Everything you need to know about VPD

“New and experienced growers that want to reap the benefits of an advanced growing strategy should familiarize themselves with the relationships between environmental factors that affect the efficiency of photosynthesis; leaf temperature, humidity, carbon dioxide concentration, and light intensity.”

1. Introduction

2. Leaf Temperature vs. Ambient Room Temperature

3. Temperature Requirements for Photoshynthesis

4. The Importance of CO2 Enhancement for High Temperature Growing

5. Calculating the effect of CO2 dosage

6. Infrared Radiation & Leaf Temperature Down the Canopy

7. Air Circulation and CO2 Uptake

8. Temperature and Light Intensity

9. Ideal CO2 Levels for different stages

10. Climate control

11. What is Vapor Pressure (Deficit)?

12. Managing Humidity

13. How to influence VPD?

14. Under control

15. VPD for Different Stages of Growth

16. VPD during Flowering

17. Practical use of VPD

18. What VPD do I want in my grow room?

19. Lightplan

20. Connecting the dots

Connecting the dots ?!

How are Air temperature, Leaf temperature, Relative Humidity, Air movement, CO2, Vapor pressure Deficit (VPD), e.c. values, and light related?

Growing tropical production plants in a closed and controlled environment is a complex and challenging task. Mastering it requires knowledge, both technical, agricultural and logical. This article is a composition of several different topics, online articles and calculations found in several scientific studies. It may or may not be suitable for beginners but definitely will help advanced and even some professionals. Most (experience based) articles you find online focus on a single topic or a group of topics but lack overview. Knowledge based articles give good in depth information and technical information we all crave for, but also lack overview. However finding information connecting these ‘dots’ and creating more of an insight can be challenging. I will try to attempt to connect some topics that don’t seem related at first sight, but when looked at more closely, will appear to have great impact on each other.

Let’s start with the basics. Photosynthesis is the chemical process of binding CO2 to turn light energy and water into sugars using Chlorophyll. This process takes place using an enzyme called RuBisCo. Under different temperatures RuBisCo can bind more CO2. The sugars produced have a much higher energy level than the CO2 and water so they require ATP. (ATP is the same energy source or carrier we humans use to flex our muscles.) Afterwards these sugars are being turned into starch and sucroses. Energy can not be created as we all know, it’s just being changed from light energy into chemical energy. But let’s not get too deep into plant chemistry at this moment.

First we’ll discuss the effect of temperature on photosynthetic rate; how temperature and CO2 concentration are intimately linked to plant growth at higher temperatures; and compare the differences between ambient temperatures in HPS and LED grow rooms.

2. Leaf Temperature vs. Ambient Room Temperature

When scientists discuss photosynthesis and temperature, they typically reference Leaf Temperature, not the ambient room temperature; this makes sense given that the biochemistry of photosynthesis takes place inside plant’s leaves. In contrast, when grow room designers discuss temperature, they usually reference the Ambient room Temperature. In most growing environments, the leaf temperature will be higher than the ambient air temperature surrounding the plant. This is especially true for plants grown under high-pressure sodium (HPS) lights, which emit infrared radiation that is absorbed as heat by the plant.

3. Temperature Requirements for Photosynthesis

RuBisCo is the plant enzyme responsible for the chemical reaction that is the first step of carbon fixation. This chemical reaction is seen as the conversion of CO2 and water into simple sugars during photosynthesis. The chemical reaction that RuBisCO performs is temperature dependent!

With full sunlight and ambient CO2 concentrations of about 300 ppm, as well as a temperature range of 5°C-27°C, the rate at which CO2 is absorbed by the plant and converted to sucrose increases as the temperature increases, leading to increasing gains in net photosynthesis.

If the internal leaf temperature rises above 27°C, RuBisCO enzymes begin to perform the reverse reaction, with some of the RuBisCO enzyme converting sucrose and oxygen into CO2 and water in a process known as photorespiration. As leaf temperatures approach 40°C, net photosynthesis will become negative as the plant burns more carbon than it gains. So, under normal ambient CO2 levels, a grower will achieve the greatest growth with leaf temperatures just around 25- 26°C.

Fortunately, an indoor grower can adjust their environment to achieve optimal growing conditions. Controlled environments allow growers to maintain optimal temperatures, carbon dioxide concentrations, light intensity and relative humidity. So, let’s explore how a grower can adjust the growing environment to take advantage of high rates of growth that occur at high temperatures.

4. The Importance of CO2 Enhancement for High Temperature Growing

Increasing CO2 concentrations will extend the temperature range in which RuBisCO may fix CO2 into sugar. With increased CO2 we see that as temperature increases, so does the rate of the chemical reaction that RuBisCO performs. This works because an increase in CO2 concentration means that the ratio of chemical substrates to products is being increased. If the CO2 concentrations are increased from ambient 300 ppm to 1400 ppm, the change in the ratio of reactants to products will allow plants to continue to fix CO2 into sucrose at leaf temperatures well above 27°C, all the way up to about 35°C! (see Figure 1)

And as the temperature increases, so does the rate of carbon fixation and plant growth. This means that if growers are careful with their environmental controls, they may achieve very high rates of carbon fixation and plant growth at leaf temperatures well above 30°C (90℉).


[Pro Tip] When for instance going as high as 32°C  (90℉) while using CO2 levels up to 1200ppm always adjust rh to meet vpd values in your grow room for the various phases of plant growth to see optimal crop performance.

Semi professionals better stay on the safe side by not going over 1000 ppm.

5. Calculating the effect of CO2 dosage:

“Closed system”-growing rooms (using air conditioners, and only ventilating to maintain “low pressure”) can experience dropping CO2 levels upto 200 ppm. Raising this to a standard 350 ppm can make a huge difference. Here is a simple calculation to prove our point:

Added growth (%) per 100 ppm added CO2=1500×1000/(cxc)
c being the original CO2 concentration.

Example 1; raising CO2 levels from 350 to 450 ppm 1500×1000/(350×350)=12.2%
Example 2; raising CO2 levels from 1000 to 1100 ppm 1500×1000/(1000×1000)=1.5%

6. Infrared Radiation & Leaf Temperature Down the Canopy

HPS bulbs emit a large infrared peak between 800 nm and 900 nm. This infrared peak significantly increases leaf temperatures at the top of the canopy, where most of the infrared light is absorbed. When examining the differences between leaf temperatures of plants grown in the same room under either HPS or LED lights, we will see significant temperature differences that infrared light causes.

When we look at the photosynthetic activity and internal leaf temperature of leaves at different distances from the light source, the internal leaf temperature measurements are very clear. The leaves of plants under LEDs did not show an increase for internal leaf temperature significantly above the ambient room temp at any distance between 2’ and 4’ from the lamp. In contrast, the leaves of plants under the HPS lamps showed a wide range of internal temperatures; the highest temps were apparent at the top of the canopy and lowest internal leaf temperatures were at the bottom of the canopy. This partially explains why HPS lamps produce top-heavy crops while LED lighting creates a more uniform canopy.

Since the rate of carbon fixation by RuBisCO is affected by leaf temperature and CO2 concentration, increasing the ambient temperature in LED-lit rooms will increase the rate of photosynthesis and plant growth. 

7. Air Circulation and CO2 Uptake

As RuBisCO uses up CO2 inside the leaf, more is drawn in through diffusion—the natural movement of molecules from higher to lower concentrations. To enter the leaf, additional CO2 must pass through tiny pores called stomata. Since this is a passive process, only CO2 contained in the air that immediately surrounds the leaf, (known as the boundary layer), is available. Poor air circulation leads to stagnant boundary layers that are rapidly depleted of CO2. This concept is critical to maximizing CO2 enrichment. Without fans actively mixing and replenishing the air in contact with your plants, they will run low on CO2, no matter how much is available in the surrounding room.

In addition to facilitating the passage of CO2, stomata also regulate water loss through transpiration. Leaves close stomata to reduce water loss, but doing so reduces CO2 uptake. It’s a dry world out there, and C3 plants constantly regulate stomatal openings to balance CO2 uptake against water loss. Due to the large moisture gradient between leaves and the surrounding air, taking in CO2 is costly in terms of water.  for more info check https://www.ralight-holland.com/en/products/wall-fans/

When CO2 concentrations are raised well above ambient, cannabis responds by partially closing its stomata. Without the need for CO2 driving them to open, the stomata naturally close to conserve water. This is important for several reasons. It means that cannabis’ water use, per unit area, may decrease with CO2 fertilization. It also makes air mixing even more important, since partially closed stomata will slow CO2 uptake. Finally, this can lead to higher leaf temperatures by restricting transpiration. 

8. Temperature and Light Intensity

CO2 fertilization allows cannabis to thrive at higher temperatures and utilize higher light intensities, but these two factors need to be considered together. Light comes with more heat, especially in HID illuminated environments. Both parameters shift the photosynthetic machinery into higher gear and CO2 enrichment allows it to run faster and cleaner. However, even with CO2, pushing too hard with light and/or temperature can send your plants into stressful conditions.

The general recommendation for maximizing CO2 fertilization in greenhouse crops is to raise the growth temperature by five to 10 degrees Fahrenheit above the ideal temperature in the absence of CO2 enrichment. For cannabis, this means that the ideal bloom temperature is shifted into the mid to high 80s. It is important to note that ambient grow temperature does not usually represent the temperature that the plant canopy is experiencing.

A room temperature in the low 80s (℉) will translate to canopy temperatures closer to the ideal for growth, with CO2 enhancement. Some strains may enjoy an even higher temperature, but I don’t recommend running your space above 83°F, unless you know your strains will respond favorably and you have tight control of other environmental parameters. Be cautious when pushing the temperature envelope, the difference between ideal and harmful can be a few degrees.

9. Ideal CO2 Levels for different stages

As CO2 concentrations are increased well above ambient, the law of diminishing returns applies to the benefits. This means that the degree to which additional CO2 increases productivity drops as ppms increase, ultimately reaching the point at which plant stress occurs. As with most things, too much CO2 can have negative effects, leading to lower yields and leaf death at extremely high levels.

The concentration at which CO2 becomes detrimental to plant health varies widely between species. Tomatoes, for example, have an upper threshold of about 2,000 ppm, while chrysanthemums experience stress at concentrations greater than 1,200 ppm. In my opinion our favorite plant likely falls on the higher side of the continuum, around 1400 ppm, as cannabis is a highly productive annual capable of explosive growth.

Another consideration is that, over time, many plants fail to maintain the productivity gains that they initially experience with CO2 fertilization. Understanding this goes back to RuBisCO activity.

Scientists studying the phenomena have found RuBisCO levels in many plant species lowers over time in response to elevated CO2. This occurs because the environmental cues that drive RuBisCO production are suppressed under enhanced CO2 conditions. The degree to which acclimation to enhanced CO2 occurs is species-specific, and few studies have evaluated this response at CO2 levels higher than 700-800 ppm. In cannabis it may be more effective to gradually increase CO2 concentrations over the crop cycle, as opposed to raising them to the highest level immediately.

My advice is to enhance bloom in the 900-1,400 ppm range, with 1,200 ppm as a good rule of thumb for pro’s and no higher than 1000 ppm for semi pro’s..

If you are running CO2 in the vegetative phase, I don’t recommend exceeding 800 ppm. This level provides your vegetative plants with a good boost and ensures that they see a significant benefit as they move into higher CO2 in flower. If able, I also recommend experimenting with different levels of CO2 fertilization and with gradually increasing CO2 concentrations across the bloom cycle.

10. Climate control

Air Temperature: or ambient temperature needs no introduction, as we discussed this earlier, but humidity and relative humidity are a little trickier.

Humidity: the amount of water vapour in the air (not visible, unlike mist) The thing is, the warmer the air, the more water it can hold. So what your humidity DOESN’T tell you is how full of water your air is.

Relative Humidity. As the name implies, it tells you how humid your air is, relative to the temperature. RH is what % of water vapour is being held (of the total amount the air could hold at that temperature). Basically – it tells you how saturated your air is, as a percentage. Remember, the warmer the air, the more water it can hold.

The ideal humidity depends on your temperature. 50% RH at 25°C is VERY different to 50% RH at 28°C. To really understand how your humidity affects transpiration, here’s what you need to know.

One of the most important things that RH influences is transpiration (water loss through leaves). A low RH (not much water vapour) increases the rate of transpiration. A high RH (plenty of water vapour) decreases the rate of transpiration.

The rate of transpiration is VERY important.The more plants transpire, the more water and nutrients they both absorb and transport. But a balance is needed.

Too much transpiration causes undue stress.

Too little transpiration leads to poor growth.

So, to get the rate of transpiration right, your grow room RH also needs to be right. To work out the right RH, you need to know what your Vapor Pressure Deficit is…

11. What is Vapor Pressure (Deficit)?

Remember , VPD is not an actual measurement of plant stress or water loss , it is only an indirect indicator. VPD alone can’t tell you if your crop is currently ‘happy’’ or wilting due to underlying problems such as root disease or acclimatization issues.

VPD Does Not Measure Plant Water Use!!

The VPD is currently regarded of how plants really ‘feel’ and react to the humidity in the growing environment. From a plant’s perspective the VPD is the difference between the vapor pressure inside the leaf (VPS) compared to the vapor pressure of the air (VPA). If you look at it with an RH hat on; the water in the leaf and the water and air mixture leaving the stomata is (more often than not) completely saturated -100% RH. If the air outside the leaf is less than 100% RH there is potential for water vapor to enter the air because gasses and liquids like to move from areas of high concentration (in this example the leaf) into areas of lower concentration (the air). So in terms of growing plants, the VPD can be thought of as the shortage of vapor pressure in the air compared to within the leaf itself (VPS-VPA=VPD).

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 values, 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.

12. 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. for more info check: www.ocl-lighting.com

VPD Does Not Measure Plant Stress!!

Remember, VPD is not an actual measurement of plant stress or water loss, it is only an indirect indicator. VPD alone can’t tell you if your crop is currently “happy” or wilting due to underlying problems such as root disease or acclimatization issues.

VPD Does Not Measure Plant Water Use!!

VPD can only tell you about the potential for water to evaporate from the leaves. There are several other factors that affect water transport including salinity in the rooting media, root health, and whether the leaf stomata are opened or closed. Although the actual rate of water loss is not directly proportional to VPD, there is a general relationship.

It can tell you whether the crop is experiencing drying conditions and you can then make some assumptions based on this. However, the actual rate of water movement through the plant is controlled by three major contributing factors, and VPD has a role in only the first one:

1. Transpiration losses

Transpiration losses caused by the leaf (stomata) responses to the immediate surrounding environment. Contributing factors include : VPD, temperature, solar radiation, wind speed, and CO 2 levels.


2. Water availability

Water availability and water uptake. This is affected by soil water availability, salinity (osmotic pressure ) and root system structure and health.


3. Transport mechanisms

Transport mechanisms between the “root and the shoot” including the structure and health of the vascular system.


“So, yes, working with VPD does require you, the grower, to consider a couple more variables… as usual, with greater power comes greater responsibility.”

13. How to influence VPD?

It can be a challenge to find the right VPD level for crops.

The right VPD depends on the stage of the growing process.
If you’re controlling an indoor growing environment, you’re having the advantage of pushing the buttons.
You can steer towards the optimum VPD.

There are a few ways you can influence the VPD:

Increase the VPD ↑
Increase the temperature = increases the VPD
Increase the light intensity = increases the VPD
Decrease humidity = increases the VPD

Decrease the VPD ↓
Decrease the temperature = decreases the VPD
Decrease the light intensity = decreases the VPD
Increase humidity = decreases the VPD

14. Under control

Controlling these variables, air temp, leaf temp and Relative Humidity is not as straightforward as it seems, or is it?

  • Air temperature is usually controlled by a thermostat connected to a ventilator/heater or more ideal to an air conditioner, maintaining the air at a perfect 26-27 degrees Celsius.
  • Optimal Leaf temperature for photosynthesis is 25 degrees Celsius .The Leaf temperature can be influenced by keeping the right distance between the lamp and the canopy. We always advise for HPS the wattage in mm. So 1000mm (1 meter) for 1000 watts. And double that for young bulbs or seedlings. The other option to control leave temperature is to dim the lights, but from the perspective of photosynthesis that should be the last resort, because the amount of light being absorbed by the canopy is the biggest contributor to a good harvest. A healthy difference between air and leaf temperature is 1 or 2 degrees under HPS and can be 4-5 degrees Celsius under LED.
  • The relative humidity or RH is therefore the most ideal variable to adjust VPD values. Considering that air temperature and leaf temperature are under control we can easily humidify the air during the daylight period and use a dehumidifier during the night. Always keep in mind that during the last 3 weeks a RH above 70% is unwanted because of the risk of fungi like mildew or botrytis. But more often these fungi are developed during the high RH values in the night. As we discussed before, the temperature difference between day and night must be held within a 8 degrees celsius range to avoid condensing of the “cooling” -air once the lights switch off. That is why a good Dehumidifier is an absolute must during the night time, and a good Humidifier during the daytime.

Plant scientists and experienced growers tend to agree that the optimum vapor pressure deficit value is somewhere around 0.80 kPa (that’s kilopascals, a common pressure unit for VPD). Plants can grow somewhat acceptably in a fairly wide range of VPD, loosely ranging from 0.40 kPa to 1.35 kPa. VPD during Vegetative Growth.

Let’s say a grow room’s air temperature is 24 °C when the lights are on. We’ll use the full 2° C temperature drop for inner leaf temperature (recall- inside the leaf is the saturated location where the transpired water evaporates from, and the evaporation cools the leaf).

The SVP of 24°C (vapor pressure inside the leaf) is 2.53 kPaIn order to get a VPD around 0.80 we need the actual vapor pressure of the room environment (AVP) to be approximately 1.71 kPa. At 24°C that requires a relative humidity of 57%.This 24/57 combination provides a VPD value very close to the 0.80 target value.

Turning to night conditions. Let’s say we get the recommended 5°C drop and have a 19°C air temperature. There’s not as much leaf evaporation, so not as much leaf cooling. Let’s use 1°C for the leaf temperature reduction.

The SVP of 19°C is 1.92 kPa The AVP of 19°C/52%RH is 1.11 kPa.

These conditions also provide VPD just slightly greater than the 0.80 target value. So VPD tells us we should run our 24°C room at 57%RH. When the lights go off and the room drops to 19°C, we want the humidity to be at 52%RH.

15. VPD for Different Stages of Growth

So now you know what VPD is, how it impacts plants, and how you can change VPD. All that’s left is to figure out how to use this information in your own grow. Ideal VPD, as a general rule for plant growth, is around 0.8 – 1.2 kPa (kilopascals).

However, your plants have different needs during different stages of growth. You need to tailor your grow environment to the stage of growth that your plants are in. Below are some general recommendations, but keep in mind that these recommendations can vary strain to strain, and setup to setup. As always, observe your plants and tweak things accordingly.

Ideal VPD for Seedlings and Clones
Clones and seedlings are baby plants, they can’t handle a lot of stress because they are still just trying to form roots. Target a higher humidity and VPD closer to the lower end of the general range.

The ideal VPD for seedlings and clones is closer to 0.8 kPa.

Ideal VPD for Vegetative Stage
In veg (vegetative) stage the plants are bigger and more robust. You can reduce the humidity in your environment to increase VPD. This will increase water and nutrient uptake, but you don’t want to increase VPD too much. This will cause the stomata of the plants to close, causing them to absorb less CO2.

CO2 is particularly important in the vegetative stage, because that’s the main ingredient that plants use to grow large.

The ideal VPD for the vegetative stage is close to the middle of the general range, around 1.0 kPa.

16. VPD during Flowering

Okay, those were the calculations with the focus on minimum plant stress. Now let’s go through the same exercise for recommended flowering conditions. Here, we’re going to use the reduced humidity values recommended by many growers with the aim to maximize resin (terpene) production and minimize the chance of fungal infection.

Let’s say, again, that the room’s air temperature is 24°C when the lights are on. We’ll use the full 2°C temperature drop for leaf temperature. This time we’ll target 45%RH and then see what the VPD comes out to.

The SVP of 22°C is 2.53 kPa The AVP of 24°C/45%RH is 1.35 kPa

These conditions provide a 1.18 VPD value, which is getting dry but is still within the recommended growing range.

For night conditions. Use the recommended 5°C drop for a 19°C air temperature. Let’s once again use the 1°C leaf temperature reduction.

The SVP of 17°C (inside the leaf) is 1.92 kPa The AVP of 19°C/45%RH (the grow room ambient) is 0.96 kPa. These night conditions provide VPD of 0.96 which actually gets us closer to the optimum VPD, so all is good with these temperatures and 45%RH.

17. Practical use of VPD

Realize that at this level you are doing some serious high performance fine-tuning of your gardening operation. You could be adding a few percent to the final weight of your yield, but it’s going to take some work and you are going to need the proper equipment to measure and control your garden at this level.

Keeping the focus here on water vapor, you’ll need a way to add moisture to your environment and a way to remove it (humidification and dehumidification). You will need to accurately measure %RH and temperature and you’ll need a good oscillating fan system (for example RALIGHT) to jiggle the leaves. We especially want to avoid condensation; this means watch out for uncontrolled temperature drops between daytime and night.

The dehumidification system is required because (especially in the growth phase) there will be a lot of water vapor in the air during the lights on period. Much of this moisture will need to be removed as the lights go off and the temperature drops. A 24°C room at 57%RH, when cooled to 19°C goes to around 80%RH. That isn’t acceptable. You will need to remove water vapor from the room at lights out. Air conditioners are the shortest way to climate control for cooling and dehumidification.

The humidification system is required because at night’s end when the lights come on and the temperature climbs back up to 24°C what was 52%RH becomes 37%RH. Again, at least for the growth phase, definitely not acceptable. You must get water vapor into the air, usually growers do this with a fogging/misting system. For a moderate sized room and fairly effective misting, we’re talking about getting a couple liters of water up, into the air as quickly as possible. Use a professional humidifier that doesn’t spray but dissolves the water into the air!

When you have the air moisture environment optimized, repeatable, and otherwise pretty much under control you can move on to adjust the other variables for best yield. Plants that aren’t stressed from too high or too low air water vapor content are much better able to respond to high performance optimizing of CO2, nutrients, and lighting.

18. What VPD do I want in my growroom?

Danger Zone <0,4
Low transpiration 0,4-0,6 KPa (Propagation, Early vegetative, wk 1,2)
Healthy transpiration 0,7-1,0 KPa (Late vegetative, early flower stage, wk 3,4,5,6)
High transpiration 1,1-1,3 KPa (Late Flower stage wk 7,8,9) Danger Zone >1,4

(Indica strains prefer slightly lower vpd than sativa strains)

19. Lightplan

So now the climate is under control, humidity sorted out, temperature at a perfect level for optimum photosynthesis and ec levels are steady, what’s next? Often overlooked is a good Lightplan that takes into account the wattage, beam angle, footprint and light color, all corresponding and tailor fit to the specific situation your grow room desires.

A carefully considered lightplan creates an evenly distribution of light, at the right intensity with the right color and at the right mounting height. The target light level at the crop canopy should be the starting point of a decent lightplan. All other factors that will affect the layout and performance of the grow lights should be considered to determine how to achieve the desired light level.

If there is an overlap in footprint, the doubled intensity of the light will create hot-spots resulting in higher VPD and thus more transpiration locally. So you are in fact creating a “micro-climate” within the growroom. The water use will be significantly higher where the light intensity is double. Most irrigation systems focus on giving all plants the same amount of water, resulting in the fact that the hotspots may be too dry while at the same time “ in the normal light intensity zone” the hydration is normal or even on the wet side of the spectrum. Make sure you know the beam-angle of your light source and hang them at an altitude that the footprints line-up. Industrial light fittings (like OCL) don not need to be lowered to achieve optimal results. Once you know the right height and space in between them, you can “draw a rectangle on the floor to visualise your footprint. If it gets a bit too technical ask advice from your local lightsource.

For more information on PAR (photosynthetically active radiation), THD (total harmonic distortion), CRI (color rendering index) and more check out the OCL website www.ocl-lighting.com

20. Connecting the dots

So now we have this “new” insight of how VPD is affecting our plants, we can relate this to the use of e.c. values, irrigation and CO2 and on how, when and where to dose in combination with little or more air movement. 

The influence of e.c. (electro conductivity) value on the plants ability to transpire and how it affects the drying out or burning of the canopy is something we have to keep in mind. The underlying phenomenon is called osmosis. Water will move from a less concentrated solution (through a semi-permeable membrane) to a more concentrated one. So if the ec value in your growing medium is higher than that of your plants, the osmosis is working against the natural “draft” of the plant upwards, making it very difficult to transpire. The added ec values that have built op over time, are a direct result of not enough irrigation quantity wise, in combination with high transpiration of the plant possibly by a high VPD value.

So playing with different ec values can be useful. Lowering e.c. will make it easier for the plant to absorb water and create a higher cel pressure, very useful under demanding circumstances. Higher ec values can help us to keep the plants “short’ but also create smaller, more dense flowers.

Air movement is a good method for improving transpiration when needed, for example for cooling or for the uptake and transport of nutrients. I hope you are getting the picture, because that is what connecting the dots is all about.

As we spoke about earlier, plants bind CO2 using the lights energy and chlorophyll through photosynthesis to create sugars. This means CO2 is the main building block for plantlife. Creating air flow results in a thinner layer of air around the leafs and makes it easier to absorb CO2 leading to more plant growth.

Air movement is crucial for good CO2 absorption. The CO2 value in standard atmosphere is between 350 and 400 ppm. The plant absorbs CO2 meanwhile lowering CO2 to values below 250 ppm while slowing down photosynthesis.


  • Improving the use of CO2 can be found in the timing (always lights on) and higher values for example when boosting the lights! Plants have to get used to higher CO2 values, therefore we advise to start from day one with a 350 ppm CO2 value and raising it every week with 100 ppm. Considering you know a lot about VPD now and all your setpoints are being met, you could go as high as 1200 ppm, although our experience tells us that above 1000 ppm the gained advantage is lower than the risks.
  • Plants get used to higher CO2 levels. About ⅔ of photosynthesis takes place in the top ⅓ of the plant. This is the reason CO2 must be dosed in the top of the canopy because CO2 is heavier than air, although good air circulation can prevent this potential problem.


Relative Humidity + Ambient temperature = Vapor pressure Air


RH (100%) + Leaf Temperature = Vapor pressure Leaf or VPS


The difference between the two is the VPD as discussed…


To show how CO2 dependant this whole discussion is, I would like to add CO2 to the VPD equation

To make things a little more insightful:

… But the common variable ‘Temperature’ is dependant on the amount of CO2 available due to the activity of RuBisCo… So let’s say that during the growth stage the CO2 level is 380 PPm (standard atmosphere) or the CO2 level is elevated to 800 ppm, the temperature must be elevated from 26 to 30-32°C completely changing the vpd variables. However the target value remains the same 0.7-.0.8 for Indica strains, 0.9-1.0 for Sativa strains. So that means you have to raise the RH Value even more!!!!!

To show how CO2 dependant this whole discussion is, I would like to add CO2 to the VPD equation.


  • Use VPD as a tool to perfect your Indoor climate Without it

 “no matter how good you think you are, your plants will never get as good as they can without making use of VPD values!”

  • Use CO2 fertilisation, but use it sensible in the right quantities and in the right way
  • Use an air conditioner, for temperature control/dehumidification
  • Get a professional humidifier! (No suggestions yet)
  • Get a professional de-humidifier (we suggest Quest dehumidifiers)
  • Get professional lighting equipment. (we suggest OCL lighting)
  • You need a VPD monitor/ controller to make sure your VPD values are met, no matter how much or how little CO2 you use, With CO2 allowing higher temperatures and raising RH levels even more! Make sure the controller changes RH instead of dimming the lights! 
All this can seem like a lot of work, and it is sometimes, but if you truly wish to experience your plants’ full genetic capability the effort is well worth it.