[DamnUglyDogE]

What is Vapor Pressure Deficit (VPD)?

“Think like a plant.”

Have you ever been given this odd-sounding advice? Even when we are encouraged to try and understand how plants work, our inherent tendency to personify the natural world is inescapable.

Growers often like to draw parallels between humans and plants, after all, there’s no doubt that plants are marvelous, highly specialized and well-adapted organisms. You might even go as far to say they are “intelligent.” But let’s be honest here. Plants are totally different from us, especially in the way they react and respond to their environment. However, if we can get our heads around the world from a plant’s perspective, we become what is commonly referred to as “green-fingered.” We become … better growers.

Have you ever wondered how plants “feel” humidity? An understanding of what humidity is, what it means to plants, and how you can manage it in your indoor garden will help you and your plants stay happy all year round.
The humidity of the air is basically the amount of water in the air. Water can only truly stay in the air when it is the invisible gas – water vapor. Small droplets of water in air, such as fog or mist, are not water vapor; they are simply larger particles of water temporarily suspended in the air that are ready to be turned into water vapor by evaporation.

Temperature plays an important role when it comes to humidity. The warmer the air, the more water vapor it can hold. This means the maximum amount of water that air can hold is directly related to the temperature of the air. As the amount of water air can hold constantly changes with temperature it is difficult to pin an absolute or fixed amount of water that can be held by air. So what’s the best way to quantify humidity if the goal posts are changing all the time? The answer is something called Relative Humidity (RH) – this is a measure in terms of percentage, of the water vapor in the air compared to the total amount of water vapor that the air could potentially hold at a given temperature.

Why is RH so important?
Generally, plants do not like to lose lots of water through transpiration. Plants have some degree of control of their rate of transpiration through management of their stomata but the general rule is the drier the air, the more plants will transpire.


What is Vapor Pressure Deficit (VPD)?
VPD can be defined as the difference (or deficit) between the pressure exerted by water vapor that could be held in saturated air (100% RH) and the pressure exerted by the water vapor that is actually held in the air being measured.

Another way of thinking about VPD is the atmospheric demand for water or the ‘drying power’ of the air.

VPD is usually measured in pressure units, most commonly millibars or kilopascals, and is essentially a combination of temperature and relative humidity in a single value. VPD values run in the opposite way to RH vales, so when RH is high VPD is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant.
As mentioned above, VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control. The only problem with VPD is it’s difficult to determine accurately because you need to know the leaf temperature. This is quite a complex issue as leaf temperature can vary from leaf to leaf depending on many factors such as if a leaf is in direct light, partial shade or full shade. The most practical approach that most environmental control companies use to assess VPD is to take measurements of air temperature within the crop canopy. For humidity control purposes it’s not necessary to measure the actual leaf VPD to within strict guidelines, what we want is to gain insight into is how the current temperature and humidity surrounding the crop is affecting the plants. A well positioned sensor measuring the air temperature and humidity close to, or just below, the crop canopy is adequate for providing a good indication of actual leaf conditions.

Managing Humidity
Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant’s water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth.

So what should you aim to keep your humidity at?

Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but it’s not the whole story as it fails to take into account the air temperature.

Humidification systems to increase RH.
Table 1 shows the VPD in millibars at various air temperatures and relative humidity. Most cultivated plants grow well at VPDs between 8 and 10, so this is the green shaded area. Please note that the ideal VPD range varies for different types of plants and the stage of growth. The blue shaded are on the right indicates humidification is needed where the red shaded area on the left indicates dehumidification is needed.




By looking at this example we can see that at 70% RH the temperate should be between 72-79°F (22-26°C) to maintain healthy VPDs. If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.)


The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress.

What’s important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can.

If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:

1.Measure the leaf temperature and look up the vapor pressure at 100% RH on table 2 below.

2.Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on table 2.

3.Subtract the air vapor pressure from the leaf vapor pressure

Example:
Leaf Temperature = 24°C (100% RH) Leaf VP: 29.8
Air Temperature = 25°C @ 60% RH Air VP: 19.0
VPD= 10.8



Humidity’s Effect on Plants
Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.
Humidity’s Effect on Plants
Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.



Leaf roll on Thai basil- Localized humidity stress causes by the lights being too close.
When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.

For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavorable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant – the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.

Quick reference chart:

Low VPD / High RH High VPD / Low RH
Mineral deficiencies Wilting
Guttation Leaf roll
Disease Stunted plants
Soft growth Leathery/crispy leaves

So hopefully now you are not just ‘thinking like a plant’ – you’re ‘feeling it’ too!

( After reading a few dozen really confusing articles on VPD I found this and simplified it even more.lol )

Gave me a different perspective on what I see in my grow room,
Thought I would share it.

These are not my words- just my notes

If there isn't already a sticky on VPD, this should be made the sticky.

[DamnUglyDogE]

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Thankz...

[TLoft13]

Thanks AD, quite interesting. May i ask, does guttation occur with cannbisplants? Never noticed it...

[overgrowit]

That is a good explanation of VPD, I tried to understand it through google searches, but didn't quite get it.


Still, I have a hard time thinking that the plants are that touchy. You would think if you followed the recommended humidity and temp levels that you wouldn't need to calculate anything.

It is cool having the humidity and temp in one measurement, but i wonder how many gardens this would make a big improvement.

To measure the leaf temp do you need one of those lazer temp guns? Does anyone know how much different the leaf temp can be from the room air, considering you have proper circulation fans and exhaust/cooling?

I'm going to try and follow these parameters and see if it makes improvements on my garden.
Am I understanding it right, at 79F the humidity would need to be 70-75% to have air VPD of 8.4-10.1? Normally I would have the humidity at 55%,

[DamnUglyDogE]

Originally Posted by TLoft13

Thanks AD, quite interesting. May i ask, does guttation occur with cannbisplants? Never noticed it...

Though rare it does happen.. I will post some examples.



dried guttation on bud

[DamnUglyDogE]

Originally Posted by overgrowit

That is a good explanation of VPD, I tried to understand it through google searches, but didn't quite get it.


Still, I have a hard time thinking that the plants are that touchy. You would think if you followed the recommended humidity and temp levels that you wouldn't need to calculate anything.

It is cool having the humidity and temp in one measurement, but i wonder how many gardens this would make a big improvement.

To measure the leaf temp do you need one of those lazer temp guns? Does anyone know how much different the leaf temp can be from the room air, considering you have proper circulation fans and exhaust/cooling?

I'm going to try and follow these parameters and see if it makes improvements on my garden.
Am I understanding it right, at 79F the humidity would need to be 70-75% to have air VPD of 8.4-10.1? Normally I would have the humidity at 55%,

Unfortunately these are general indoor and outdoor charts- none specific to plant type-

So I take it more as a general area to shoot for.

I am looking for cannabis specific charts and if
I can not find them.I might go ahead a make a couple.
Indica and sativa specific.

Of course if someone where to beat me to this.
I would be happy if ya post them here.

When I use the above chart (without leaf temperature) using the ambient canopy temperature that I usually use for growing sativas indoors (82F) the chart says I must have 75-79%RH.

I find this is too humid for indoor plants that flower as there is high risk of mold.

Outdoors plants can grow with higher RH levels and not mold as there is always a lot of fresh air.

So the My ideal VPD RH recomended values in an indoor room for cannabis with 82F ambient temp would be closer to (50-60%RH 82F) when it comes to flowering.

For veg I agree with the values proposed although still indoors I think its always safer to be on the drier side of things.

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I simply like to find out why things are - what they are.
I do how ever try and back up my post and thoughts with logic
and hope the topic comes to conclusion with a solid answer.

[DamnUglyDogE]

Not going to get any simpler than this...lol

Figure 1 shows how VPD relates to the customary thinking about humidity.

Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition.

Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition.




Yep- that's about all I can say on the subject. Thank you all so much for the props.
Enjoy and forever enhance. Peace...

[DamnUglyDogE]

So having tried higher RH levels and getting a battle with PM as a result. I'm left wondering how ta achieve an environment these studies and charts suggest.
Temp 79-85 With RH 70-80s without PM issue's ?

My Idea..

In Veg....

Sulfur vaporizing x2 a week apart...
Then...
Eagle 20 treatment x2 a week apart...

Flower raise RH and flower a few weeks later.
Keeping RH in the 70s

This will be a
(Vert for seeds)

Im hoping to use the VPD theory to achieve bigger healthier and PH resistant seeds.

Any thoughts would be appreciated...

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[jammie]

excellent article AD. i feel the key to avoiding PM is good air circulation. i use an intake, exhaust and small circulation fans in an 8x8x10 grow room and have never had pm in spite of humidity levels like yours (70-80)