Experiencing Grow Issues? VPD May Solve Them

Experiencing Grow Issues? VPD May Solve Them.

What Vapor Pressure Deficit (VPD) Is and Why It’s Important

Sluggish growth, nutrient burnout, mildew, dried-out plants-all of these issues can be addressed by tracking and optimizing VPD (vapor pressure deficit) for each stage of plant growth.

What is VPD?

• Measures the relationship between air temperature, leaf temperature, and relative humidity
• Helps to identify ideal temperature and humidity range for your grow room
• Influences plant processes such as plant transpiration rates, stomata opening, CO2 uptake, nutrient uptake, and plant stress

Too High VPD: Air around the plants may dry them out.

Too Low VPD: The plants can’t breathe. Moisture will build, and the plant will grow much more slowly. If this state goes on for too long, the plants may develop mold and fungi.

Key Terms to Know

SVP: Saturated Vapor Pressure, or SVP, is the maximum amount of water vapor that air can hold at a certain temperature.

• Hotter air = SVP increases
• Air cooling down = SVP decreases

This concept explains morning dew. When the air is too full of water, the water condenses out into the environment.

RH: Relative Humidity, or RH, is the amount of moisture in the air.

Why is VPD Important For Plants?

VPD can influence:

• Transpiration Rates:
  • As VPD increases, the plant transpires (evaporates from leaves) faster due to the larger difference in vapor pressure between the air and leaf.                       
  • Background: Transpiration is similar to how humans sweat. Plants release moisture to optimize nutrient intake levels to fuel photosynthesis.
• Stomata Opening:
  • As VPD increases, stomata become smaller in size.Background: The plant will die if it can’t release moisture through its stomata.
• CO2 Uptake:
  •  As VPD increases and stomata get smaller, CO2 uptake decreases.
• Nutrient Uptake:
  • As VPD and transpiration increases, the roots are able to take in more nutrients.
• Plant Stress:
  • As VPD increases, stress on the plant rises. The activity of VPD increasing affects the whole plant, from the leaves to the roots.

How Do You Control VPD In A Grow Room?

We recommend maintaining these systems:

Air Cooling System:

Keeps the fungi at bay. Fungi may establish between 50 and 70°F, in an above 55% RH.

Humidification System: 

When the lights off period is over and the lights activate, and the temperature hikes up, the humidity decreases. This is not ideal. Water vapor must enter the air, which is often accomplished with a fog or misting system. A medium-sized room requires about a few quarts of water being misted into the air ASAP.

Dehumidification System:

During the time of day when the lights are on, there will be a lot of water vapor in the air, especially in the growth phase. A lot of this moisture will need to be removed as the lights go off and the temperature drops.

Tracking System

Keeping track of your results/data/moisture quantities will assist you in calculating the perfect grow conditions for the size of your set-up, strain, and environmental factors. For example, log your data in a spreadsheet.

VPD Chart: Optimal VPD at Every Stage of Growth

Assuming leaf temperature is at 5° F below room temp.

Note that various strains require different humidity and temperatures. Also, triple-check your measuring instruments for accuracy and proper calibration.

Room VPD Chart

Instead of calculating VPD conditions in your grow room, you can follow this VPD chart, which shows possible VPD readings across common temperatures and relative humidity levels.

Ideal VPD for Different Stages of Gowth

General Optimal VPD = ~0.8 – 1.2 kPa (kilopascals).

Below are general VPD recommendations, but observe and adjust per your operation.

Clones

• Lower Range: ~0.8 kPa
• Why? Clones can’t handle a lot of stress because they are still forming roots. A higher humidity and lower VPD are ideal to keep the stress levels under control.

Vegetative Stage

• Mid Range: ~1.0 kPa.
• Why? Plants are more robust at this stage. By reducing humidity to increase VPD, water and nutrient intake will increase. However, you don’t want to get too much VPD, which will cause the stomata to close–causing them to absorb less CO2. CO2 is important at this stage because it’s the main factor that helps plants grow bigger.

Flower Stage

• Higher Range: ~1.2kPa – 1.5kPa.
• Why? Plants are robust, but the flowers are sensitive. Excessive humidity is not ideal.

How to Calculate VPD

In order to calculate VPD in the air, you need to know the temperature, relative humidity, and saturated vapor pressure (SVP) values for a given temperature.

How to calculate air VPD:

1. Calculate SVP
   • SVP = 610.78 x e^(T / (T +238.3) x 17.2694))
     • T is in degrees Celsius
     • SVP is in pascals (divide by 1000 to get kPa)
     • e is a mathematical constant called Euler’s Number, = ~2.71828
2. Calculate VPD
   • SVP x (1 – RH/100) = VPD
   • RH = Relative Humidity

There is a different calculation for figuring out the temperature of the plant. To determine this, you will need to know the air temperature at canopy level and leaf temperature. An IR thermometer may be used.

How to calculate leaf VPD:

1. Calculate air SVP (ASVP)
   • Use the same formula above
2. Calculate leaf SVP
   • Use the same formula as ASVP, but use the leaf temperature instead of the air temperature (typically 1-3 °C or 2-5 °F cooler)
3. Leaf VPD = LSVP – (ASVP x RH/100)

If your VPD is NOT in the optimal range, you should:

• Increase the air temperature to increase the leaf temperature to the optical temperature that satisfies the VPD value
• Augment the RH in the room to bring the VPD into the ideal range

Because VPD is so influential in affecting the plant environment, VPD is an extremely useful metric in optimizing growth. For the best results, you will need to identify the best VPD range for each stage of growth.

For more assistance on how to utilize VPD to optimize your yield, call the Grow Pros grow experts at 818-818-8088