Greenhouse humidity control is a challenge every greenhouse operation faces. When things get out of hand, the plants usually pay the price.
Plants don’t transpire effectively in high-humidity environments. They struggle to move water and nutrients through their systems.
Condensation can form on glazing, leaves, and other surfaces. It interferes with light transmission, drips on crops, and opens the way for fungal diseases like botrytis and powdery mildew to take hold.
Plants dehydrate when the humidity is too low. The stress they experience interferes with their growth and development, resulting in yield loss and poor plant quality. Plants also lose turgor (the pressure of water inside a plant cell) and may show signs of wilting or drooping despite a good watering.
Managing humidity and condensation effectively in your greenhouse starts with understanding the relationship between relative humidity, temperature, and vapor pressure deficit.
Greenhouse Humidity Control at a Glance
Successful greenhouse humidity control is about balancing temperature, relative humidity (RH), and vapor pressure deficit (VPD).
Together, these factors determine how efficiently plants transpire and how likely condensation and diseases are to develop.
In this article, you’ll learn:
- The relationship between RH, temperature, and VPD
- How humidity affects plant growth and disease development
- Eight practical ways to control greenhouse humidity
How RH, Temperature, and VPD Interact
Here’s a simple way to think about temperature and relative humidity:
- Temperature=how hot or cold the air is
- Relative humidity (RH)=how full the air is with moisture
When temperatures rise, RH falls (and vice versa), even if the amount of moisture in the air stays the same. This occurs because air expands as it warms up. RH drops because warm air can hold more moisture.
When the air can’t hold any more moisture, the excess water appears as condensation on leaves and other surfaces. The temperature at which this occurs is the dewpoint.
VPD measures how strongly the air pulls moisture from plants, and it is a more accurate way to determine how humidity affects your crops.
When VPD is too high, it’s an indication humidity is too low. Plants lose more water to transpiration than their roots can pull in. In response, they close their stomata, which means they can’t take in CO2. This interferes with photosynthesis.
Low VPD levels cause plants to transpire less. They struggle to move nutrients through their systems. Deficiencies can develop that affect plant growth and yields. There is a greater risk of mold and other plant diseases due to the high humidity.
It’s important for greenhouse humidity control to know how to adjust variables such as temperature and humidity in response to VPD to optimize plant growth and yields.
To raise VPD, you must increase temperature or decrease RH. To lower VPD, decrease temperature, or increase RH.
VPD tells you how thirsty the air is around your plants. Lower humidity means a stronger pull; higher humidity means a gentler pull.
8 Strategies for Greenhouse Humidity Control
There are several approaches to humidity control in greenhouses:
1. Keep Air Moving
Opening your roof and side vents allows warm, humid air inside the greenhouse to exchange with cooler, drier air outside.
When warmed, the cool, dry air absorbs moisture. That’s why one strategy growers use to lower RH is to open the greenhouse vents and turn on the heater(s) to warm the incoming air and increase its water-holding capacity.
Your vents need to be large enough to promote good air exchange to control moisture, prevent disease, and strengthen your plants.
Exhaust fans help remove hot, stale air. Adding HAF or circulating fans can keep air moving when passive ventilation just isn’t enough.
2. Avoid Overwatering
Cultural practices can go a long way toward greenhouse humidity control.
Avoid overwatering and late-afternoon or evening irrigation. Give your foliage time to dry out and excess moisture time to escape the greenhouse before nightfall. Overwatering also leads to soggy soil, which raises humidity.
Drip irrigation is a great way to deliver water to the plant roots while keeping the foliage dry.
Stale, dead-air pockets can form when plants are spaced too close together, with their canopies overlapping. Proper spacing allows air to flow freely around them.
Standing water evaporates, adding moisture to the air. Keep an eye out for problem areas. Floors and other surfaces should be kept dry.
3. Add Shading
When ventilation isn’t enough to control temperatures on its own, shade curtains are a low-cost way to boost your cooling.
Shade systems come in different percentages that tell you how much sunlight the fabric blocks and can be customized to fit your greenhouse.
While it’s difficult to apply them uniformly, shading compounds can be brushed, rolled, or sprayed onto glazing in spring to help with cooling, but be sure to remove them by the first frost.
4. Use Anti-drip Poly
Condensation is a sign of high humidity. It’s important to control condensation because condensed water on the glazing reduces light transmission and can cause damage when it drips onto plants.
Anti-wetting agents, or surfactants, on the underside of poly reduce water droplets by spreading moisture into thin sheets that drain down the sides of the greenhouse.
Just be sure to install this type of poly correctly with the coated side facing toward the inside of the greenhouse.
Moisture in the air condensates more quickly on single-layer poly than on double-layer poly because it gets cold faster.
Inflating the air space between two layers of poly film acts as insulation, helping the inside surface stay warmer and reducing condensation.
Getting the correct roof pitch is also important for humidity control in greenhouses with other coverings to encourage moisture runoff without a wetting agent.
5. Run a Mist or Fog System
Fog and mist systems can be used to increase humidity in dry environments and reduce air temperatures.
High-pressure nozzles spray ultra-fine water droplets into the air, which increase moisture and provide cooling when they rapidly evaporate.
6. Put Evaporative/Fan and Pad Cooling to Work
Growers have used fan-and-pad systems for many years to provide evaporative cooling in greenhouses. With this method, fans draw air through wet pads to cool and humidify it.
Evaporative cooling uses heat to turn water into vapor. The process absorbs a large amount of heat, which is then exhausted from the greenhouse and replaced with cooler, drier air from outside.
Because evaporation is the driving force, the efficiency of fan-and-pad systems is determined by the outside humidity. Lower humidity allows for a greater reduction in temperature. Higher humidity won’t allow for quite as much, but evaporative cooling can still have great benefits.
7. Integrate Your Climate Control Systems
A climate computer automatically manages humidity for you.
After collecting data from sensors measuring humidity, light levels, CO2, etc., it compares this information to setpoints you enter. Then it sends commands to activate fans, vents, and heaters to adjust the environment as needed.
8. Try a Dehumidifier
You can use a dehumidifier for greenhouse humidity control to remove excess moisture from the air. These units can come in handy for tightly sealed greenhouses, such as indoor grows, where humidity can rise quickly.
They come with various capacities and can be installed as a floor mount or suspended from the ceiling.
Frequently Asked Questions
What is the ideal humidity for a greenhouse?
The ideal relative humidity (RH) depends on the type of crop and its stage of growth. In general, most recommendations advise maintaining relative humidity ranges from 65% to 70% through the night and 80% to 90% during the day for healthy plant growth.
Monitoring vapor pressure deficit (VPD) gives you a more accurate picture of how humidity is affecting your plants than RH alone.
Why is my greenhouse so humid?
High humidity is often caused by:
- Overwatering
- Poor ventilation
- Crowded plants
- Standing water
- Poor air exchange
Cold weather can also increase condensation when warm indoor air comes into contact with cooler glazing and other surfaces.
How do I lower humidity in a greenhouse?
The fastest way to lower humidity is to exchange humid indoor air with drier outdoor air through roof vents, side vents, or exhaust fans. Heating incoming air can also help because warm air can hold more moisture.
What causes condensation in a greenhouse?
Condensation forms when warm, moist air cools to its dew point. Water vapor changes into liquid water on cool surfaces such as glazing, structural members, or even plant leaves.
Is VPD more important than relative humidity (RH)?
Relative humidity tells you how much moisture is in the air compared to its maximum capacity. VPD goes a step further by showing you how strongly the air pulls moisture from the plants. Many growers use VPD to make irrigation and climate decisions because it better reflects how plants respond to their environment.
Can high humidity damage plants?
Yes. Excess humidity can be very damaging to plants. Specific consequences of high humidity include:
- Reduces plants’ ability to ability to take in CO2 for photosynthesis
- Gives fungal and bacterial pathogens such as botrytis and powdery mildew a place to breed
- Condensation damage (water dripping on plants)
- Disrupted transpiration (slows down the release of water vapor)
- Opens the door for pest infestations
How can I increase humidity in a greenhouse?
If the air in your greenhouse is too dry, misting or fogging systems can add moisture while helping to cool the greenhouse.
Reducing ventilation for short periods may also increase humidity, but conditions should be monitored carefully to avoid creating disease problems.
Whatever you do, don’t let poor humidity management hold your crops back from reaching their full potential. We can help. Give us a call today.
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