Greenhouses are engineered to capture and retain solar energy, providing an extended growing season and protection from cold weather. This design creates a challenge during summer when excessive heat must be actively managed to prevent plant damage. The enclosed structure can rapidly reach temperatures far exceeding those outside, leading to scorched foliage, sterile pollen, and failed fruit set. Successfully utilizing a greenhouse in summer relies on creating a functional system that efficiently expels heat and moderates intense solar radiation.
Mastering Ventilation and Passive Cooling
Controlling the internal temperature begins with aggressive air movement to replace the superheated air inside the structure. Passive cooling leverages the natural tendency of hot air to rise, known as the stack effect. This method requires opening roof vents to allow the hottest air to escape, while simultaneously opening lower side vents to draw in cooler outside air.
For most structures, passive ventilation is insufficient during peak summer heat, necessitating mechanical systems. Exhaust fans should be sized to achieve approximately one complete air change per minute, translating to 7 to 10 cubic feet per minute (CFM) of capacity for every square foot of floor area. This rapid air exchange limits the temperature rise from the intake to the exhaust end of the greenhouse to a maximum of 8 to 10 degrees Fahrenheit.
Circulation fans (HAF fans) are necessary to move air horizontally and eliminate stagnant pockets of hot, humid air. HAF fans provide about 2 CFM of movement per square foot of floor area, helping reduce fungal diseases by drying the plant canopy. In arid climates, evaporative cooling systems (swamp coolers) are effective, drawing air through water-soaked cellulose pads to cool the air by 10 to 20 degrees through evaporation.
Managing Solar Intensity with Shading
Reducing the solar load is the first line of defense against heat buildup, as sunlight is the primary source of thermal energy. Shade cloth is the most common solution, with densities ranging from 30% to 50% suitable for most sun-loving vegetable crops. Installing the shade cloth on the exterior is significantly more effective, reflecting radiation before heat is absorbed by the glazing material.
For growers seeking a temporary and reflective option, specialized exterior whitewash can be applied directly to the glass or polycarbonate panels. This substance reflects solar energy and provides a shading factor around 40%. Whitewash scatters light, improving penetration into the lower plant canopy, but it must be manually removed or allowed to wash off in the fall.
Regardless of the method chosen, it is important to monitor light reduction to ensure plants still receive their necessary Daily Light Integral (DLI). Excessive shading can lead to weak, stretched growth, even if the temperature is controlled. The goal is to block the surplus of intense summer light that causes heat stress without diminishing the light required for robust photosynthesis.
Adapting Watering and Nutrient Delivery
High heat and intense light dramatically increase plant transpiration rates, requiring a complete overhaul of the irrigation schedule. Plants may need watering daily or multiple times a day, particularly those grown in porous containers. The best time to irrigate is early morning, ideally between 6:00 AM and 10:00 AM.
Watering in the morning ensures the plant canopy and soil surface have time to dry before evening, preventing fungal and bacterial diseases. Deep, targeted watering methods like drip irrigation are preferable over overhead watering, as they deliver moisture directly to the root zone while minimizing leaf wetness.
High temperatures severely impact root function, reducing the plant’s ability to absorb nutrients like calcium and phosphorus. This manifests as physiological disorders such as blossom end rot on tomatoes or tip burn on leafy greens. If plant growth is stunted due to heat stress, the nutrient concentration may need to be reduced, although the overall water volume demand remains high.
Misting systems can be employed to cool the air through evaporation, but they must be used cautiously in humid climates. When relative humidity is above 75%, a misting system will not cool effectively and can increase the moisture level, encouraging pathogen growth. Misting for cooling must be carefully balanced with adequate ventilation to prevent a high-humidity environment.
Selecting Productive Summer Crops
The controlled environment of a greenhouse in summer is suited for crops that thrive in sustained high temperatures. Heat-loving species like tomatoes, peppers, and eggplants flourish, often yielding better and more consistently than field-grown counterparts. Tropical varieties such as okra, melons, and certain types of basil also thrive, benefiting from extended heat and protection from pests.
Conversely, intense heat causes cool-season crops to prematurely complete their life cycle through a process called bolting. Leafy greens like lettuce, spinach, and cool-season brassicas rapidly send up a flower stalk, making the leaves bitter and unpalatable. Growers can extend the season for some leafy greens by utilizing a heavier shade cloth and ensuring constant ventilation, but these crops often require a dedicated, cooler section. The deliberate choice of heat-tolerant varieties is the most effective way to ensure a productive and continuous harvest throughout the hottest months.