The VPD chart below shows VPD levels widely accepted and recommended for growing cannabis. The top of this chart is the RH or Relative Humidity in your grow room. The left displays the temperature in both F and C of the air in and around the canopy of the plants. VPD (Vapor Pressure Deficit) is a control value for evaporation and plant transpiration.
What should my VPD be?
In the vegetative stage, your VPD should be 1kPa. In the flowering stage, your VPD should be 1.2 – 1.5kPa.
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Results suggest that decreasing VPD improves plant water status, which ultimately enhances photosynthesis and growth under water stress.
The above values measure VPD in KpA or kilopascals. The higher the value in the chart, the higher the VPD is, and the lower the calculated value, the lower the VPD. Ideally, you want your numbers to be in the green zone for optimal vegetative and flowering growth.
The higher the VPD, the faster the plants will transpire and the quicker your root zones will dry out. In high VPD situations, nutrients accumulate more immediately, and plants favor water over food. If you’re stuck in a high VPD situation and can’t fix it, it’s always better to feed on the lighter side of things to avoid over-feeding your plants.
If VPD is too low, plants will start showing Calcium deficiencies, and more Calcium will not fix the problem. In low VPD environments, plants struggle to move Calcium through their tissue, and spots will begin on the leaves, starting at the top and moving down. Low VPD environments are also notorious for breeding fungal diseases like powdery mildew. Generating the above VPD chart for LED grow lights contain the ideal parameters if you grow using LED Grow Lights.
If the VPD of your grow environment is off, plants may exhibit traits of nutrient deficiency.
What is the ideal VPD for cannabis clones?
The ideal VPD for cannabis clones is around 0.8kPa
What is the perfect VPD for cannabis vegetative stage?
The ideal VPD for cannabis in the vegetative stage is around 1kPa. (Ideal VPD for veg stage and mother plants)
What is the ideal VPD for cannabis flowering stages?
The ideal VPD for cannabis in the flowering stage is 1.2 – 1.5kPa. (Ideal VPD for flower and best VPD for flowering stage)
What is VPD? VPD stands for Vapor Pressure Deficit. VPD consists of air temperature, leaf temperature, and relative humidity.
VPD dictates how efficiently a plant balances its internal energy with the broader environment. How well a plant can pick up or lose water, pump nutrients throughout, remain turgid and upright, and grow. VPD directly influences a plant’s hydraulic capacity and this, in turn, impacts growth rates and overall plant health.
Plants use a lot of water while growing, so the air in the growing space is usually pretty humid. But since the air in the plant’s leaves is usually much more humid than the air in the growing area, the pressure inside the leaf is typically much higher than the pressure inside the growing space. This difference in pressure causes water vapor to diffract (spread out) into smaller and smaller droplets until it’s almost all vaporized.
How is VPD calculated?
To calculate VPD, subtract the actual vapor pressure of the air from the saturated vapor pressure (VPsat – VPair). VPD (Vapor Pressure Deficit) in kilopascals (kPa) in grow rooms and cannabis-related gardens.
To find out how aggressively the environment is pulling air from the plant, we must compare the plants’ Saturated Vapor Pressure (if leaf temp is known) and the Vapor Pressure of the air.
To calculate VPsat, we need the temperature of the saturated environment -> the plant’s leaf. Measure this with an infrared temperature gun.
The formula for VPsat (in Kilopascals kPa) is:
To get VPair, we must know the temperature and humidity of the air, known together as relative humidity. We can measure this with a thermometer/hygrometer in the growing room.
The formula for VPair (in Kilopascals kPa) is:
VPD = (VPsat – VPair) is how VPD is calculated. See below:
And VOILA, you have VPD. Now it’s time to consult the VPD chart and your plants’ ideal growing temperature range to see if your VPD is within the correct range and your plant is transpiring healthily. Remember, each plant has its own correct VPD, so it’s essential to stick to the plants’ native temperature and consult the chart from there.
*Important Note: Prevent relative humidity from going over 80% or below 50%. If you cannot hit your desired targets, use the chart to find a combination that gets you as close to the green area as possible. Most growers understand that higher humidity is acceptable during the veg stage of growth. If you can control your environment with a humidifier and a dehumidifier, we suggest you shoot for a higher VPD during the vegetative growth stage. VPD and powdery mildew go hand-in-hand; maintain proper VPD to avoid powdery mildew infection.
The leaf surface temperature of your cannabis plants is critical to ensuring the best VPD for your environment and dialing your grow to optimal production.
Best VPD Monitoring Device
- Accurate: Premium sensing component with accuracy of ±3%RH, ±0.3°C / 0.5°F. Professional-grade components, assembled into devices you can trust. For even more demanding applications, the device is easily calibratable to a trusted reference, or try the SensorPush HT.w or HTP.xw for even greater out-of-the-box accuracy. Whichever SensorPush you choose, you'll enjoy reliable, quality monitoring of environmental conditions including air temperature, relative humidity, dewpoint and VPD
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Final Word
Proper HVAC equipment design → consistent temperature and relative humidity in the space → consistent vapor pressure deficit → plant transpiration → plant growth.
Humidity and temperature are essential for plant growth. Small changes in humidity or temperature can significantly impact the vapor pressure differential, which is how much water vapor is present in the air. The HVAC system’s size, cost, and efficiency and how you control your environment to encourage optimal plant growth are determined by how much air and water need to be added and removed from the growing area in a specific time range.
Best LED Grow Lights for Cannabis
A great VPD resource and exciting concept for VPD – vpdchart.com
Some of the data and info in this article was original research found here:
L Breit, M Leavitt, A Boyd, Cannabis Science and Technology 2(2), 52-61 (2019).