Jessica McKeil   |   June 05, 2018

Mastering VPD Control for Plentiful Harvests

Why should cannabis growers concern themselves with vapor pressure deficit (VPD) in their grow operation? It is an interesting approach, and while it is definitely an advanced environmental measurement technique, it's worth exploring for indoor grow rooms.
Jessica McKeil is a freelance writer focused on the medical marijuana industry, from production methods to medicinal applications. She personally found relief through cannabis for the treatment of her panic and anxiety disorder. She is lucky…

Chris Vaughn, a 15-year industry vet, now growing with Higher Minds Horticulture, was recently interviewed by the Growers Network for their Growers Spotlight. Vaughn dives in-depth about why VPD is a crucial new tool for nutrient and water management in cannabis farming.

The Science Behind VPD

Vapor pressure deficit, or VPD, is a measurement describing the vapor pressure differential from inside a leaf, compared to the air pressure of the room. It rolls air temperature, leaf temperature, and relative humidity into an easy to manage number. Growers experienced in VPD management understand optimal control of a grow room environment is about finding the sweet spot between humidity and temperature.

As plants grow, they transpire water vapor through the stomata (special cells located on the surface of the leaf) out into the atmosphere, thus boosting relative humidity levels in their environment. A basic component to the growth cycle for any plant is the movement of water and nutrients drawn up by the roots, through the body of the plant, and eventually transpired out by the canopy.

This upward movement of nutrients and water is nature's way of seeking balance. There is naturally a differential, or VPD,  between the pressure of the rooms environment and the vapor pressure within the interior of the plant. Any gases trapped inside the plant are 100 percent saturated with water, and eventually, these saturated gas molecules are drawn out into an external environment.

Getting an extreme VPD under control will also control water and nutrient use. With a high VPD  and low relative humidity, cannabis will naturally transpire water much more quickly. A dry environment will simply pull more transpiration out of the plant itself, drawn up through the roots. If the pressure imbalance goes on too long,  and the atmosphere is too dry, plant's close off their stomata as a final protective mechanism to reduce moisture loss.

In a relatively humid environment, the VPD is low. The vapor pressure internally and externally even out. Finding a  perfect balance of humidity and temperature, guarantees water and nutrient movement, but prevents aggressive waste seen in low relative humidity environments. Plants are generally less stressed, and more receptive to CO2 in the grow room.


The Benefits of VPD According to Vaughn

VPD management is an advanced growing technique, however once mastered can be especially beneficial for large-scale warehousing operations relying on massive AC units for temperature control. Air conditioning is a perpetually dehumidifying process, and can quickly throw the VPD out of balance if not counteracted with some sort of humidifying process.

Even greenhouses, which are generally humid environments can benefit from VPD monitoring. It can help growers maintain a perfect relative humidity throughout the growing season, and through the phases of development.

Once conquered, VPD monitoring enhances the usefulness of other elements of a grow room as well. A low VPD prevents cannabis plants from closing their stomata in an attempt to protect its water reserves; this naturally allows increased CO2 absorption rates.

Understanding the VPD at any given time also puts a grower closer in touch with the stress level of their crop. Remember, a high VPD generally means a stressed out plant, as it struggles to manage water transpiration, uptake, and low relative humidity. A happy plant applies nutrients more effectively, including any additional CO2, and will produce a better overall harvest. Vaughn believes it is crucial for trichome development.

Tips for Proper VPD Control

Lowering the VPD value generally means increasing the relative humidity, especially in large industrial warehouse operations. Vaughn advises readers of the Growers Network that higher humidity levels aren’t without risks. Growers should be wary of festering pathogens in a damp environment. Cleanliness is an increasingly valid concern in high humidity but low VPD atmosphere.

He also suggests operating a series of smaller grow rooms (or using a dome technique on clones), to maintain absolute and optimized control over the atmosphere. To manage VPD levels is to make miniscule adjustments in temperature, airflow, and humidity.

In perfect conditions, a grower will bring the cannabis crop through the vegetative stage with low VPD and high relative humidity. As the plants enter into the flowering stage, Vaughn suggests gradually moving the plants into moderate and then into high VPD. A dryer climate is essential during the flowering stage to prevent mold, mildew and other pathogens from taking hold.


Article courtesy of Growers Network



1 Comment

  • Bryan Sherman 2018-06-08
    Granted VPD is very important and everything you say is true! And therefore, cultivators and more importantly engineers should get out of their “boxes” and learn how to control the psychrometric properties of the space. Or better yet Hire an Engineer that does understand. “Air conditioning is a perpetually dehumidifying process and can quickly throw the VPD out of balance if not counteracted with some sort of humidifying process.” This is where EVERYONE so far has gotten it wrong! There is a better way to control the environmental conditions that does not rely on the “Comfort Cooling” process. THIS IS NOT COMFORT COOLING!! Anyone who understands the equations they use to determine the “cooling load” knows that “Cooling” is incidental to Dehumidification. THINK about what you are doing; the whole process is designed around a leaving coil condition of saturated air at 55OF, that is 100% Relative Humidity. Why? Because in “Comfort Cooling” you are trying to maintain around 50% Relative Humidity with Temperatures in the 70’sOF and when that air mixes with the room air the desired Relative Humidity is achieved. Most of the time the loads are weather dominant. Of course, in a Dry Climate “swamp coolers” are popular and effective but NOT for this process. Cultivation is an internally load dominated application! Done properly, most of the time this process, would not need any refrigeration at all. It is Possible to maintain Psychrometric conditions within 1OF & 1% RH while providing a sterile environment using half the Energy of the most efficient system used anywhere today. There is no magic and nothing new it has been done for decades. Just not yet in the Cannabis industry and as soon as someone has the brains to do it, the industry will be revolutionized. I am happy to educate anyone who would like to understand. Bryan Sherman, CEM, EBCP, MFBP [email protected] 516-767-0525