With rain and cloudy weather returning, it is good to review methods for reducing humidity in greenhouses. The following is a short article on the subject from the New England Greenhouse Update.
Cool, cloudy or rainy weather creates an ideal environment for Botrytis infections during production in a full greenhouse.
Once Botrytis develops, it cannot be effectively controlled with fungicides alone. The key to suppressing Botrytis is to keep the plant canopy dry especially from dusk until dawn. This can be accomplished by managing the greenhouse environment.
Ventilation allows the exchange of moist greenhouse air with drier air from outdoors. Heating is necessary to bring outdoor air up to optimum growing temperature, and also increases the capacity of the air to carry moisture, thus avoiding condensation. Neither practice alone is as efficient as both combined.
The method and time it takes for heating and venting will vary according to the heating and ventilation system in the greenhouse. To vent the humid air in greenhouses with vents, the heat should be turned on and the vents crack open an inch or so. When doing this the warmed air will hold more moisture (RH), escape from the greenhouse through the vents and be replaced with outside air of lower RH. This natural rising of the air will result in a greenhouse of lower relative humidity.
In houses with fans, the fans should be activated and operated for a few minutes and than the heater turned on to bring the air temperature up. The fans should then be shut off. A clock could be set to activate the fans. A relay may be needed to lock out the furnace or boiler until the fans shut off so that both the fans and heating system do not operate at the same time and flue gases are not drawn into the greenhouse.
The venting and heating cycle should be done two or three times per hour during the evening after the sun goes down and early in the morning at sunrise. The time it takes to exchange one volume of air depends on several factors including whether or not fans are used and, the size of the fans and vents. For some greenhouses it may take as little as 2-3 minutes air exchange. For greenhouses using natural ventilation, it may take 30 minutes or longer. Heating and venting can be effective even if it is cool and raining outside. Air at 50°F and 100% RH (raining) contains only half as much moisture as the greenhouse air at 70°F and 95% RH.
Information from Tina Smith, University of Massachusetts and Leanne Pundt, University of Connecticut in the New England Greenhouse Update: http://www.negreenhouseupdate.info/greenhouse_update/?p=2569