Humidity

During Cutting Establishment

At the time of transplanting, providing supplemental humidity is critical for limiting stress to the cutting and good cutting establishment. Misting the potted cuttings 3 to 6 times a day will help the cuttings make the transition from the propagation benches to the growing benches. Shading (from 25% to 50%) will also help the plants acclimate to the new higher light and lower humidity environment.

During Production

Humidity is one aspect of the greenhouse environment that we tend to ignore, and many growers could benefit from paying closer attention to humidity control. High relative humidity reduces evapotranspiration in plants. This means the plants take up less water, less nutrients are transported to the shoot in the transpirational stream, and less water is removed from the pot. High levels of relative humidity during October and November have been implicated as a contributing factor to leaf loss. High relative humidity has been implicated as a contributing factor in bract necrosis, possibly relating to less calcium being accumulated under very humid conditions, since calcium travels in the transpirational stream. High relative humidity increases the chances for Botrytis. High humidity reduces evaporation of water off of leaf surfaces and increases the problem with condensation and dripping; both favor Botrytis. Ideally for the second half of the production season, humidity should be less than 70% and good air circulation should be provided.

Bract edge burn of poinsettias.
Click image for larger version. (image ©Brian Whipker, NCSU)

Botrytis infection of bracts.
Click on image for larger version. (image ©Brian Whipker, NCSU)

Venting

Evening venting can help reduce relative humidity at a very low cost to the grower. As cool air is warmed, the relative humidity (RH) will be decreased because warmer air can hold more moisture than cooler air.

As an example, assume it is 40°F outdoors and raining; RH outside is 100% (there are 37 grains of water per pound of air given these conditions). It is 65°F in the greenhouse and RH inside is 90% (there are 83 grains of water per pound of air). Exhaust fans are turned on and vents are opened to exchange half of the volume of air in the greenhouse, and the greenhouse is heated back to 65°F. The resulting RH is 65% (there are only 60 grains of water per pound of air). How much did the venting and heating of the outside air cost? Our example greenhouse has a volume of 39,348 ft³ (it is an 86 ft x 36 ft Quonset style greenhouse). Therefore, 19,674 ft³ of air must be vented, replaced with 40°F air, and heated to 650F. For the temperature range of a greenhouse, it is safe to assume that 1 BTU of energy can raise approximately 52 ft³ of air 1°F. The total BTU output required to heat the air in this example is [19,674 ft³ ÷ 52 ft³ /BTU/°F] x 25°F = 9,459 BTUs of heater output. Assuming a 70% efficient natural gas heating system, the cost to produce 9,459 BTUs of energy is about 6¢. If we vent the greenhouse every evening from 1 September until 15 December, the cost of heating after venting would be $6.36 for this greenhouse.

If we modify our example to a 20°F outdoor temperature (RH still at 100%), the total cost for heating after venting from 1 September to 15 December is only $11.48. Given this temperature differential, the resulting RH in the greenhouse would be 55% (50 grains of water per pound of air); even lower than the 65% in the previous example. How much would it cost to treat this same greenhouse just once with a fungicide for control of Botrytis? Venting is a cost efficient method for reducing relative humidity in a greenhouse.

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