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 its 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 Vapor Pressure Deficit 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 Vapor Pressure Deficit 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 Vapor Pressure Deficit 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 Vapor Pressure Deficit levels 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 Vapor Pressure Deficit (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 Vapor Pressure Deficit 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 Vapor Pressure Deficit 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 Vapor Pressure Deficit range, somewhere between 0.4 and 1.6 kPa.

The ideal level of Vapor Pressure Deficit changes during a plants life cycle. Clones need a super low Vapor Pressure Deficit to grow roots, otherwise they’ll just dry up and die. Large plants in late flowering, tend to produce much better harvests when the Vapor Pressure Deficit 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 Vapor Pressure Deficit 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.

DOWNLOAD THE PRINTABLE VPD CHARTS PDF

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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 Vapor Pressure Deficit, but the specific formulas are beyond the scope of this article. Especially since we made that really nice Leaf Vapor Pressure Deficit 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 Vapor Pressure Deficit 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?

32 Responses

  1. This is now my go to for vpd reference. I’m a science guy. Meters and controllers for everything. Your chart and the wave of change over time are great.
    The leaf vpd calculator(I was using an IR thermometer already, low 80s with a QB works well. ) is awesome.

    1. No doubt Greg !!! This is the best reference I’ve seen so far.. Such a clearer understanding for sure.. NICE WORK GUYS!!!!

  2. This is awesome! I’m a total beginner and I didn’t know much about VPD before, other than leaf temp should be lower than room temp. Thank you for doing this, I think it will help me a lot.

  3. Can you tell me how to calibrate the temperature probes for the dimlux controllers ? I’ve tried steam and ice but this didn’t work .

    I have two controllers to control my HPS and CMH lights seperately but the probes give me different temperatures readings (around 2 oC difference, and around 4 oC difference from my opticlimate probe !)

  4. How’s it going . I’m still trying tony derstsnd the VPD chart . Say my lights are on , temp 78 degrees early veg , humidity 49% . That’s good or bad .
    Would I adjust the A/c temp or Light Percentage Higher or lower( Using Gavitas) . To change the humidity ??

    1. Usually we use Humidifiers and Dehumidifiers to adjust the humidity in a room. And we use AC or heaters to adjust temperature. In extreme cases we adjust the light intensity to cool a room. Optimal Light intensity is another conversation. Basically, there is a range where plants grow best (can be different for different species), anywhere in that range will produce excellent growth for you plants. Check out our article on Half Force for information on light.

  5. There is a large temperatures / humidity variance for optimum VPD.
    In flower is it better to start off at a higher/temp and slowly reducing the temp/humidity? say starting at 80degrees/60% humidity and then ending at 70degrees 35% Humidity in some linear fashion? Or is it better to start at 75 degrees 60% humidity and just reduce the humidity to 40% by harvest?
    What should the night temp and humidity be? Just follow the vpd chart?
    Thanks

    1. Hi Pete,

      Follow the recommendations on the upper right of the chart for the various plant stages. You should think in terms of kPa instead, but yes, either one of those scenarios you proposed fit in with what I recommend in the chart. Keep things under control at night too. Follow the chart!

      K

  6. Is it possible to create a positive offset on the chart as the hps bulbs heat up the plant leaves 2-3C more than the room temps

    1. Hello Andrew,

      Have you considered taking leaf temperatures from other parts of the plant? Or conversely taking the air temperature from the canopy where you’re getting your leaf temp from?

      The leaves should always be cooler due to transpiration.

      Let me know if this clears it up for you.

      K

      1. I’ve tried many different areas on the plants. still, right now, I am getting 1C difference higher temps on the veg plants. From 23-26C room temp, and the leaf temp follows the room temp. The humidity in the room is about 70% daytime and 80% at night. And the calculator shows the leaf RH to be around 1.0vpd which is ball park of where we want to be. I just cannot get the leaf temp lower than the room temp. MH 1000DE (6k color) are 3’ away with about 280umoles. I really think it’s the Infrared heating up the plants leaf temp. Not sure what else to think…

        1. This is really odd. I understand what you’re saying in regards to the radiant heat of the lights heating the leaves up, but in my experience the leaves have always been cooler than the air due to transpiration. Have you verified this with different thermometers? I’ve had the misfortune of using defective thermometers before. And I’m sure you’re taking the air temperature around the leaves as well, right? Rather than the air temp somewhere in the room that’s cooler. We want to measure the plant’s living environment… I’m running out of ideas to help though…

        2. I would definitely recommend redundancy with your measurement instruments. Double checking accuracy with a second instrument should help clear things up, or at least confirm that your readings are correct. I use LED and they’re obviously quite a bit cooler but I’ve never had leaf Temps be closer than 2 degrees below room temp. I have my LEDs at 14″ from canopy.

  7. I would love to have someone explain this chart to me, trying to get a handle of using this chart. The chart mentioned here shows numbers that is confusing me is it the room temp. or the leaf temp that I go by?? For ex. my plants are 3 1/2 weeks old from seed the room temp is at 72F, humidity is at 60% and the leaf temp. is at 70-73f???

    1. Hi Ray,

      The temperatures on the chart represent the room’s temp. There are six charts, each representing a different leaf temp. The leaves should be cooler than the room due to transpiration. So this difference is represented at the top of the chart, “Leaf cooler than the room by” 0, 1, 2, 3, 4, and 5.

      The room temperature will be different in different places. It’s probably a good idea to take the air temperature near where you’re taking your leaf temperature readings to get an accurate difference. It won’t make much sense to take the ambient room temperature someplace that’s cooler, and then a canopy leaf surface temp directly under the light.

      Let me know if that helps!

      K

        1. Several measurements and some averages. Use fans to circulate the air so the temps across the room are more consistent. This is usually done with many plants in a large greenhouse.

  8. Would all these charts (6) all work for various tropical plants , to greenhouse vegetables, to house plants , to outdoor field crops/ berries etc. , or are these VPD charts geared to cannabis only ?

    1. Good question. It’s going to be different for different plants. Tropical plants will likely have a different VPD range that they do best in.

  9. I’ve read many interpretations on VPD. This is, by far, the clearest and most informative. More importantly, it is as layman’s as possible. I especially like the chart variations when taking leaf temps into account. Excellent write up!

    1. It’s better to have good air circulation so the temperature is more consistent around the room. But if this isn’t possible, you want to get an idea of the temperature the air is immediately around the plants. Canopy is the place temps are usually measured. VPD is a guide and the temps are another guide to understanding what the plants are experiencing. These aren’t hard rules. So you’ll need to use some intuition along the way. And you’ll get better with time.

  10. Does nighttime VPD matter as well? It’s hard to keep the temps and humidity the same once lights turn off. the air conditioners drop the ambient air a lot and the dehumidifier setting doesn’t change, which will create a pretty low vpd unless a heater is brought in. Thoughts?

    1. For sure! Keeping VPD in check is important all of the time. We don’t want moisture gathering on the leaves overnight! That will create perfect environment for powdery mildew (or other fungi) to take hold. And I don’t know anyone that wants powdery mildew. Gross.

  11. Hello, if I already have my vpd given to me on my master control , but I don’t have my leaf temp what chart do I use? Or do I have to get the temp of the leafs

    1. Good question. Choose a chart where the leaf temperature a 3 to 5 degrees below the room temp. This is the typical range you’ll measure the leaf temps once you get an infrared thermometer.

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