Humidity and Greenhouse Climate Control: A Brief Introduction

March 9, 2017   

When growing in a greenhouse, humidity can become a serious factor in the success of your crop. Because of dense vegetation packed into limited, enclosed space, plants transpire moisture. This is why humidity tends to run high for many farmers.

First it must be specified that when one refers to humidity, the kind of humidity that is important for greenhouse growers is relative humidity. Relative humidity (RH) is expressed as a percentage of the amount of water vapor present in the atmosphere at a certain temperature, in regard to the amount of moisture the air can optimally hold at that temperature. In other words, relative humidity is the amount of water vapor in the air. Humidity should be managed according to the needs of different crops.

As a rule, when air warms, it expands and has the ability to hold more vapor. As air cools, it contracts, allowing the vapor to fall as water.


Vapor Pressure Deficit is the measured difference between the amount of moisture in the air and how much moisture the air has the potential to hold when saturated. In other words, for the plant, VPD is the shortage or deficit of vapor pressure in the air as compared to the vapor pressure in the leaf. When there is an imbalance of vapor pressure between the atmosphere and the plant, growth and development are affected negatively.

Low VPD leads to low transpiration, restricting the transport of minerals as they move through the plant’s xylem. If Vapor Pressure Deficit is seriously lacking, plants lose the ability to transpire, and the pressure usually released by transpiration builds up inside the plant. Combined with a wet root zone, this can result in guttation, which is when pant pores secrete water.

Low VPD or high RH can cause mineral deficiencies, disease, guttation and soft growth. While high VPD and low RH can result in leaf roll, stunted plants, wilting or crispy leaves.


Transpiration is when the leaves of a plant start to accumulate moisture on the surface. While around 90% of a plant’s water uptake is transpired leaving only 10% of water uptake for growth. It is therefore very important to help create favorable humidity conditions. When transpiration or “sweating” occurs, the plant’s stomata open and release water vapor. To conserve moisture, the stomata close. Stomata open wider with a VDP decrease and high RH, and close as VPD increases and RH lowers. The amount of moisture released by a plant is somewhat self-regulated by the opening and closing of the stomata. Nevertheless, issues can arise with too much or too little humidity.

 In high humidity atmospheres, the plant’s ability to cool itself or transport nutrients and minerals can be limited. Further, moist environments can be a breeding ground for powdery mildew or mold, which gone unnoticed can spread throughout, rotting the fruit and destroying produce. Many plant diseases require moisture on leaves to initiate an infection.  For farmers, the fear of mildew, mold, virus and disease should be a primary concern. Controlling the humidity can greatly minimize the risk of these issues arising. High humidity can be very beneficial during certain growing periods of the plant’s life cycle and disastrous at others.


Temperature and humidity are correlative. As the temperature of the air increases, the relative humidity decreases even though the amount of moisture remains constant. This is because the water-holding capacity of the air doubles with each 20 degree Fahrenheit increase in temperature. So, if your temperature increases from 60 degrees to 80 degrees it can hold twice the moisture at 80 degrees than it had at 60 degrees.

Leaf temperature is a way to measure VDP. Measure the temperature within the plant’s leaf canopy to get a closer reading of the leaf temperature

The rate of photosynthesis is affected by leaf temperature. The leaf temperature should remain above the ambient air dew point to inhibit condensation from forming on the surface.

Downfall of Overhead Water Fall

Water vapor accumulates on the foliage of plants, so try to refrain from watering before closing the greenhouse at night, as humidity is higher during the cooler parts of the day or nighttime.

Avoid overhead watering methods like sprinklers and consider a drip tape system to reduce surface moisture on plant leaves. Space your plants appropriately to circumvent the risk of overlapping canopies, which create a microclimate that is different to the rest of a plant. Water in the mornings, so there is plenty of time before evening falls for additional moisture in the greenhouse to escape.

As a rule, remember that high humidity and low temperature promotes mold, low humidity and high temperature may stretch the growth or slow down growth. Keep in mind soggy soil raises humidity, dehumidifiers pull water from the air and humidifiers put moisture into the atmosphere. If conditions become too humid, plants cannot transpire or breathe. Extreme humidity or lack thereof make pollination challenging.

Control Humidity

Maintaining optimal humidity requires monitoring and adjustments. Cultivating agreeable climate conditions for your greenhouse crops will reduce your risk of pests and disease, which will essentially cut down costs and use of pesticides or fungicides.

If your plants are growing in a dry environment, the greenhouse might benefit from a humidifier to boost the humidity. If you are concerned about the environment being too humid, maintain a humidity range by adding a dehumidifier.

Even if humidity is low or at a safe level, air exchange in your greenhouse is imperative. Use fans to keep air moving, but keep the fan from blowing directly onto the plants. Combine fans with ventilation to create airflow that reduces fast change in temperature difference and creates a gentle buoyant movement in the air. If temperatures are too cool, consider using heat mats to maintain your soil warmth.

To achieve favorable greenhouse conditions, consider what your plant needs during its life cycle. Consider growing seasonally for best results. Invest in a few items that will fit your space and maintain the ideal environment for your plants. A digital thermostat with a humidity meter is a primary tool to keep an eye on your atmosphere. If you would like something a bit more advanced, electromechanical de-humidistats, prewired humidistats or the iGrow 800 and 1400 are tools that can make your greenhouse operation less labor-intensive and time consuming. Take a look through our greenhouse growers section of the FarmTek or GrowerSupply websites to see an array of products available for your environmental control needs.

Contributed by Amanda Williams