Photos courtesy of Dr. Greenhouse
Vapor Pressure Deficit (VPD) is a measure of how much more water vapor air can absorb at a given humidity and temperature. A low VPD indicates the air can’t hold much more water vapor, and a high VPD indicates that the air can hold a lot more water vapor. VPD combines information from relative humidity and temperature into a single number.
Many gardeners are more familiar with the related term “relative humidity” (RH), which is often expressed as a percentage of how much water vapor is in the air versus how much water vapor air can hold. For example, 50% RH describes air that holds half as much water vapor as it can hold. At 100% RH (the dew point), the air is saturated with water vapor, and any more will precipitate out as dew, condensation, or rain. One issue with relative humidity is that it changes with temperature. Hot air holds more water vapor than cold air does. Air with an RH of 55% at 65 degrees Fahrenheit is in the right ballpark (VPD of .95), but 55% RH at 75 degrees (VPD of 1.33) is dry enough to be stressful.
Vapor Pressure Deficit calculations describe the difference between water vapor pressure of the plants and the water vapor pressure of the air. Vascular plants (like cannabis) take in water through the roots, which travels through the xylem (tubes that carry water up through the plant) to openings in the bottoms of leaves called stomata where it evaporates into the air in a process known as transpiration. If there is too much water vapor in the air (too low of a VPD) then transpiration is slowed and conditions are favorable for fungal pathogens or other moisture loving problems to develop. If there isn’t enough water vapor in the air (too high of a VPD), then the transpiration can accelerate to the point that the plant will close the stomata to try and conserve moisture (to prevent drying out).
One benefit to using VPD is that the “sweet spot” for any growing temperature or RH can be found. The calculations involved to convert RH and temperature to VPD can be daunting, but fortunately there are charts and apps that will simplify the process considerably, such as those found at doctorgreenhouse.com. They recommend a VPD of .80 to .95 for vegetative growth, .96-1.15 for flowering and 1.16-1.35 for stress conditions. Dr. Greenhouse Inc. is a Sacramento-based agriculture and mechanical engineering design firm providing state-of-the-art HVAC design and controls solutions for indoor grows. Enter the RH and temperature into the app and it gives you VPD, along with charts. Luckily for us, Dr. Nadia Sabeh, president and founder of Dr. Greenhouse and a recognized subject matter expert in controlled environment agriculture (CEA), agreed to answer a few questions on the subject. Here are her responses, lightly edited.
Where or how is the best place to take the temperature?
If we’re specifically talking about leaf temperature, or canopy temperature, then typically the temperature is measured on the upper surface of the leaf. A grower might use a handheld infrared thermometer and take measurements by hand of several leaves on the same plants, preferably from sort of top to bottom, as well as in different locations within the room to give an idea of what the overall canopy temperature is.
Some growers have an infrared camera that is positioned somewhere above the canopy, pointing at a certain location within the room. And that infrared camera then takes images and can sometimes feed it back to the monitoring and control system. For high-tech indoor and vertical farms, sometimes that infrared camera might even be on a track or boom, where it’s actually moving through the room and getting an average or identifying potentially where there are hot spots or heat stress in specific locations of the room.
But I will say that most commonly air temperature is used as a proxy for measuring leaf temperature. Leaf temperature would give you a more direct and accurate measurement of the leaf VPD and canopy health but it can be impractical as it’s a laborious effort.
So air temperature is a good substitute if you know that in general, the canopy or leaf temperature is usually one to two degrees lower than what the air temperature reading is.
When transitioning from vegetive growth to flowering, the suggested VPD increases. This can be achieved by changing either the temperature or humidity. How does one determine which is easier and/or more cost effective to change?
Raising the temperature of the room can be a lot easier than decreasing the relative humidity of the room. What it basically means is that if you let your temperature increase from say 78 degrees to 82 degrees, then you need less cooling in order to achieve that same temperature for the lights and the plants and everything that’s going on in the room. However, because so many cooling systems also dehumidify, if you allow that temperature to rise by performing less cooling, you will also inadvertently get less dehumidification from that air-conditioning system.
Lowering the air temperature means we also need to reduce the relative humidity and the moisture content of the air enough that we have a higher VPD. Just like allowing the temperature to rise reduces how much dehumidification our cooling system does, by reducing the air temperature and doing more cooling, we also remove more moisture because those coils are working harder to remove the heat from the air.
If you are going to let the temperature rise, or if you are going to drop the temperature late in the flowering stage, you may need additional dehumidification equipment to help boost that dehumidification you don’t get when the air conditioner is working really hard at low temperatures, or not hard enough at higher temperatures.
VPD is helpful in optimizing transpiration conditions. How important is it to maintain during dark periods?
I would say the jury is out, at least for cannabis. We know that there are some crops, such as strawberry, where having a really low VPD, basically zero, can help with the translocation of nutrients and minerals to the strawberry fruiting bodies, making them sweeter and prettier. We don’t necessarily know that’s true for other crops, including cannabis, and I would say, for cannabis, the risk of having such a low VPD is obviously having condensation on the flowers and mold growth and quality detriments, but there might be some benefit to having a lower VPD at night, which means a higher relative humidity.
The charts used on the website list values down to the hundredths, in a real world setting with normal operating fluctuations, how much variation would be considered acceptable?
This is a very good question. I love it when people recognize the validity or invalidity of too many significant digits. Our chart is based on pressure units called kilopascals, and if we were in pascal, we would multiply everything by 1000. I don’t think having values down to the hundredths offers much useful information. Down to the tenths, potentially. We really want to be looking at a minimum to the tenths, which would be 1.0, 1.1, 1.2.
Also, this question is good, because a lot of people put maybe too much stock in those extra decimal points and really, HVAC equipment can’t control to that level of precision. Your plants probably also are not responding to those minuscule changes or minuscule variations in the temperature and humidity environment. Really, it’s more important to set a target and be plus or minus 0.05 or 0.1 kilopascal to maintain consistent crop growth and development.
The charts emphasize Vegetative, Flowering and Stress. Is there a time period when it may be beneficial to stress a cannabis plant in this way?
The stress vapor pressure deficit levels that we have in our charts, and that you can calculate with our calculator, is usually focused on the last two to three weeks of the flowering cycle so it produces quality characteristics that we as humans find very attractive.
I will also say when we’re talking about cannabis, that most growers want to raise the VPD in that late flowering stage and also reduce the air temperature in the room. The most common reason I hear for doing that is because at lower temperatures you’re sort of simulating that late fall weather when the plants are getting more stressed, and they start producing more trichomes, more terpenes, and more coloration.
Grubbycup was raised on an organic dairy farm and has 40 years experience with personal use and larger gardens. Articles he has written have been translated into seven different languages and published in a variety of countries around the world.