How Does Ventilation Affect Relative Humidity (RH)?

Relative Humidity (RH) refers to the water carrying capacity of air and is dependent on temperature. Cool air will hold less water by volume than hot air. At any temperature, a maximum amount of water (RH=100%) can be held by air. At this maximum, your plant’s ability to expel water “transpiration” will be compromised. If your plants can’t expel water through their leaves, they also stop absorbing water through their roots. Once this happens, the plants will also stop absorbing nutrients which slows growth and puts them at risk of disease and pathogens. 

Quick Tips for Greenhouse Humidity

  • Plants require different humidity levels for different stages of growth. 
  • A young plant will thrive at 70-80% RH 
  • A plant that is well established in the vegetative stage will enjoy 60-70% RH
  • Plants that are in the first 4 weeks of the flowering stage will prosper at 50-60% RH
  • Plaints in their final stages of maturing should be kept at 40-50% RH to minimize risks of pathogens but preserve terpenes

The Heat-Purge De-Humidification Method

The most common method of greenhouse de-humidification is through a heat and purge process. This process is directly related to the concept of relative humidity. To remove the humidity from a greenhouse the air should be heated above 75 and then pulled out with an exhaust fan. Although this seems like a waste of energy, its truly a far better trade-off when comparing the cost of dealing with mold, mildew, or total crop failure.

Greenhouse Dehumidifiers

Greenhouse dehumidifiers are another option for controlling humidity in your greenhouse. There are many options, but units are costly.

Quest Dual 105 Overhead Dehumidifier

  • May be placed overhead or at ground level
  • Energy-efficient greenhouse dehumidifier performs at over 8.8 pints per kWh (compared to common 3.8 pints/kWh)
  • Optional condensate pump and ducting kits
  • MERV-11 filtration is standard on these units, capturing particles (including mold spores) down to 1 micron in size

greenhouse dehumidifier system

Greenhouse Humidity Management

Greenhouse humidity, like heat, can exceed optimal crop production levels. Much of the humidity in a greenhouse comes from the plants. At night, plants open pores in their leaves called stomata to release oxygen (the waste product of photosynthesis). While releasing oxygen, some water escapes the plant as well and is released into the environment.

The outdoor temperature and dew point will also play a role in humidity and condensation formation. When air is cooled to a particular temperature (the dew point), it can become saturated with water. If that air cools further, condensation forms. Some plants do thrive in high humidity, but excessive humidity encourages the growth of pathogens like mold, mildew, and fungus8. 

Greenhouse Humidity Reduction

Managing humidity in the greenhouse is very important for preventing pathogens from infecting plants. Humidity related diseases like Botrytis and powdery mildew are at their peak in spring and fall. The sunny days of spring and fall increase moisture release from transpiration in plants and warm air holds the water vapor until the air cools in the evening, causing it to turn back to water droplets. Pathogens in the environment can travel to plants through droplets of condensation so minimizing it is key.

Practices to Reduce Greenhouse Humidity

  • Have floors that drain well and clean up any standing water, keep the greenhouse as dry as possible
  • Avoid watering plants excessively, to do this water early in the day to make sure the surface of the plants is dry by the evening
  • Space plants far enough apart that there will be good airflow through the canopy where humidity is highest
  • Plant leaves should move slightly when fans are running, this signals that there is enough air circulation
  • Plants in warmer temperatures can tolerate more humidity. Ideally, relative humidity is kept near 83% at 50°F, 89% at 61°F, 91% at 68°F, and 95% at 86°F
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