Growing vegetables in a greenhouse during the summer is possible, but success depends entirely on managing the intense heat. A greenhouse is designed to capture and retain solar energy, which is beneficial in cooler months but becomes a liability in summer. The primary challenge is solar gain, where the glazing traps heat, quickly raising the interior temperature far above ambient outdoor levels. This rapid heat buildup causes plants to suffer from heat stress, prevents pollination, and can lead to crop failure if not actively controlled.
Environmental Control: Cooling and Airflow
Managing the high temperatures generated by solar gain requires moving hot air out of the structure. Passive ventilation is the first line of defense, relying on the natural tendency of hot air to rise, known as the stack effect. This system uses low-level vents, louvered intake shutters, or roll-up sides to draw cooler air in. Roof vents then allow the accumulated hot air to escape through the highest point of the structure.
For more rapid cooling, active ventilation systems are implemented, often requiring enough fan capacity to achieve one full air exchange per minute. Exhaust fans are mounted high on one wall to pull hot air out, while intake vents are placed on the opposite side to draw in fresh air, creating a powerful cross-breeze. Inside the structure, horizontal airflow fans circulate the air constantly. This prevents stagnant, superheated pockets and ensures a uniform temperature distribution around the plants.
When outside temperatures are extremely high, simple ventilation may not be enough, necessitating evaporative cooling. This involves drawing warm air through a wet medium, such as a water-soaked pad, using an exhaust fan. As the water evaporates, it absorbs latent heat from the air, which can lower the temperature inside the greenhouse by several degrees. This method is significantly more effective in drier climates.
Light Management and Protection from Sun Scorch
While heat is one issue, the intensity of summer sunlight presents a separate problem, leading to sun scorch and stress in plants. Direct, unfiltered summer light can bleach leaves and stunt growth, even for heat-loving plants, by overwhelming their photosynthetic capacity. Managing this requires reducing the amount of solar radiation that enters the greenhouse.
The most common solution is exterior shade cloth, a woven material available in various densities. A 30% to 50% shade rating is suitable for sun-loving vegetables like tomatoes and peppers, offering relief without drastically cutting the light needed for fruit production. This material is often mounted on a system that allows it to be rolled up or removed when conditions become cloudy.
Another effective, temporary method is applying a reflective coating, often called whitewash or shading paint, directly to the outside of the glazing. This coating reflects incoming sunlight and heat before it can penetrate the structure, and it can be washed off at the end of the season. Interior shade curtains can also be deployed to provide targeted relief during the hottest hours of the day.
Water Management and Humidity Control
The combination of high temperatures and increased ventilation rates causes rapid water loss through plant transpiration and soil evaporation. Plants must be adequately hydrated to maintain turgor pressure, as water stress in high heat leads to wilting and reduced nutrient uptake. Consequently, watering frequency must be increased compared to cooler seasons.
It is recommended to water plants early in the morning so moisture is available to the roots before the sun reaches its peak intensity. Watering during the midday heat is inefficient due to rapid evaporation and can be detrimental. Evening watering can increase overnight humidity, promoting fungal growth. Automated drip irrigation systems are beneficial because they deliver water directly to the root zone, minimizing waste and ensuring consistent moisture levels.
Managing humidity is a delicate balancing act, as high moisture levels in stagnant air can create an ideal environment for diseases like powdery mildew. However, slightly higher humidity can also help cool the plants through transpiration. Applying organic mulch around the base of plants helps keep the root zone cooler and reduces moisture evaporation from the soil surface.
Selecting Heat-Tolerant Crops
Successful summer greenhouse gardening relies on selecting varieties that can withstand high temperatures and intense light without bolting or dropping flowers. Cool-season crops such as lettuce, spinach, and broccoli quickly bolt—prematurely flowering and becoming bitter—when temperatures exceed their optimal range. This makes them poor choices for the summer greenhouse.
Instead, warm-season vegetables are ideal. Excellent choices include tomatoes, particularly indeterminate varieties, as well as peppers and eggplants, all of which benefit from consistent warmth. Okra is exceptionally heat-tolerant, able to handle temperatures up to 95°F, and is a strong producer in the hottest conditions.
Cucumbers and melons also flourish, enjoying the warm, humid microclimate maintained with proper management. Other productive crops include bush and pole beans, which grow quickly and continue to yield throughout the summer. Focusing on these heat-loving species ensures the controlled environment maximizes production.