A greenhouse creates an ideal setting for accelerated plant growth by maximizing light exposure and maintaining elevated temperatures, allowing growers to extend the natural growing season. However, intense summer heat requires specialized planning to prevent the greenhouse from harming plants. Maximizing the summer harvest depends on regulating the climate and selecting crops that thrive under sustained warmth.
Managing High Temperatures
The greatest challenge of summer greenhouse cultivation is heat management, as temperatures can quickly rise 10 to 20 degrees Fahrenheit above the outside air if left unchecked. Implementing a robust ventilation system is the primary method for preventing internal temperatures from reaching plant-damaging levels. Fan systems should be sized to provide approximately one complete volume air exchange per minute for the growing area during peak summer conditions.
This high rate of air movement, measured in Cubic Feet per Minute (CFM), ensures hot, stagnant air is constantly replaced with fresh, outside air. A temperature difference of 8 to 10 degrees Fahrenheit between the intake and exhaust points is still common. For smaller operations, manual side vents and roof openings provide natural cross-ventilation. However, powered exhaust fans are necessary for consistent climate control in larger structures. Proper air circulation, achieved using horizontal airflow fans, also helps eliminate localized hot spots and strengthen plant stems.
Shading is another effective technique to limit solar gain and reduce the heat load within the structure. Applying an external shade material, such as a shade cloth with 30 to 50 percent density, can significantly lower internal temperatures. Some growers opt for a temporary whitewash coating applied directly to the exterior glazing, which reflects incoming solar radiation. Combining mechanical ventilation with shading is the most reliable way to keep the greenhouse in the optimal temperature range for summer crops.
High-Yield, High-Heat Crops
Once the internal temperature is managed, high-value, long-season crops that benefit from the sustained warmth are the optimal choice for summer production. Tomatoes are a popular greenhouse staple because the controlled environment protects them from pests and ensures consistent development. However, the daytime temperature must be maintained between 70 and 82 degrees Fahrenheit for optimal growth and fruit development.
Temperature control is particularly important for tomatoes during the fruiting phase. Exposure to temperatures above 104 degrees Fahrenheit for even short periods can cause fruit set failure. Lycopene, the pigment responsible for the red color in the fruit, fails to form when temperatures exceed 86 degrees Fahrenheit, resulting in poorly colored tomatoes. Indeterminate, or vining, tomato varieties are best suited for greenhouse culture as they continue to produce fruit over a long season, maximizing the use of vertical space.
Cucumbers are highly productive during the summer, performing best when temperatures are maintained between 80 and 85 degrees Fahrenheit. Varieties like English and Beit Alpha are commonly grown because they are parthenocarpic, meaning they produce seedless fruit without the need for pollination. For the hottest summer months, specialty types like the Armenian cucumber, which endures greater heat without developing bitterness, can be chosen. These vining crops can produce a rapid harvest cycle, sometimes yielding fruit within seven weeks of seeding.
Quick-Turnaround and Companion Plants
To maximize space and efficiency, growers can integrate quick-maturing plants that occupy the areas beneath or around the primary long-season crops. These plants provide an early harvest while the main crops are still establishing themselves vertically. Heat-tolerant leafy greens offer a harvestable crop within weeks and can be planted in the cooler, shaded areas at the base of the tomato or cucumber plants.
Malabar spinach, a vine-like plant that thrives in heat and humidity, is a good alternative to traditional lettuce, which bolts quickly in high temperatures. Fast-growing bush beans can fill spaces temporarily and are harvested quickly before the larger vine crops require the full extent of the light and root zone. Planting herbs, such as basil, alongside fruiting plants can also provide a steady yield and may offer benefits like attracting beneficial insects or diversifying the harvest.
Radishes are an excellent, low-commitment, fast-turnaround crop that can be seeded directly into the soil between rows of larger plants. These root vegetables mature in less than a month and utilize soil space that would otherwise be unused during the initial growth stages of the main crops.
Water and Nutrient Needs in Extreme Heat
The high summer temperatures that benefit fruiting crops also significantly increase their metabolic rate and water demand. Plants transpire heavily to cool themselves, which means the frequency of irrigation must increase to compensate for the rapid moisture loss. Monitoring the moisture level in the growing medium, rather than relying on a fixed schedule, is necessary to prevent plant stress and issues like blossom end rot in tomatoes.
When using a hydroponic or soilless substrate system, the increased watering frequency directly impacts the concentration of the nutrient solution. Since plants are taking up water more rapidly, the Electrical Conductivity (EC), which measures the total salt concentration, must often be adjusted downward. Diluting the nutrient solution slightly prevents the buildup of salts in the root zone, which can otherwise lead to nutrient burn or block the uptake of water.
A typical adjustment involves reducing the target EC in the summer compared to winter, such as moving from 1.4 to 1.2 mS/cm for certain crops, to account for the higher volume of water delivered. Maintaining the pH of the nutrient solution between 5.5 and 5.8 is also important during periods of high demand. This ensures nutrients remain soluble and easily accessible to the roots.