While greenhouses are designed to capture solar energy, this solar gain becomes a major liability during the summer. Temperatures inside a closed greenhouse can easily exceed the outside ambient air temperature by 20°F or more, creating conditions that cause rapid heat stress for plants. Maintaining a stable temperature is necessary for plant health; excessive heat disrupts photosynthesis and quickly leads to desiccation and death. Successful summer greenhouse management relies on a strategic, multi-step approach that proactively mitigates solar heat, actively removes warm air, and uses water to condition the environment.
Blocking Solar Gain With Passive Shade
The most energy-efficient defense against summer heat is preventing solar radiation from entering the structure. This passive technique requires no mechanical energy. Applying exterior shade significantly reduces the heat load before it can be trapped by the glazing.
External shade cloths are the most common solution and are rated by the percentage of sunlight they block. A shade percentage between 30% and 50% is suitable for sun-loving crops like tomatoes and peppers, offering sufficient relief without severely limiting light for growth. Placing the shade cloth on the outside is better than internal shading because it stops the heat from radiating into the interior air.
Alternatively, liquid shade coatings, sometimes called whitewash, can be sprayed or painted directly onto the glass or polycarbonate panels. These materials reflect a portion of the solar radiation while allowing a controlled amount of light to pass through. These coatings are designed to wear off naturally by the end of the season, eliminating the need for manual removal before winter. Internal shade systems, such as retractable roller blinds, intercept the heat after it has already passed through the glazing, making them less effective than exterior options.
Maximizing Airflow Through Active Ventilation
Once passive shading is in place, active ventilation replaces the hot air inside the greenhouse with cooler outside air. Simply opening vents often provides insufficient airflow in hot conditions, necessitating mechanical systems to ensure a rapid air exchange rate. Natural ventilation relies on the chimney effect, where hot air rises and escapes through roof vents while cooler air is drawn in through side vents.
For effective summer cooling, the entire volume of air within the greenhouse must be exchanged approximately once every minute. This high rate, often translating to 60 air changes per hour, requires properly sized exhaust fans installed on one end of the structure and corresponding motorized intake louvers on the opposite end. Sizing the fans correctly is important; an inadequate system will fail to remove heat quickly enough, leading to temperature stratification.
Internal circulation is necessary to prevent pockets of stagnant, superheated air. Horizontal Air Flow (HAF) fans are mounted high up to gently move the air in a continuous, circular pattern parallel to the ground. These fans eliminate hot spots and ensure uniform temperature and humidity throughout the plant canopy, which helps maintain consistent transpiration rates and reduces the risk of fungal diseases.
Utilizing Water for Evaporative Cooling
When the outside temperature is too high for ventilation alone to maintain acceptable internal conditions, using water to cool the air becomes necessary. Evaporative cooling exploits phase change physics, where water absorbs latent heat from the air as it turns from liquid into gas. This process can reduce the air temperature significantly, sometimes by as much as 10 to 20°F, depending on the environment.
The most common system is the fan-and-pad cooler, which works by drawing outside air through a thick, porous cellulose pad kept constantly wet by circulating water. The exhaust fans on the opposite side of the greenhouse pull the air through the wet pad, causing the water to evaporate and cool the air before it enters the growing area. A disadvantage is that it often creates a temperature gradient, with the air being coolest near the pad and warmest near the exhaust fans.
High-pressure fogging or misting systems offer an alternative, using specialized nozzles to atomize water into extremely fine droplets, often between 10 and 20 microns in size. These micro-droplets are small enough to remain suspended in the air and evaporate quickly before falling onto the plants, which avoids wetting the foliage and reducing disease risk. Evaporative cooling is most effective in arid climates with low relative humidity, as dry air has a greater capacity to absorb the moisture and the associated cooling effect.
Regulating Internal Thermal Mass and Humidity
Supplementary strategies that manage the internal environment, such as utilizing thermal mass, can help stabilize temperature swings. Large water containers, such as 55-gallon drums, possess a high heat capacity and can be strategically placed within the greenhouse. During the day, these water barrels absorb excess heat from the air, and they slowly radiate that stored heat back into the space at night, moderating the rapid temperature drop.
For summer cooling, the goal of the thermal mass is to absorb heat. Painting the containers a light color, or shading them, can help them absorb surrounding air heat rather than direct solar heat. Managing humidity is also a factor, as high humidity levels reduce the effectiveness of evaporative cooling and can promote plant disease.
Adjusting watering schedules to the early morning allows plants to absorb water before the peak heat of the day. This maximizes the natural cooling effect of plant transpiration while allowing the leaves to dry before nightfall.