When we think of running a greenhouse, we often focus on temperature, light, and irrigation. But there’s one hidden factor that plays a massive role in plant health—and it's often underestimated: humidity.
Humidity management is one of the most critical elements in greenhouse operation. If not handled properly, it can lead to plant stress, lower yields, and widespread disease, even if temperature and lighting are under control.
What Exactly Is Humidity, and Why Does It Matter?
Humidity, especially relative humidity (RH), is the percentage of moisture in the air compared to the maximum amount it can hold at a given temperature. For plants, this number is more than a weather detail—it affects their ability to breathe, transpire, pollinate, and stay disease-free.
Too much humidity can cause moisture to build up on leaves, making conditions ideal for fungal diseases like gray mold and downy mildew. On the other hand, low humidity causes plants to lose water rapidly. The result? Leaf curling, dry pollen, and poor fruit set, especially in crops like tomatoes and cucumbers.
Some greenhouse growers in cold regions heat their space in winter to maintain warmth. But as temperature rises, humidity drops fast—often leading to dehydrated plants and flower abortion. This is how humidity becomes a silent stressor, even in a temperature-controlled environment.
What Factors Affect Humidity in a Greenhouse?
Temperature Swings Change Humidity Levels
Warmer air can hold more moisture, which means relative humidity actually drops when the temperature goes up. If you raise the heat in your greenhouse without increasing humidity, the air dries out. In cooler periods, moisture in the air condenses and raises humidity levels, often causing condensation on plants and surfaces.
This balance between heat and moisture is delicate and requires active monitoring—not just a thermostat.
Poor Ventilation Traps Moisture
Ventilation isn’t just about cooling; it’s essential for managing moisture. Roof vents, side vents, and exhaust fans help remove excess humidity and circulate fresh air. Without proper airflow, moist air stays trapped, increasing the risk of fungal outbreaks.
In many modern greenhouses, automated fan-and-pad systems can reduce RH from 90% to 75% in just minutes. Smart systems like the one used by Chengfei Greenhouse (成飞温室) integrate humidity sensors with ventilation controls to respond quickly and efficiently.
Irrigation Method Impacts Air Moisture
Sprinklers and fogging systems may distribute water evenly to plants, but they also increase moisture in the air. If the greenhouse is already humid, these systems can make things worse.
Drip irrigation delivers water directly to the root zone with minimal evaporation. When combined with timed ventilation, it helps keep the air dry while ensuring plants stay hydrated. Growers switching from overhead irrigation to drip systems often report lower disease rates and better yields.
Plant Density Affects Transpiration
Plants release water into the air through transpiration. The more densely you plant, the more moisture gets released, turning the greenhouse into a natural humidifier.
Reducing crop density—even slightly—can help regulate RH and reduce disease pressure. For example, lowering cucumber density by 20% can noticeably reduce fungal issues and improve air circulation within the canopy.
Covering Materials Influence Humidity Retention
Some greenhouse films are excellent at retaining heat—but they also trap moisture. Materials with poor permeability lead to higher night-time RH levels and morning condensation.
In colder climates, using high-insulation film like EVA can boost temperature retention. However, if paired with poor ventilation, it creates an environment that encourages condensation buildup and fungus-friendly microclimates.
How to Control Humidity Effectively?
Use Real-Time Monitoring Tools
Guessing isn't good enough. Use digital humidity sensors and connect them to a smart control system. With real-time data, the system can automatically activate fans or dehumidifiers when RH gets too high or low.
In some agricultural zones in China, automated systems are programmed to turn on fans for 5 minutes whenever RH exceeds 85%. These systems dramatically reduce disease risks by keeping air quality in check.
Adjust Strategies Based on Time of Day
Humidity isn’t constant throughout the day, so your management should adapt.
In the early morning, RH is usually high—ventilation is critical.
At midday, temperature peaks and RH drops—preserve moisture, but don’t overwater.
At night, balance insulation and moisture to prevent condensation and fungal growth.
Some greenhouses schedule automatic roof vent openings at sunrise, close them at noon, and activate thermal screens in the evening. This timed control approach is more effective than manual ventilation all day.
Use Dehumidifiers When Necessary
If ventilation and temperature control aren't enough, mechanical dehumidification can help. Heating and venting moist air is a proven method. Some growers even install heat-assisted dehumidifiers to maintain RH around 65%.
This method is commonly used in high-value tomato production in Japan, where stable humidity means fewer diseases and higher productivity.
Schedule Irrigation Strategically
When you water matters as much as how much you water. Morning irrigation can worsen high RH levels. Instead, schedule irrigation between 10 AM and 2 PM, when air is warmer and drier. This timing reduces lingering moisture and allows humidity to balance naturally.
Don’t Fall for These Common Myths
“If temperature is right, humidity will take care of itself.”
→ False. Temperature and humidity don’t always move in sync.
“High humidity helps plants stay moist.”
→ Not exactly. Excess moisture disrupts transpiration and can suffocate plants.
“No condensation means humidity is fine.”
→ Wrong. RH above 80% is already risky, even if you don’t see water droplets.
Final Thoughts
Controlling humidity isn't a “nice-to-have”—it’s essential for greenhouse success. From smart sensors to timed irrigation and strategic ventilation, every part of your system plays a role.
Managing humidity well means fewer diseases, healthier plants, and higher yields. It's one of the most important steps toward smart, efficient, and sustainable agriculture.
Welcome to have a further discussion with us.
Email:Lark@cfgreenhouse.com
Phone:+86 19130604657
Post time: Jun-26-2025
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