What is Vapor Pressure Deficit (VPD) in Cannabis Cultivation?

by | Jan 15, 2020

Detroiter Karhlyle Fletcher is the host of High Lit, a cannabis research and classic literature podcast featuring leading voices and independent music. In addition to years in written and video cannabis journalism, he is also a traditional author.

Vapor Pressure Deficit, or VPD, compares the vapor pressure inside a plant to the air pressure of its surroundings. 

At first, it's essential to VPD low or to keep the environment humid. Once the flowering stage begins, it's better to keep VPD high. These different stages and shifts in VPD allow crops to get as many nutrients as they need while also preventing the spread of pests.

How Does VPD Work? How Is It Managed? 

The stomata of plants control VPD as they are how plants release water vapor into the air. Stomata work as pores for plants, releasing gases and absorbing CO2. Advanced cultivators understand that lowering VPD prevents the plant from closing their stomata. By keeping the stomata open, farmers improve their crops’ rate of CO2 absorption. 

This measurement is an excellent way to gain control over cannabis and hemp, but it's an environmental measurement. The best way to address issues with VPD in cannabis cultivation is by controlling the environment, which is why it's easiest to work with separate and small rooms when analyzing VPD. Shifts in VPD can occur from slight changes, so growers must be vigilant if they aim to master VPD. Any changes in airflow, humidity, or temperature can change the VPD of a grow. 

Of course, large warehouses still benefit from understanding VPD. Any environment is manageable, but it is simpler to work with smaller rooms. A common issue in warehouses is dehumidification from air conditioning. If the VPD is too high, cultivators can humidify the environment so that their crops are less stressed. 

High VPD generally corresponds to a stressed plant because it makes it more difficult for the crop to manage water transpiration, uptake, and low relative humidity. After the plant enters the flowering stage, it's better to move to a dryer environment. 

Having a higher VPD during flowering is preferable as it helps prevent the growth and spread of mold, mildew, and other pathogens. The transition of this advanced technology may seem complicated, but it's logical. When cannabis is growing, it needs a humid environment so that it doesn't feel like it has to protect its water supply. Once it's flowering water content isn't as critical, but managing pathogens is. Start with low VPD so cannabis plants can freely access and release nutrients. Shift to a higher VPD once it starts flowering to prevent undesirables from spreading. It's a complicated procedure, but one that's easy to grasp once the stomata of plants are understood. 

Getting Real Control Over VPD 

The ideal ranges for VPD for cannabis cannot be too high or too low. Plants are essentially water pumps, and when the VPD is too high, then the plant will lose too much water. If the VPD is too low, then the plant will choke. Keeping the vapor pressure deficit in an ideal range allows plants to cycle water and nutrients effectively. 

For successful VPD management, it is imperative to chart temperatures and humidity during the night and day cycles. Environmental control is the only way to address VPD. There are several options farmers have to manage the environment. 

  • Moving lights. Sometimes the heat given off by lights can upset the balance in an environment. Switching lights, or even raising them above the crops, can address issues with high VPD. HPS lights, in particular, can upset VPD. LEDs paired with heatsinks are excellent for controlling temperature.
  • Using domes. By creating a smaller closed environment around a plant, farmers can gain better control over VPD. Doing so is an ideal solution in a large environment.
  • Limiting fans and air conditioning. Air movement can exacerbate the effects of poor VPD control. Controlling airflow is crucial to managing VPD. 
  • Measure often. Once you have enough experience with any crop, you'll learn what VPD works for your plants at what points in time. Practicing diligence with data will speed up this process. 

Why Focus On VPD?

When farmers grow with an understanding of vapor pressure deficit, they enjoy increased cyclical rates and more substantial development of their crops. Their plants are less stressed, healthier, and more bountiful. Managing VPD properly also allows for plants to distribute nutrients more effectively. 

VPD measurements reveal information about humidity, temperature, and airflow. While VPD may seem complicated to novice growers, it's unnecessary to be intimidated. Working with it allows for a greater understanding of the environment and the strain one is growing. Mastering this information will make growing less painless. Afterward, all you'll have to worry about are soil content, nutrient balance, and managing pests