Vapor pressure deficit (VPD) is the difference (deficit) between the amount of moisture the air can hold and the amount of moisture in the air at the specific time. VPD is related to temperature and relative humidity (RH), which can be tweaked to create the optimum cannabis-growing environment. For indoor or greenhouse cultivation, understanding and using VPD effectively can lead to healthier plants and boost yields. A relative humidity monitor, an infrared thermometer gun, and a VPD chart are all that's needed, theoretically. But, as with all things cannabis, there are many levels of detail for those who are interested.
What is VPD in plants?
VPD is a mathematical equation dealing with relative humidity. In cannabis cultivation, VPD can also be looked at as the difference between the vapor pressure within a plant and the vapor pressure of the air around the plant. It's important to plants because of transpiration, or the way water and nutrients travel from root to leaf. The process allows plants to cool down, by releasing water, and take in the carbon dioxide they need from the atmosphere. When plants release water vapor, they take in more from the roots, along with whatever nutrients are available. That water makes its way up the plant to the leaves, distributing nutrients along the way. Transpiration is essentially the engine of plant growth and VPD fuels that engine.
Why is VPD important?
A vapor pressure deficit that's too far out of whack is often the cause of unknown plant problems, whereas VPD in the right range for the plants' growth stage creates perfect growing conditions. The proper environment makes a difference in quantity and quality of the cannabis harvest. Understanding VPD and using it to tweak the indoor cultivation environment can be the difference between sickly plants that deliver a mediocre yield or big, robust, healthy plants that reward the dedicated gardener with stellar yields of terpene- and cannabinoid-rich buds free from mold and mildew concerns. Ideal VPD ranges from 0.8 to 1.5 kiloPascals (kPa). But it's better to think in terms of changing the temperature or humidity than the VPD.
How can I use VPD?
Getting back to the infrared thermometer gun, RH monitor, and chart for a second — these are the minimal tools you'll need to employ VPD in the service of cannabis health. Since VPD is the difference between the moisture present in the air and what it can hold at a set temperature, you need to know the temperature of your canopy first. Aim your thermometer gun at the top of the canopy in several spots, then average the numbers to get the canopy temp. You'll also need the RH level, which you can get from your humidity monitor. (Most indoor and greenhouse growers have them. If you don't, stop now and get on that.) Then visit a chart to see if your numbers are in the optimal range. Remember, optimal range will vary slightly by strain and a bit more depending on the plants' stage of growth. If your numbers are out of range, adjust temperature or humidity to correct. If you're more of a do-it-for-me type, look into a system that monitors all this and adjusts temperature or humidity as needed to keep your plants in the optimal VPD zone.
That's the basic level. If you want to know how to do your own calculation, read on.
How do you calculate vapor pressure deficit?
VPD is calculated using the temperature and relative humidity of the cultivation space as well as the saturated vapor pressure (SVP). Convert the temperature to Celsius and use this chart to find the SVP for a given temperature.
Next use this formula: ((100-RH)/100) x SVP. That's 100 minus relative humidity divided by 100 then multiplied by the SVP you get from the chart above. The result will be in Pascals, which you can convert to hectoPascals (hPa) for use in other charts.
For example, if your room is 75 Fahrenheit, that's 24 Celsius for an SVP of 2983, according to the chart. And say the RH is 60%.
VPD = ((100-60)/100) x 2983 or 0.4 x 2983 = 1193 Pascals or 11.93 hPa
If you want to dive deeper, Cannabis Science and Technology has you covered.