation relies on natural air movement, utilizing the principle that hot air rises to escape through roof vents while cooler air is drawn in through lower side vents, creating a chimney effect.
For larger structures or during intense heat, forced ventilation uses exhaust fans to actively pull air out and intake shutters to draw in fresh air. To maximize cooling efficiency, the total volume of air should be exchanged every minute or two, depending on the outside temperature. This constant air exchange removes excess heat and prevents the buildup of humidity and stale air.
Shading reduces the amount of solar energy that enters the structure. Temporary solutions include deploying shade cloths, typically rated by the percentage of light they block, such as a 50% density cloth for most common vegetables. These are most effective when installed externally, intercepting solar energy before it converts to heat inside the growing area.
Alternatively, growers can apply a temporary whitewash or reflective paint directly onto the glazing material to reflect sunlight during the hottest months. In dry climates, evaporative cooling systems offer significant temperature reduction by harnessing the energy-transfer properties of water. Fog systems or fan-and-pad systems cool the air by causing water droplets to evaporate, which absorbs heat energy and lowers the ambient temperature. This technique is less effective in naturally humid regions.
Methods for Providing Supplemental Heat
When outside temperatures fall, strategies are needed to raise the air temperature and prevent cold damage to sensitive plant roots and foliage. The most direct approach uses active heating systems. Electric fan heaters are popular for smaller spaces due to simple installation, while larger commercial operations rely on natural gas or propane heaters, which are more cost-effective for heating significant air volumes.
When using fossil fuel heaters, proper venting is mandatory to prevent the accumulation of combustion byproducts, such as ethylene and carbon monoxide, which are harmful to plants and people. Beyond active heat generation, retaining the heat that is already present is an energy-saving measure that minimizes the demand on heating systems. This is achieved by insulating the greenhouse walls with materials like thermal bubble wrap or specialized poly film during the winter months.
Heat retention can also be achieved by utilizing a thermal mass, which absorbs and stores heat energy during the day and slowly releases it at night. Large water barrels painted black are an excellent choice because water has a high specific heat capacity, allowing it to store a substantial amount of solar warmth. Positioning these thermal reservoirs where they receive maximum direct sunlight helps buffer the rapid temperature drop that typically occurs after sunset.
Monitoring and Automation
Consistent temperature regulation depends entirely on accurate measurement and prompt response, which is managed through monitoring and automation systems. The placement of temperature and humidity sensors (thermo-hygrometers) is important, as the air temperature near the roof can be substantially different from the air temperature at the plant canopy level. Sensors should be positioned near the center of the growing area, shielded from direct sunlight and drafts, to reflect the actual environment surrounding the plants.
Environmental controllers and advanced thermostats use the data collected by these sensors to trigger heating and cooling equipment automatically. By setting specific temperature set points, the controller can activate exhaust fans, open motorized roof vents, or turn on supplemental heaters without constant human intervention. This automation ensures a narrow range of thermal fluctuation, protecting the plants from sudden, damaging temperature shifts that might occur overnight or while the grower is away.