Using a greenhouse during the summer months is entirely feasible, but it transitions the structure’s primary function from heat retention to heat rejection. While the enclosed environment provides protection from pests and weather fluctuations, the intense solar radiation of summer quickly turns a greenhouse into an oven if management techniques are not employed. Successfully navigating the warmer season requires understanding the atmospheric physics at play and implementing specific adjustments to temperature, air movement, and plant selection. This comprehensive approach ensures the growing environment remains productive for heat-sensitive crops.
Understanding Rapid Heat Buildup
The fundamental challenge in a summer greenhouse is a concentrated form of the physical process known as the greenhouse effect. Short-wave solar radiation easily penetrates the glazing material, but once this energy strikes surfaces inside—such as soil, benches, and plants—it is converted into long-wave infrared radiation, or heat. The greenhouse structure then traps this infrared energy, preventing its escape and causing temperatures to climb rapidly.
This trapped heat can quickly create lethal conditions, often exceeding 120°F (49°C) on a warm day. Temperatures above the optimal range of 70–85°F (21–29°C) cause significant plant stress, leading to physiological failures. High heat slows down photosynthesis, and if temperatures exceed 95°F (35°C), many fruiting plants, like tomatoes and peppers, will experience flower drop and pollen sterility, directly reducing yield. Heat stress also causes wilting, sun scorch on leaves, and excessive transpiration that can severely stress the plant’s root system.
Essential Cooling and Shading Strategies
Managing the summer heat requires a combination of passive and active cooling methods, with the first step being to reduce the solar energy entering the structure. Shade cloths are the most effective way to accomplish this, ideally placed on the exterior of the greenhouse to block heat before it passes through the glazing. For sun-loving crops like peppers and cucumbers, a shade percentage between 30% and 50% is generally sufficient to lower the ambient temperature without compromising the necessary light levels.
Passive cooling relies on maximizing natural ventilation, often utilizing the “chimney effect” where warm air naturally rises and escapes through high-point vents. Roof vents, especially those equipped with automatic openers that respond to temperature changes, should be opened wide to allow hot air to exit. Lower side vents or doors must also be opened simultaneously to draw in cooler, outside air, creating a consistent cross-breeze.
Active cooling systems provide a reliable way to maintain set temperatures even when passive methods are insufficient. Exhaust fans installed on one end of the greenhouse pull hot air out, requiring intake louvers or vents on the opposite end to draw in replacement air. A common guideline suggests sizing the fan capacity to exchange the entire volume of air inside the greenhouse at least once every minute during peak heat.
For gardeners in dry climates, evaporative cooling is an extremely effective active strategy. A fan-and-pad system works by pulling air through a water-saturated cellulose pad, where the process of water evaporation absorbs a significant amount of heat energy from the incoming air. This method, often referred to as a swamp cooler in smaller setups, can lower the internal temperature by 10 to 20 degrees Fahrenheit, but its effectiveness decreases substantially in high-humidity regions.
Air Circulation and Humidity Management
Air movement within the greenhouse is crucial for plant health and disease prevention, separate from exhausting hot air. Horizontal Airflow (HAF) fans are placed strategically to create a continuous, gentle circulation pattern, preventing stagnant air pockets. This constant internal movement strengthens plant stems and aids in crop pollination.
Stagnant, warm air contributes to high humidity, increasing the risk of fungal diseases such as Botrytis and powdery mildew. Plants naturally release moisture through transpiration. If this water vapor is not moved away from the leaf surface, it creates an environment conducive to fungal spore germination. Circulating fans continually mix the air, preventing moisture from condensing on leaves, especially during cool evening hours.
Managing watering timing is another important technique for humidity control. Watering plants early in the morning allows excess moisture to evaporate throughout the day when ventilation systems are running. Avoiding watering late in the afternoon or evening is critical, as trapped moisture overnight raises the relative humidity to damaging levels when temperatures drop and the greenhouse is closed.
Selecting Plants for Summer Greenhouse Conditions
Choosing the right crops allows the gardener to take maximum advantage of the managed summer heat. Heat-loving crops, native to warm climates, thrive under these conditions and benefit from the extended season.
Heat-Loving Crops
Examples include fruiting vegetables such as tomatoes, peppers, and eggplants, which require consistent high temperatures to set fruit and mature fully. Cucumbers and various types of melons also flourish in the warm, humid environment. For herbs, varieties like basil, rosemary, and oregano respond well to the intense heat and light. These plants generally perform best when daytime temperatures are consistently between 70°F and 85°F (21–29°C).
Avoiding Cool-Season Crops
Cool-season crops should be avoided unless significant shading and cooling are implemented. Leafy greens like lettuce and spinach, along with many cool-season herbs, will quickly “bolt,” or go to seed, when exposed to high heat, making them bitter and ending their harvest. Selecting heat-set varieties of common greenhouse crops, which are genetically bred to produce flowers and fruit even when temperatures exceed 90°F (32°C), is a practical choice for ensuring a successful summer yield.