Understanding Vapor Pressure Deficit (VPD)

VPD is a very important concept to understand when you're looking to take your growing skills to the next level. Getting it "just right" will help maximize the growth and production of your plants. However, we have frequently seen people misunderstand and mis-use VPD to the detriment of their plants, so it is important to understand its limitations.

What is VPD?

VPD is a measurement of how much pressure difference there is between completely water-saturated air (100% relative humidity) and air at the current relative humidity. VPD is expressed in units of pressure, usually kilopascals (kPa), and essentially represents how easily water will evaporate. A low VPD means that water will evaporate slowly, with higher VPD levels indicating more and more rapid evaporation. At a fixed relative humidity, lower temperatures yield a low VPD while higher temperatures will result in a higher VPD.

* Note: this is different than VPD between leaves and the air; read on to see why.

If you're aiming to keep VPD fixed, lower temperatures require less relative humidity than higher temperatures to get the same VPD.

VPD is very relevant when cultivating plants; healthy leaves are saturated with water (100% relative humidity) so looking at VPD tells you how quickly water can escape from leaves into your growing environment. You don't want the VPD too high as you would dry the plant out too quickly, but too low means the plant can't transpire water from the leaves to pull more nutrients from the roots.

Most plants grow well at VPDs between 0.45 and 1.25 kPa, though different plants have different ideals within that range and even during different parts of their lifecycle. There are also times when slightly higher VPD (up to about 1.6) may be desirable to try and maximize nutrient uptake, but too high can cause stress and nutrient burn.

For Cannabis plants, the ideal VPD depends on the stage of the lifecycle:

As you can see, the VPD generally increases throughout the plants' lifecycle. Plants in general, including Cannabis, don't appreciate rapid changes in environmental conditions, so it is always best to make changes to VPD slowly over a period of 2-3 days to avoid shock.

How to determine VPD

To calculate the vapor pressure deficit you need to know the temperature and relative humidity. The formulas for calculating VPD are given below, but it is usually easier to look up values on a VPD chart.

One major consideration when determining VPD is that the temperature of a plants' leaves is almost never the same as the air temperature. This is the first major issue we've seen when growers are trying to hit the "perfect VPD"- assuming that just using air temperature is enough to determine the plants' actual VPD. There are many other factors changing average leaf surface temperature:

• Transpiration: Water evaporating from the leaves cools them down, in the same way that sweat evaporating from your skin cools you down. How much cooling leaves get depends on the VPD- higher VPD means more evaporation and therefore more cooling. Air movement also affects this, as stagnant air will cause humidity immediately around the leaf to increase, slowing down evaporation.
• Light Intensity: Higher light levels will warm up the leaves more, just as direct sunlight warms your skin more than being in the shade. The top-most leaves in the canpoy are exposed to more-intense light, but they also progressively cast shade on the lower leaves.
• Light Spectrum: Different wavelengths (colors) of light warm leaves differently. Infrared radiation (IR) above 800nm is not used by the plant and only serves to warm up leaves, but even in the photobiological active radiation range (PBAR, 280-800nm) different wavelengths have different effects. Chlorophyll can directly absorb light in the blue and red areas of the spectrum, so these colors don't warm leaves as much. Yellow light is captured by other pigments and converted to red light that chlorophyll can absorb, but this convertion releases heat so yellow light warms leaves more than red or blue. See our research on how spectrum affects leaf surface temperature for more information.
• Leaf Morphology and Distribution: Thicker leaves are better able to retain heat as they have more mass. Larger leaves shade out lower leaves more than smaller leave, and branching / leafing habits vary and have different effects on average leaf temperatures.
• Plant Spacing: When plants are competing with neighbors for light, the average leaf surface temperature will tend to drop as the thin, tall plants rush to out-grow each other and shade each other out, even as the top-most leaves heat up more as they get closer to the lights.

Many VPD charts available online have no mention of the assumptions they have made with respect to how warm the leaves are relative to air temperature. A VPD chart made for use with HPS lights will give horribly incorrect values if you're growing with LEDs, as the light spectrum from HPS warms up leaves much more than the same intensity from an LED grow light. Even within LED grow lights the leaf temperature is going to vary depending on the exact spectrum. Some VPD charts have an "leaf temperature offset" that can be specified to indicate the difference in temperature between the leaves and the air. VPD charts that don't have this feature are really not applicable to different growing situations, as there are so many variables that can effect average leaf surface temperature.

So how should you determine leaf temperature to get an accurate VPD? This is the second major issue we've seen with cultivators trying to get to the ideal VPD, because determining leaf temperature is very, very hard.

We see lots of people recommending using an infrared thermometer to measure leaf temperature. This sounds like a perfect, easy solution- just aim the thermometer at a leaf, and you know the temperature! Right?

Infrared Thermometers don't Give you the Whole Picture

Infrared thermometers measure the temperature at a particular point, but this does not tell you how the temperature varies between leaves and even across a single leaf. Below is a picture of a plant from a forward-looking infrared (FLIR) camera, showing 57,600 different temperature readings in the image:

Taking point temperature readings can be misleading

In this picture, we can see that there are leaves that are 74 °F, all the way up to 86.4 °F- a 12 degree range, just on one portion of one plant! The number in the upper left shows the average temperature inside the central circle- the temperature that an infrared thermometer would report if it were aimed at that spot. Plugging this 86.4 °F leaf temperature into the VPD calculator with the 81 °F air temperature and 50% relative humidity shows the leaf VPD is 2.49 kPa- much too high according to the charts. But this isn't the full picture, and taking action to adjust VPD based on this or any single reading would be unwise.

The unfortunate truth is that leaf temperature varies over a single leaf, and much more across different leaves on a plant. These are all FLIR images of the same plant, in the same conditions as the image above:

Same plant showing great variation in leaf surface temperature

This variation in leaf surface temperature makes it impossible to maintain the perfect VPD for every part of every leaf. Aiming for the ideal VPD based on the average leaf surface temperature would seem to be an ideal solution, but it is very hard to determine the average leaf temperature. It isn't as simple as taking readings from the top leaves, middle leaves and lover leaves and avergaing them together- the older / lower leaves tend to be larger in area and count for much more total surface area than the upper-most leaves. How much more varies depending on the strain, lighting spectrum and intensity, plant spacing, CO₂ enrichment, air flow through the canopy, and numerous other things.

So, shouldn't it be possible to just take a FLIR-like image and average it out to determine the average leaf surface temperature? When taking an infrared picture from overhead the top-most, warmest leaves are more visible and prevent some of the lower leaves from being seen, leading to an inferred average temperature that is higher than it should be. In addition, anything in the image that is not actually a leaf can interfere with getting a true average leaf temperature:

The floor shows through the canopy in this FLIR image

Chasing Ideal VPD

We have known many growers that try to chase an ideal VPD by taking measurements of leaf temperature using many different methods, changing the relative humidity, temperature or both daily or even multiple times a day to try to get a perfect VPD. More often than not this causes problems and actually hurts their grow. Trying to maintain an ideal VPD is a worthy goal, but changes to VPD should be made slowly and gradually over the lifecycle of the plant, not every few hours or daily.

It is far better to determine a leaf temperature offset- the difference in average leaf temperature from air temperature for the light intensity and spectrum in your growing area when lights are on- and use that offset consistently when determining VPD than it is to try and determine average leaf temperature on a daily basis.

Recommended Leaf Temperature Offset

For Black Dog LED's Phyto-Genesis spectrum, this average leaf temperature offset is usually -6 °F (-3.3 °C), meaning that the leaves on average are 6 °F cooler than the air temperature. For really intense lighting (over 1500 μmol/m²/s PBFD) the average leaf temperature offset is usually -4 °F (-2.2 °C). For other light spectrums, the leaf temperature offset is usually smaller (just a couple of degrees cooler than air) but how much depends on the particular spectrum.

Night-Time VPD

It is best to keep roughly the same VPD at night to keep conditions coziest for the plants. However, at night the air temperature drops, so relative humidity will have to drop as well to maintain the same VPD. Most plants, including Cannabis, close their stomata at night so evaporation doesn't cool the leaves as much, leading to a close-to-zero average leaf temperature offset. When calculating ideal humidity levels or ideal VPD at night it is therefore generally best to use a 0 ° or -1 ° leaf temperature offset.

VPD is not the Only Thing to Consider

While maintaining an ideal VPD is important, it is critical to remember that it is not the only environmental parameter that needs to be optimized. Keeping your grow too cold will slow down plants' metabolism and restrict growth; too warm stresses the plants. Having high relative humidity encourages the growth of mold and mildew, possibly leading to a crop loss.

Far too often we have seen growers blindly following a VPD chart and allowing their grow to get far too cold, too warm or too humid for ideal plant growth, even though the VPD may be in the ideal range. If your VPD chart tells you that you need 70% humidity anytime in flower, something else is wrong- either your grow room is too warm, or more likely you are misunderstanding the true average leaf surface temperature or are using a VPD chart that is assuming a different leaf temperature offset that what you have.

VPD Calculators

Formulas for Calculating VPD

There's a simple formula for calculating air VPD:

VPD = SVP * (1 - (RH / 100))

where:

SVP is the saturation vapor pressure (100% relative humidity);
RH is the relative humidity in the air (in percent)

Determining the saturation vapor pressure is a bit more complicated, but a formula known as the Tetens equation gives a very good approximation in the range of 32 - 167 °F (0 - 75 °C).

Saturation vapor pressure given by Tetens equation:

SVP = 0.61078 * e^{(17.27 * T / (T + 237.3))}

where:

e is Euler's number (≈2.7182819);
T is the temperature (in degrees Celsius)

Finally, the formula for calculating Leaf VPD (LVPD):

LVPD = SVP_L - (SVP_A * RH / 100)

where:

SVP_L is the saturation vapor pressure in the leaf;
SVP_A is the saturation vapor pressure in the air;
RH is the relative humidity in the air (in percent)