How to Maintain Humidity inside a Greenhouse?

The role of humidity in a greenhouse

Greenhouses have been long used by farmers and growers to extend the growing season of various crops or flower to year-round. From azaleas to zinnias, from cannabis to cucumbers, greenhouses are also used to grow plants that would otherwise never survive in a particular climate or geography. We often think of greenhouses are special structures that keep plants warm, but equally important, they help ensure proper humidity too.

Usually, when you walk into a greenhouse, you are blasted with warmer and thicker air, due to the added humidity from having so many plants in a confined area. There are times however when humidity may be too high or too low and it needs to be moderated back to a normal greenhouse range for plants to thrive.

It is essential for greenhouse growers to monitor humidity with tools that help them determine if action is needed to stay out of the extremes and keep humidity within range in the greenhouse.

It’s all relative—relative humidity, that is

We use general words like stuffy, muggy, dank and so forth to describe the climate in a greenhouse, but to get precise greenhouse humidity readings, the key metric to monitor is relative humidity or RH. This term is used to describe the percentage of actual water vapor in the currently in the air compared to the maximum amount of water vapor that could exist at a given temperature. (Absolute or specific humidity is a measure of humidity in the air irrespective of temperature.)

That connection to temperature is key: if temperature goes up or down, the air can hold more or less moisture, respectively. Air at 70°F with 60% relative humidity is actually less humid than air at 80°F and identical RH because the air’s total capacity for moisture is lower.

Monitoring humidity is vital for greenhouse plants

Knowing the temperature/RH relationship is important when making adjustments in a greenhouse to maintain humidity. Plants love humidity. Not enough and plants falter with slowed growth. Too much humidity and you may increase risks for mold growth or other pathogens. Plus, plants aren’t as able to absorb some vital nutrients like calcium when humidity is excessive.

Larger, commercial greenhouses use sophisticated, automated systems to monitor and change humidity without manual intervention. For smaller greenhouses, having the right tools can be just as effective.

Monitor, monitor, monitor with a humidity meter

If the goal is to ensure relative humidity remains at a healthy 50-70%, one key device is to monitor RH is a humidity meter. Do I really need a humidity meter to do that? Human perception of humidity is limited to dramatic changes. Small changes over time may not be noticed and can lead to critical conditions that are missed. Humidity meters can be handheld units that can be taken anywhere to measure ambient humidity and temperature. They are sometimes called hygrometers, hygro-thermometers or psychrometers, depending on added functions they have (hygro- is a prefix meaning moisture). Some hygro-thermometers are meant to be placed on a tabletop or shelf surface or hung on a wall. They continually monitor and display humidity and temperature readings. Some include jumbo-sized digits to read from a distance.

Identifying greenhouse trends over time with datalogging and alarms

Among both the handheld and stationary kinds, some meters also serve as dataloggers, to record readings at regular intervals to keep track of humidity and temperature in a greenhouse. This might help identify how external temperature changes impact internal temperature and humidity, so that interventions can be planned with increased ventilation for example.

To make it easier for immediate interventions, some humidity meters and hygrometers also have High and Low alarms with visual and audible alerts to notify you if conditions have exceeded normal thresholds.

Plan a response for high greenhouse humidity

Imagine coming into the greenhouse and you hear a beeping from the back left corner. You walk over and see that the hygro-thermometer on your potting bench is beeping with a HIGH alarm flashing. It may help to increase ventilation and reduce the temperature of the greenhouse.

Open windows, skylights and doors to increase airflow and accentuate that with fans if needed. Air exchange is generally important but can be a fast aid in a high humidity situation. Keep in mind that this approach may require close monitoring of humidity and temperature, especially in colder climates.

Another alternative is to use a high-volume greenhouse-grade commercial dehumidifier, typically with over 100-pint capacity.

Be ready for low greenhouse humidity

Greenhouses can also get dangerously dry. Consider this: a grower is heating the greenhouse overnight. When the sun rises and bumps the temperature up, fans turn on and relative humidity drops dramatically. Humidity meters or monitors with high and low alarms are useful for alerting growers to this condition and override any automatic equipment.

Depending on the size of the greenhouse, a grower can start with increased misting of plants and if soil conditions warrant, increased watering to raise overall RH. Smaller greenhouses can benefit from a domestic-grade humidifier or vaporizer to introduce moisture. For a faster result, wetting the floors down is a helpful way to raise humidity. Spraying or wetting a floor introduces moisture over a very large surface making it easier to raise RH quickly through evaporation. Evaporative cooling systems that use moisture to wick away heat may also help raise humidity.

Humidity meters: your 24/7 greenhouse sentinels

Hands-on growing in a greenhouse can be very rewarding but it takes a lot of work too. Monitoring and responding to ambient conditions like temperature and humidity play a big role in a greenhouse’s success. But you can’t do it alone. It’s a no brainer to use humidity meters or hygrometers to put your mind at ease with a bigger picture of relative humidity conditions. They can alert you when things go amiss, almost like 24/7 sentinels for your greenhouse.