A common VPD misconception

Many growers know about vapor pressure deficit (VPD) and why it is important, but I am constantly seeing one variable left out, which throws off the whole basis of VPD and optimal plant transpiration as a result.

Now, I’m not going to explain in detail what VPD is, or recommend that you should follow it. In fact, many growers don’t follow it at all, and still produce top-quality cannabis. VPD is not needed, and therefor, this missing variable I will soon talk about is also not needed. We all grow different ways, and learn from each other, which strengthens the collective growing power. In other words, grow how you want to, and improve yourself with each grow, no matter the methods and tools you use or do not use.

However, when talking about cannabis cultivation, we are talking about the optimization of plant growth and improving as many factors as we can that results in more preferred harvested crops. VPD is one such parameter we can utilize to optimize our cultivation, but there is a slight misconception about it that I will try to fix here, after I have seen countless occurrences of plant stress when taking VPD at its face value.

Without going into too much detail in the science or math, which I am willing to do later if there are any questions, VPD is a function of dry bulb (air) temperature and relative humidity. We use it to estimate the transpiration rate of plants, which directly correlates to growth rate.

The main problem here is, that is partly a lie. We shouldn’t be using dry bulb temperature in the equation, but instead, factor in the offset of the average leaf temperature. Because of moisture present in your cannabis leaves (I hope), evaporative cooling and transpiration mechanisms alter the temperature of the leaves, as compared to the air temperature, even when measuring the air properties at canopy level.

Just a single degree (F) will have a fairly large effect on the resulting VPD value, and with a cooler leaf surface temperature, and taking the air temperature at the canopy level, we are not doing our plants any good – plants do not care about the air temperature and humidity directly, only the effect the ambient environment has on its leaves and other photoreceptive parts.

This is why some automation controllers, such as AC Infinity’s, have a leaf offset parameter for you to plug in. VPD with a leaf temperature offset of zero, is not a good indicator of VPD under most conditions.

So how do we fix this? There are several ways, but a simple method is to take a laser or IR thermometer, and take samples of the canopy - several different leaves at the canopy level, and take the average of this. These values will change over time, especially after the plant receives water, or during night when it is doing less transpiration and more respiration, so it is a good idea to periodically check up on them.

Now, if you have an automation controller with a leaf temperature offset parameter to specify, it is not a good idea to enter in a value derived from the probe’s air temperature and your thermometer leaf readings. This basically boils down to because all thermometers are different. Instead, it is a better idea to also take samples of different objects in your grow space with your handheld thermometer, after measuring the leaf surfaces, around the leaf surfaces, and averaging these as well.

With the average canopy leaf temperature, and the average surrounding air temperature, you can derive a LTO value by subtracting the former from the latter. That is, if your surrounding air temperature sample average is 82F, and your leaf surface sample average is 79F, your LTO is -3F. This LTO value is what should be used in the calculation of VPD, adding it to the air temperature, rather than the direct air temperature at any given point.

Now, this is a little tedious, and these values will change over time, especially with diurnal changes. But the truth is, we are trying to optimize transpiration rate, which can only be done by taking fairly accurate samples of the moist leaves themselves, in addition to the surrounding air.

There is more I would have liked to write here, but just this should help optimize your grow just a little more if you were not familiar with any of this, and you happened to be following the concept of VPD.

I’ll happily answer any questions in the comments below if there is any confusion, or you’d like me to go more in-depth on something.

I’ll conclude this with mentioning that in my first couple of years growing indoor, I didn’t do this extra step, and saw many dozens of plants become diseased with (mostly) powdery mildew, as they were stressed, making them more susceptible to serious issues.

I hope this helps at least someone else. Happy growing!

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Awsome… Where can I pick up a copy of your book!!! Seriously your post was excellent information and extremely well written and explained. I will definitely refer back to this for future reference. Cheers !!! Well Done …

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Thank you good sir, kind sir. Take this blessing :raised_hands::sparkles:

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Vpd taken at leaf.
Vpd graph is ideal.
I grow in 110f, plant swamp cools, leaf can be 75 to 80 but still not ideal vpd but still grows fine a bit slower. That’s all i know.

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What does “canopy level” refer to? The top leaves?

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Room VPD is a proxy for leaf VPD.
I have 2 Govee loggers in my greenhouse, one high, one low…just more info.
“The perfect is the enemy of the good…”

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Basically, yes. The leaves not shadowed by other foliage. In an indoor environment, maintaining as close to a level canopy as possible (you can never grow a perfectly flat plane) spreads out the optimal photosynthesis. VPD goes hand in hand with DLI, as both the optimal photosynthetic radiation and vapor pressure deficit are needed for good transpiration.

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I agree to an extent. You can never be perfect, but cultivation is different from natural growth. I actually have had 3 Govee hygro-thermometers in each tent for years, placed at different extremes. In my first 2 years, I have stressed many plants by taking the average of these sensors as inputs into the VPD eqation. It makes sense now though, as stomata determine the transpiration rate.

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I only have one sensor for this, but would 3 sensors be better at 3 different heights of canopy, averaged?

Trolmaster has averaging built in and im wondering. Thanks.

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Good info. And the reason I built in adjustable leaf temperature offsets for the VPD calculations into my potnanny automation.

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The LTO (leaf temperature offset) is the difference from the surrounding air temperature. I mention this, because as I pointed out in the above, you shouldn’t calculate it from a sensor or sensors. Since a sensor is only sampling the air, and you are probably going to get a leaf temperature (LT, not LTO) from a handheld pointing device, you should calculate the LTO with this pointing device also, by getting some samples of surrounding objects near the LTO samples, such as pointing it at your existing sensors. LTO is a value relative to the air temperature and the leaf temperature, so it is best obtained from a single temperature measuring device.

About your multiple sensors, I would recommend placing them just below the canopy if not covered by something, as direct exposure to light/heat will throw temperature or humidity readings off from the actual value. If they are not wireless sensors, covering them in foil works well to avoid this. In my case, I have 3 wireless sensors to monitor the air properties, and place them slightly below the canopy, hidden in the shade.

Optimal placement of air temperature/humidty sensors is a topic all in itself unfortunately. As it depends on a lot of factors specific to your growing environment, such as, but not limited to, your exhaust system placement and air circulation.

But the takeaway is, these air sensors should be used to monitor your current temperature and humidity levels, and for calculating the VPD by adding this separate LTO to its temperature readings. Depending on where the sensors are should tell you whether you should average them. There may be a large gradient if you are placing them at the extremes of your tent, where photosynthesis isn’t taking place, for example.

This time, as short as possible: 2 sets of devices for temperature: 1 for measuring the leaves and surrounding objects (a pointing device) and one or more for measuring the air (which you then add the difference from the first one to when you need a VPD reading).

I hope this helps. Let me know if you need anything else.

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Yes I understand the LTO concept just asking if it’s better applied to 3 sensors at different heights in the canopy or 1 sensor at canopy level. I’m not really talking about extremes just saying different heights within the canopy. Yes my sensor is covered in an upside down funnel, I’m aware of the heating effect (despite trolmaster saying it’s negligible)

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For the air temperature sensors that are always hanging in your tent, I would recommend them being spread equidistant apart, and at slightly different elevations. This might depend on your training style and plant structure though.

For the LTO readings, taking samples (with the pointing device) of as many unobstructed leaves at the top of the canopy is best, in addition to a few non-plant objects surrounding them in order to derive the LTO.

While you are doing the latter, you may be able to gain insight into your other sensors, and place them in better locations, and/or calibrate them if they are able to.

Where your always-present sensors are really depends on where the most photosynthesis is taking place, but should always be close to the canopy in my honest opinion.

In my case, they are all slightly below the canopy, at slightly different height levels, and I periodically check up on them to see if any one has a large difference in temperature. In fact, I have software I wrote that will send me an alert if they do, so I can place them better, or fix my air circulation devices, etc.

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I just realized I avoided the question. In my case I do take the average of the 3 sensors in the canopy, but I pay attention to their individual readings to ensure that they are all within some tolerance from each other.

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Keep in mind, that these sensors in the canopy are for your temperature and humidity readings to derive the VPD from. It is still beneficial to add additional sensors elsewhere, such as at the base of a plant, to ensure humidity levels aren’t too high, especially in mid-late flower. Or perhaps to monitor the soil temperature for an optimal rhyzosphere. There are a lot of variables, but I was specifically answering the question in terms of VPD.

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The Cannafused Vpd calculator has been in the app store for years. Not sure why people still post those silly graphs.

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Greetings @resimax,

With the greatest respect for your well written analysis of VPD application to cannabis cultivation, I must point out a flaw in your presentation and advice to growers.

As is evident in this sample chart of calculated VPD values based on observed temperatures and relative humidity, the difference in a single degree temperature only has a modest difference in VPD value, on the order of 0.03 (kPa) per degree (F) change in temperature. There are very few cases where the difference between air temp and leaf-surface temperatures will result in a “Danger Zone” VPD result.

Your suggestion that calculations of VPD will be more accurate using Leaf surface temperatures is certainly and technically correct, but I think it is unhelpful in that it inserts yet another variable rather than using simple ambient temp near your plants, into a subject that is already so complex that many growers ignore it completely…

I closely monitor and control VPD in my grow room routinely and I generally utilize canopy air temps since it is easy with the AC Infinity controllers. VPD is clearly an important variable, but the level of fine-tuning you support is of marginal utility in real world cultivation. A grower’s time would be better spent focusing on several of the other “knobs” available such as lighting (ppfd, DLI), pH (!), nutrient concentration, timing and interactions (EC, Growth phase, mineral antagonism/inhibition), etc.

My point here is not that you are wrong, but that elevating Leaf Temperature to this level of importance is unjustified and could result in growers ignoring VPD entirely.

I trust that you will receive this in the spirit which it was written.

-Grouchy :v: :green_heart:
[edited to clarify my belief that an ambient temp at canopy is adequate]

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I’m more of a dew point man. I just make sure I’m a good few degrees away and I’m happy. For such a reactive grower I’ve only ever gotten rot twice and not for a long time now.

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As a new grower who is looking at VPD and thinking about ways to play with it — this is a fabulous read and share. Thank you!

I’ve felt like my VPD is too high but to lower it would put the plant in a lower temp range than it seems to want or a higher humidity range than it seems to want. I’ve been mentally struggling between adding the humidity or changing the temp, then having the VPD number okay, yet whacking other parameters out of bounds.

This just makes me realize I need better data and a better understanding still. Thanks for the info again!

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First of all, no offense taken at all. VPD optimization is not for everyone, and could in fact dissuade people from following it. Being approximate may work out just fine, or even just ignoring it completely and paying attention to your rate of growth (listening to your plants).

That said, I have some concerns about the presentation you provided, the same presentation I was following until I broke out the math and computed it myself. I do not know the algorithm that infographic derives its values from, but it appears different than the underlying math. I wish they provided their formula.

As an example, when inputting parameters into the VPD equation, I get wildly different results from that graph, and even other resources such as vpdchart.com differ from it, and negligibly due to floating-point rounding error from my paper math.

As a concrete example, with a temperature of 77F and a relative humidity of 55%, VPD is 1.445 kPa (rounded to thousandths) when computed by hand. vpdchart.com says that it is 1.43 kPa, and the displayed chart with no citations specifies 1.24 kPa.

These discrepencies are one thing I set out to resolve, after two years following that chart and getting dieased or stunted plants. Mind you, my leaf surface temperature is usually around -4F to -6F, which changes my calculation of 1.445 kPa to between 1.18 kPa and 1.263 kPa, using the same parameters of 77F/55% air properties. Changing it to 76F brings it to 1.397 kPa, slightly more than your claimed 0.03 kPa, which I assume was in relation to this graphic that has no backing mathematical information behind it.

To each their own, but I don’t trust magic numbers without more information. The values seem far off from other calculators, including my hand-written math, so I am curious if they are hard-coding in some constant LTO themselves in that graph.

I appreciate all of your concerns, but I think elevating leaf temperature is of extreme importance if one wants to optimize their grow and not have the potential for two years of failure as I have had when starting out, as you’ve seen with my average LTO, leaf temperature alone could downgrade to a previous stage of growth, de-optimizing transpiration rate, leading to issues.

Everyone’s growing environment is different though, and this might not be that extreme elsewhere. I encourage everyone to at the very least, do their research and peruse different sources of information, if they are not well-versed in the math, such as, but not limited to vpdchart.com.

Thank you very much for your input, and I hope that you too will receive this in the spirit it was intended to be.

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