This post is primarily for greenhouse growers. Botrytis can be especially tricky problem for greenhouse growers to solve.
When I first started growing, I didn’t realize how important a basic understanding of physics would be. Much credit goes to Plant Empowerment for educating me on this topic. If you’re serious about mastering climate control, I highly recommend reading Plant Empowerment – The Basic Principles. Prepare to be overwhelmed, but the insights are invaluable if you can push through.
Heat Emission: How Plants Exchange Energy with Their Environment
Plants are constantly balancing their internal temperature with their surroundings by either absorbing or emitting heat. This process becomes especially critical in controlled environments:
- In greenhouses, walls and roof coverings experience significant temperature fluctuations, making heat exchange a bigger challenge.
- In indoor grows, heat emission is still a factor, particularly in cold climates where insulation may not be ideal.
The Science: Why This Matters
- When lights are on (or the sun is up), plants usually absorb heat.
- When lights go off (or the sun sets), plants release heat toward the coldest surface, such as a grow room ceiling, greenhouse roof, or a poorly insulated wall.
The Problem: Hidden Microclimates
If a plant’s heat is drawn toward a cold surface, like an uninsulated wall or a clear night sky, it creates a localized temperature drop at the top of the plant. This can push that area into the dew point, even if your environmental sensors show that room humidity is well below 100%.
Most growers think of microclimates as stagnant air pockets under tables or in room corners. While those are important, heat emission driven microclimates are more dangerous because they occur at the plant’s head, right where flower sites develop.
The Solution
- Identify cold spots using thermal cameras or an infrared thermometer.
- Prevent heat loss by improving insulation (e.g., adding thermal screens in greenhouses or fixing poorly insulated indoor grow walls).
- Optimize airflow with a combination of horizontal and vertical fans. Maintain a steady airspeed of 0.5–1.5 m/s to prevent localized cooling effects and to keep transpiration moving after lights out.
- Control temperature layering by monitoring vertical air stratification—warmer air rises while cooler air sinks, so mix the air evenly.
Key Takeaway
Even with RH well below 100%, cold surfaces can force condensation onto buds. Don’t just monitor air temperature, pay attention to surface temperatures as well. A well-insulated grow space and properly managed airflow can help prevent these dangerous microclimates.
Added Note from a private chat with Peter Van Weel – One of the authors of Plant Empowerment: The Basic Principles
- He has found that one of the primary causes of Botrytis in Greenhouse production is guttation inside the flower head. At the end of the day, when energy from the sun/lights is taken away, the temperature will often drop sharply. The difference in temp between the head of the plant and the roots creates a high internal pressure in the veins. In combination with the sudden drop in crop transpiration this results in plant sap escaping at the end of the vessels (veins) which is inside the flower. That sap is not visible but sits inside the flower for a long time, resulting in botrytis spores to grow.
The Psychrometric Chart: A Grower’s Secret Weapon
The psychrometric chart is one of the most powerful tools in climate management, showing how temperature affects air’s ability to hold moisture.
You don’t need to memorize the physics behind it, but understanding its key principles can help prevent major environmental issues.
The Science: How Dew Point is Reached
- Warmer air holds more water vapor. As temperature increases, relative humidity (RH) decreases if no extra moisture is added.
Further Explanation:
Think of the temperature and relative humidity like a cup filled with water. Lower Temp = Smaller Cup, Higher Temp = Bigger Cup. Same amount of water in both cups but the water represents a smaller portion of the total volume in the bigger cup. Hence, higher temp + same amount of water in the air = lower relative humidity.
- Colder air holds less water vapor. A sudden temperature drop can push the air to its dew point, causing condensation.
Example: Why Temperature Drops Create Issues
At 84°F and 75% RH, your vapor pressure deficit (VPD) is 1.0 kPa—ideal for plant growth. But if the temperature drops just 10°F without dehumidification, the dew point is reached, leading to condensation.
This is why integrating dehumidifiers with your cooling system is critical. If AC is running but dehumidification can’t keep up, RH spikes and condensation forms—creating the perfect environment for Botrytis (bud rot).
The Solution
- Raise temperature if needed to prevent humidity spikes when dehumidification struggles.
- Ensure cooling systems trigger based on humidity, not just temperature. If humidity isn’t falling fast enough, stop cooling to let the humidity catch up.
- Maintain consistent airflow to prevent stagnant pockets of cold, humid air.
Key Takeaway
Monitor temperature drops closely—especially at night. Even if your overall humidity is in range, localized cooling can still hit the dew point, creating an ideal breeding ground for Botrytis. If you have a day night temperature differential set, be sure to stage your nighttime cooling for a slower ramp to avoid unbalanced plant cooling (head cooling faster than the rootzone). If necessary in a greenhouse, add gentle heat after lights out to stabilize temperature changes.
