Vapor Pressure Deficit
The ideal environment for your plants.

Introduction

You probably already have some idea of what Vapor Pressure Deficit is, but maybe you need a simple introduction, with no formulas or math. We got you covered, plus we’ll give you calculators, recommended ranges, and easy to use reference charts!

At some point, most indoor growers run into some mysterious plant growth problems, ranging from symptoms that look like nutrient deficiencies, when the nutrients are actually perfectly balanced and fresh. Or sometimes a cultivator sees sluggish growth, or powdery mildew, to a whole host of unanswered questions that in fact relate back to the environment! 

The amazing thing is, adjusting for this one thing, Vapor Pressure Deficit (VPD) can create the ideal environment for your plants to thrive in. This is a pretty cool and slightly advanced concept that can take you to the next level. Once you understand it and apply the concepts, you’ll see your garden improve in a big way. 

So if you’re ready to create the absolute best environment for your plants, read on and we’ll dive right in.

The VPD in this room is too low, and moisture is collecting on the leaves. This may lead to Powder Mildew or mold.

Concepts

Vapor Pressure Deficit measures the amount of drying power the air has upon the plant. Basically, it’s how much moisture is being sucked out of the plant by the atmosphere.

You probably already know the plant uses transpiration to grow. Literally, the plant will die if it can’t release moisture through it’s stomata. The process of transpiration in plants is similar to how we sweat; they have stomata which are similar to our pores. But the big difference is, they do this so they can pull in more liquids through their roots to fuel photosynthesis.

When VPD is too high, the plants might not be able to keep up with the environmental demand upon them. The air is drying them out too fast! And this will cause them to exhibit symptoms that look just like nutrient deficiencies, and the plant will grow poorly.

On the other end of the spectrum, if VPD is too low the plants can’t transpire. Moisture may build up on the leaves, and the plant will grow much more slowly. If this goes unchecked for too long, the plants might get attacked by molds and fungi, like powdery mildew.

Reading up on VPD you’ll see all sorts of measurements of Vapor Pressure. There’s Saturated Vapor Pressure (SVP), Atmospheric Vapor Pressure Deficit (AVPD), Leaf Vapor Pressure Deficit (LVPD), plus several others not worth mentioning. 

Vapor Pressure Deficit is the difference between the Saturated Vapor Pressure (SVP) and Relative Humidity. Saturated Vapor Pressure is the maximum amount of moisture the atmosphere can hold according to it’s temperature. And Relative Humidity is the amount of moisture currently suspended in the air. So to calculate the room’s VPD all you need is two measurements, temperature and relative humidity!

But this isn’t the whole story. The plant’s experience is slightly different because they are usually a bit cooler than the room.  If the plant temperature is exactly the same as the room temperature, then the plant and room VPDs are equivalent. But this is rarely the case! Usually, the leaves are between 3° and 5° F cooler than the room because they are transpiring. The evaporation on the leaf’s surface literally draws heat from the leaf, thereby cooling it.

We call this Leaf VPD (LVPD), and we need only one more measurement to figure it out, and that’s the leaf temperature of the plants. To get this measurement we use an infrared thermometer (IR thermometer), and they are rather inexpensive, there are dozens available on Amazon for under $30. It’s so easy to take the temperature of anything with an infrared thermometer, like ballasts and reflectors. So if you’re like us, you’ll start pointing it at everything in sight.

So while the room’s VPD is important, we really want to focus on what’s going on within the plant. That means we want to know the Leaf Vapor Pressure Deficit because we’re trying to grow the best and biggest harvest possible, right? Right!

Here’s a good place to give you our first gift of this post, the Room and Leaf Vapor Pressure Calculator:

Leaf VPD Calculator

 

More Details and Specifics

It’s important to note that the different parts of the plant are going to be at different temperatures. You’ll see this once you start using your IR thermometer. 

This all sounds wonderful, until you’re doing it daily, multiple times per day… and then there’s the dark period! How do you do that too!?

Well, DimLux has you covered with the Maxi Controller, which can automate the measurement process for you when it has a thermometer, humidity meter, and Plant Temperature Camera. In fact, the Maxi Controller Data Logger will keep track of all of this for you on a spreadsheet so you can see if you’re having swings in your VPD when you’re not around to catch the problem! 

We highly recommend the Maxi Controller Data Logger with all the add ons, obviously.

Plants can grow within a wide Leaf VPD range, somewhere between 0.4 and 1.6 kPa.

The ideal level of VPD changes during a plants life cycle. Clones need a super low VPD to grow roots, otherwise they’ll just dry up and die. Large plants in late flowering, tend to produce much better harvests when the VPD is on the high end of the range, stressing them a bit. Also, a slightly drier atmosphere will inhibit powdery mildew and mold.

Recommended Leaf Vapor Pressure Deficit

Assuming leaf temperature 5° F (2.8° C) below room temp.

Plant Cycle StageMin Leaf VPDMax Leaf VPDTemperatureRelative Humidity
Propagation / Early Veg0.8 kPa1.0 kPa70° F60%
Late Veg / Early Flower1.0 kPa1.2 kPa75° F50%
Mid / Late Flower1.2 kPa1.6 kPa75° F40%

Here are the Leaf Vapor Pressure Deficit numbers we recommend over the plant’s life cycle. Keep in mind, different strains are going to be a little different than one another. Also, you need to make sure your instruments are calibrated and giving you accurate readings. If you can’t trust your instruments, then the VPD number you calculate is useless.

We’ll leave you with the biggest set of VPD charts we know of, so you can just take the measurements and reference the correct color coded VPD Chart adjusted for leaf temperature.

These will help guide you to maintain the best humidity and temperature levels in your grow room for each stage the plants are in. You can select the chart corresponding to the temperature the leaf is below room temperature, anywhere from 0° to 5° F. Move the mouse over the temperatures below each chart and notice how much the chart changes based on a few degrees change in the plant’s temperature.

Leaf Vapor Pressure Deficit Charts

Vapor Pressure Deficit Charts by Leaf Temp

 0° F  |  -1° F  |  -2° F  |  -3° F  |  -4° F  |  -5° F 

Formulas

There are several ways to calculate VPD, but the specific formulas are beyond the scope of this article. Especially since we made that really nice leaf VPD calculator and the LVPD Charts for you. If you reeeaaaaalllllyyy want those formulas, check the source links we provide at the bottom of our post. A few of the formulas you can just plug right into a spreadsheet like Google Sheets or Excel.

 

Why do I see VPD represented by all sorts of different measurements?!

Because VPD is by definition a measurement of pressure, there are several numbers that represent it. The most popular is Pascals (Pa), which is usually too large of a number so we take it down to kiloPascals (kPa). For completeness, 100 Pa = 1 hPa = 0.1 kPa. Moreover, 1 hectoPascal (hPa) is equivalent to 1 millibar (mb). Why the different names? Well, the name was changed to honor the scientist that discovered it. We do the same thing with Volts, Amperes, Ohms, Watts, Joules, Hertz, Celsius, and Fahrenheit. Sometimes pressure is measured in pounds per square foot, or kilos per square meter. But in our case we’re going with kiloPascals (hPa) because it will give us clean whole numbers to work with that match the Maxi Controller.

Sources:

  1. Quest Dehumidifiers – Vapor Pressure Deficit Series: Part 1Part 2, & Part 3
  2. Desert Aire – Vapor Pressure Deficit & HVAC Design PDF
  3.  National Weather Service – Vapor Pressure Deficit formulas PDF
  4.  Alchima – Vapor Pressure Deficit in Cannabis Cultivation
  5.  The Weather Window – Hectopascals & Millibars are they different?