What is VPD? Vapor Pressure Deficit, VPD, is an indicator of the evaporation potential of water into the air. It’s a measurement that’s defined as the difference, or deficit, between pressure exerted by the moisture present in the air currently and the pressure at saturation. It’s one of several different methods that you can use to evaluate crop stress or water stress within a plant.
VPD is VPD. That’s not really gonna change. However, the ability to control the VPD will change. Solar radiation, wind leakage, the weather; all of these things affect the VPD. Controlling the temperature and humidity of the environment lets you change the VPD.
In a controlled room we effectively have control over everything. In a greenhouse, we don’t have control over the sun and its input. You can put in blackout curtains, and things like that, but there’s still going to be effects from heat in the surrounding area.
My opinion is that it would be easier to control for VPD in a closed environment because I have all the control and equipment I need, as opposed to a greenhouse where I have less control and I probably have restrictions on equipment. It’s not impossible to put an AC in my greenhouse, although it may not be economically or operationally feasible to do so.
The caveat is that while I do have 100% control in a controlled environment, it’s going to cost me. For example, I’ve have my air conditioner which can dehumidify and cool. If I want the temperature at 65 or 70 degrees, my air conditioning removes humidity. But if I want 60% humidity, I have to put the humidity back in somehow. You end up fighting yourself to get the right conditions. You have to buy extra equipment and the electrical costs add up over time.
All of this can occur in a greenhouse too, but you’re generally not going to stick an air conditioner in a greenhouse. Here in Tucson, when we go into monsoon season, humidity can jump drastically. If you’re using a swamp cooler, suddenly your cooling goes kaput. VPD control has gone out the window.
These are considerations that cannabis growers have to think about. When you’re inside, your problem is primarily equipment costs and operational issues like electricity, labor, and water. In a greenhouse, factors beyond your control may occur and your perfect VPD can get thrown out the window.
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