Climate control in a greenhouse can be very challenging. It is certainly a step up from traditional outdoor farming but the moment the decision is made to grow in a greenhouse, we are accepting the lack of true environmental control like we can have in indoor gardens. We accept that we will tolerate a range of temperature and humidity conditions that the ambient weather, in a particular location, and by season will allow us. For this reason there is not a “cookie cutter” conversation to be had. Your range of conditions and the associated challenges are going to be very different from Seattle, to Albuquerque, to Toronto, to Orlando.

So, why is any of this important? Ideal temperature and humidity allows plants to grow vigorously while discouraging pests and fungus. These ideal conditions allow plants to grow better, yield more, and have a higher quality. Too dry and we have slow growth rates and risk nutrient burn. Too wet and plants won’t uptake nutrients/water and become susceptible to mold problems. This is the essence of Vapor Pressure Deficit (VPD), when your air has just the right temperature and humidity to support ideal transpiration rates.

For the most part, greenhouse climate control centers on ventilation. Too hot, ventilate; too humid, ventilate; but then we have to accept the ambient air condition of the make-up air that is pulled into the structure on the back end. In some of these areas and at certain seasons, you have to ask, “What value does it provide bringing in that ambient air with extreme temperature or humidity profiles?”

Most common efforts to control heat are with the use of evaporative coolers, pad and fan, or sometimes known as a “wet-wall”. Cooling occurs through the temperature drop associated with evaporation. However, evaporation, is very dependent upon relative humidity of the ambient air. On a 90F degree day, you might get a 23F (7C) degree drop in temp if you are in 30% RH Albuquerque, NM; but only an 8F (4C) degree drop if you are in 70% RH Orlando, FL. As the sun sets and temperatures fall at night, we begin to hit dew point conditions and water vapor condenses on leaves and flowers. The unintended consequence is that through this evaporative cooling, we’ve also ADDED a lot of humidity into our greenhouse and provided a more ideal environment for mold. Using Quest dehumidifiers at night can help control your humidity and break the cycle required for mold to propagate.

In norther climates, during winter months, we need to add heat to our greenhouses. This is often done with a radiant boiler system to protect temperatures at root zone and with indirect fired heaters to address the rest of the air in the greenhouse. Where this falls short is that we still have to remove the moisture of transpiration. If we ventilate, we may be bringing in some very dry air. For example, a winter day in Toronto may be 21F (-6C) degrees and 80% humidity. By the time we bring that air into our greenhouse and heat it to 60F (16C), it is now only 18%RH. This is far too low for healthy transpiration. Quest dehumidifiers can help control your humidity at ideal levels and reduce the costs of heating the outside air you are bringing into the structure.

How do we get closer to true climate control while getting the low cost advantage of greenhouse growing? Enter the hybrid greenhouse model. The hybrid model uses the best of both worlds for low cost production with a unique set of “if-then” scenarios. By having the equipment in place to utilize low cost, passive, climate control; the hybrid greenhouse focuses on using active ventilation and evaporative cooling during hot days when conditions are right. At night, or when conditions are particularly oppressive, Quest dehumidifiers and possibly mechanical cooling can be used to offset heat and moisture loads in a very controlled manner.  

Quest Dehumidifiers has been committed to bringing the most energy efficient and reliable equipment to the indoor and greenhouse gardening community. With lower operating costs, Quest may provide you the return necessary to justify the move to a hybrid greenhouse model.

For more information:
Quest Climate
4201 Lien Rd.
Madison, WI 53704