If you’ve dialed in your lighting and nutrients but still aren’t seeing the results you’re looking for, you may be missing a crucial component to cultivating top-shelf cannabis indoors. Vapor pressure deficit (VPD) is an often-overlooked factor that can have a significant impact on the growth of a plant. In this detailed article, we’ll discuss what VPD is, how it affects marijuana plants, and how you can find the sweet spot for optimal plant development.
What is Vapor Pressure Deficit?
Vapor pressure deficit is a comparison between how much moisture is in the air (relative humidity) and how much water the air can hold, known as the saturation vapor pressure (SVP). In simpler terms, VPD is a measure of the air’s capacity to accept additional water vapor.
As air warms, its ability to hold water increases, thus raising the SVP. The greater the difference between the air’s current humidity level and saturation vapor pressure, the more capacity it will have to take in additional moisture.
How Does VPD Affect Cannabis Plants?
The movement of water and nutrients through a plant is known as transpiration. When water evaporates from the leaves, flowers, and stems, the plant draws in moisture from the roots, thus supplying the plant’s tissues with additional water and nutrients. This process serves essential metabolic and physiologic functions in plants.
Cannabis plants lose between 97 percent and 99 percent of the water they uptake. When this happens, the moisture is released back into the air, and the roots uptake more water.
As humidity increases, it can reach a saturation point at which transpiration is slowed or stopped altogether. It can also lead to the growth of mold, mildew, and other plant pathogens. Conversely, if the humidity is too low, this can cause the plant to lose more water than it can uptake. Therefore, it is crucial to monitor the VPD level regularly and make adjustments when necessary.
VPD levels must fall within a specific range to maximize plant growth. This range can vary greatly depending on the variety of cannabis. For example, an Indica strain from the Middle East likely will require less humidity than a tropical Sativa.
How To Calculate VPD
To calculate the ideal VPD for your cannabis plants, you’ll need to take note of the current relative humidity and temperature and use this data to determine the SVP. Simply convert the temperature to Celcius and use this chart to determine the SVP:
Once you’ve found the SVP, enter the data into this equation to determine the vapor pressure deficit:
VPD = ((100-RH)/100) x SVP
That’s 100 minus your relative humidity, divided by 100, and then multiplied by the SVP.
For example, if your grow room is 72 degrees Fahrenheit, that is approximately 22 degrees Celsius. Based on the SVP chart, the saturation vapor pressure would be 2643. If your relative humidity is 65 percent, you will plug those values into the equation:
VPD = ((100-65)/100) x 2643 = 1189 Pascals, 11.89 hPa, or 1.189 kPa
In general, most cannabis plants thrive when the VPD is between 500 and 1400 Pascals (5-14 hPa). Therefore, the temperature and relative humidity provided in this example fall within the correct range for cannabis.
Bear in mind that, throughout a plant’s lifecycle, the ideal temperature and humidity will change. This chart can help you determine the right VPD in kilopascals (kPa) based on the growth stage of the plant:
How to Change the Vapor Pressure Deficit
If your VPD falls outside the range of the chart provided above, you’ll need to adjust the temperature or humidity of your grow area. Fortunately, there are several easy ways to change your VPD.
To increase the VPD, you can raise the temperature or the humidity in the grow room. The former can be accomplished by using a heater or lowering the air conditioner. You can also increase the light intensity to raise the leaf temperature. The latter can be achieved by using a humidifier to increase the amount of moisture in the air.
To reduce the VPD, either lower the temperature or humidity level in the area by turning up the AC or using a dehumidifier. You can also reduce the light intensity to decrease the leaf surface temperature.
Final Thoughts on Vapor Pressure Deficit and Cannabis
Monitoring the vapor pressure deficit may seem like an advanced technique, but any grower can do it with the right tools and know-how. By keeping a close eye on the temperature and humidity of your grow room, you’ll be able to maximize plant growth while preventing the spread of pests and diseases.