Overcoming Heat Challenges in Summer Greenhouse Operations

Summer greenhouse production may be less intense than spring in terms of volume and diversity of plants, but it presents its own unique challenges. By understanding how high summer temperatures impact plant physiology, growers can adjust their cultural practices to better meet the needs of crops during the summer months. Additionally, it is essential to consider potential summer pests and pathogens that may target vulnerable plants, allowing for more effective management strategies.

As climate conditions intensify and summers grow hotter, greenhouse growers face increasingly complex challenges that threaten crop yield, quality, and plant health. Heat-related issues are not merely seasonal inconveniences; they have deep implications for plant physiology, cultural practices, infrastructure, pest and disease outbreaks, and overall greenhouse operations. By understanding the wide-ranging effects of high temperatures and adopting proactive management strategies, growers can maintain productivity and reduce the risks associated with extreme summer conditions.

High Heat and Humidity

Heat becomes even more difficult to manage in a greenhouse when humidity increases. When one heats the air in a greenhouse, relative humidity initially decreases because warm air holds more moisture. However, absolute humidity (the amount of water vapor in the air) stays the same unless the moisture is removed (often through ventilation or dehumidification). The dew point is the temperature at which air becomes saturated and water vapor condenses into a liquid. If the greenhouse air cools below the dew point, condensation forms, yielding wet plants and increasing risk of disease. Evaporative cooling has limited ability to reduce air temperature; therefore, cooling pads are less efficient. Similarly, high humidity decreases transpiration, making it more difficult for plants to regulate their internal temperature, and irrigation timings may need to be adjusted.

Meanwhile, in a low humidity scenario, adding water to the air can help cool the greenhouse more efficiently; there is better dry down in the root zone, and better oxygen exchange and airflow. However, in drier climates it is easier to dry out the root zone; therefore, shade is needed to protect plants.

Cultural Practices

Heat generally increases plant need for water, as higher temperatures lead to higher transpiration rates, allowing the plant to cool itself. The plant also needs water to maintain turgor pressure to stay upright. This water is being drawn from the substrate, drying it out. Growers must pay attention to the fine line between dry down and dry out. Peat moss is known for its ability to hold water; however, it is less well known that it is very hydrophobic once the material has become completely dry. Having materials such as wood fiber can help combat dry peat’s hydrophobicity. However, a keen eye and more precise irrigation scheduling are of utmost importance during this time. On another note, if using drip irrigation, hot water in the drip lines can damage roots, so caution is recommended if drip lines are in the sun or have been sitting for too long.

High heat can also affect fertilization and microbial activity. Organic fertilizers cannot be converted into inorganic forms that plants can absorb as fast as they can under cooler temperatures. On the other hand, controlled release fertilizers release is faster under higher temperatures in used and unused mixes. This can lead to high EC, and when mixed with heat and humidity, will cause the root system to shut down, opening the door to diseases. Similarly, a high EC in substrates that are drying down within the root zone can become toxic and burn root hairs, damaging plants and slowing growth.

Heat-Related Pests and Pathogens

Many insect pests thrive in warm greenhouse environments. Heat accelerates their life cycles, allowing populations to multiply rapidly. Common pests include spider mites, aphids, thrips, fungus gnats, shore flies, mealybugs, scale insects, caterpillars, and leaf miners.

Growers attempting to cool their crops through frequent irrigation may inadvertently keep media too wet, creating ideal habitats for fungus gnats and shore flies. Some pests, like scale and mealybugs, have waxy or hard coatings that make them resistant to standard chemical treatments, especially under summer conditions. Close monitoring and integrated pest management are essential.

Several fungal pathogens become more virulent in high temperatures and humidity. Botrytis flourishes during bloom under tight plant spacing and low light. Powdery mildew thrives in warm, humid conditions, and unlike many fungi, it doesn’t require free water to infect. Rhizoctonia is triggered by temperature fluctuations, while Fusarium and Thielaviopsis are persistent soilborne pathogens that can devastate root systems late in crop cycles. Water molds, such as Pythium, Phytophthora, and downy mildew, are particularly dangerous in warm, wet environments. These pathogens produce motile spores that swim through water and colonize roots, causing damping-off and root rot. They often establish in hydroponic systems, irrigation lines, and water reservoirs. Once present, they are extremely difficult to eradicate.

Heat also encourages the proliferation of weeds, which compete aggressively with crops for water, nutrients, and light. They grow quickly, develop thicker cuticles (reducing herbicide absorption), and produce more seeds under hot conditions. Inadequate weed control can lead to long-term infestations.

The most effective strategy for handling heat stress is preparation. Planning ahead allows growers to schedule irrigation, maintenance, and planting in ways that mitigate risk. Using growing degree days (often known as GDD) can help time crop cycles to avoid the hottest periods. Staff protocols and equipment test runs ensure that everyone and everything is ready when the heat hits. Key practices include optimized irrigation timing, uniform water distribution, and appropriate shading and ventilation. Adjusting pot spacing, pot color, and substrate type can also influence heat retention and root zone temperatures.

For additional information on heat-related pests and challenges, including a thorough look at plant physiology and a heat-related challenges grower checklist, read the original article from Berger Grower Advisor Jack Bobo here.