Vaper Pressure Deficit: VPD Chart

[Reave]

That’s where I run mine too. 70% humidity 72F leaf temps. Later in flower I drop to 65rh. Buddy in one of the organic pod casts said you can run high humidity in flower when you are rocking the right VPD. He said they actually get more stretch when you do it that way. Now obviously some plants don’t like humidity and are weak against PM(mold). Ride those sweet spots and the plants will grow WAY faster. Once I started dialing in my VPD my 2 week old plants looked like monsters compared to when I started growing.

 

[Reave]

This is the one I use:

I trust the source.

 

[ppm Charlie]

This is the one I use:

I trust the source.

This chart has no information on their "assumptions". If you look at the charts I posted - people make assumptions based on the difference between the leaf surface temp and room temp. I see no info here on what assumptions they made or what lighting they used. Kind of a useless chart then if you do not know how they came up with there RH vs Temp chart.

 

[ppm Charlie]

Ok so I did some digging. I still prefer growing at 22C when in late flower. But here's some research, and the studies it cites,.taken from Manic Bonantix

Light, CO2, Temperature, Relative Humidity and Photosynthesis​

The Relationship between Light, CO2, Temperature and Relative Humidity in Photosynthesis



Photosynthesis defines the process by which plants manufacture glucose which becomes the building blocks for growth. Scientists summarize the process as follows: using light, carbon dioxide + water = glucose + oxygen. The process occurs within special structures called chloroplasts located in the cells of leaves. Optimum photosynthetic rates lead to the removal of greater amounts of carbon dioxide from the local atmosphere, producing greater amounts of glucose.



While photosynthesis involves a series of complex biological and chemical processes, put simply, the rate of photosynthesis determines the rate of growth. A plant which is photosynthesizing at optimum will grow and produce at optimum levels while a plant photosynthesizing at below optimum will grow and produce at below optimum.



The key factors affecting the rate of photosynthesis are light intensity and colour, carbon dioxide concentration and temperature. In any given situation any one of these may become a limiting factor; in other words a deficit of CO2 and/or light, and/or excessive or too low air temperatures directly affects the rate at which photosynthesis can take place.



Light and rate of photosynthesis



Light is the energy of photosynthesis; it powers all aspects of plant growth. Without light there would be no green plant life on earth.



Energy = Energy



Light is primarily responsible for creating chemical energy (in the form of glucose) in plants. Glucose then becomes the fuel for other agents of plant growth.



This process is called photosynthesis. Photosynthesis is the fundamental food making process in all green plants.



Light is the energy source for the complex chemistry of photosynthesis.



In a nutshell: The plant takes in water and nutrients from the growing media and carbon dioxide (gas) from the air, and in the presence of chlorophyll it makes the simple sugar glucose, which is now the store for light energy. In this process oxygen is separated from the water and passes out into the air.



Glucose is the building block for several other sugars and complex carbohydrates that the plant makes (e.g. fructose, sucrose, starch). The glucose is stored in the leaf tissue. In addition to this, it is the leaves that catch and trap the light energy. Therefore, leaves receive light and store it as chemical energy.



The light energy required by plants is confined almost entirely to the visible spectrum of light (400nm – 700nm).



While there are key points within this spectrum (435nm and 675nm etc), growth is optimized under the entire range of the spectrum. This is because different color wavelengths stimulate different biochemical reactions within the plant. As a result of this, different physiological functions are activated and energized, which, in turn, determine plant growth rates and formation (morphological) characteristics.



Photosynthesis depends on the energy created by a combination of both light intensity and color.

Photosynthesis is a cumulative process. During the daylight hours a plant traps and stores light energy. This process can be likened to water dripping into a bucket. Over several hours (let’s say 18 hrs) the bucket slowly accumulates water. When the bucket is full, the water which continues to drip into the bucket can no longer be captured and goes to waste. However, if the tap is dripping too slowly the bucket doesn’t become full within our 18 hour timeframe.



The accumulation of light energy is similar to this. If the plant receives adequate levels of light it is able to fill itself with chemical energy. In a situation where there is not enough light the plant will only become partly filled with chemical energy. If the plant receives more light than it is able to use, the excess light goes to waste.



Most plants experience optimum growth in high light intensities. However, a single leaf is light saturated at a much lower level than is required to saturate an entire plant. The higher intensities of light are needed to compensate for light shading (light shading refers to areas of the plant that only receive low levels of light, or no light, due to foliage growth inhibiting the access of light). The plant draws on the excess light in order to fulfill the needs of the entire plant. That is, the plant is able to draw in higher levels of light than is required by the immediate leaf area and then distribute the light energy throughout areas of the plant that are shaded by foliage growth.



The parts of the plant that are open to light, therefore, can be described as receivers for energy and conductors of energy to the larger body of the plant. Too much shading or not enough light will greatly limit this process.



Again, let’s consider our bucket example: here we should think of our bucket as not a single bucket but, instead, many smaller buckets which we wish to fill. This time our dripping tap can quickly fill a single small bucket. However, in order to fill all of the smaller buckets our dripping tap still has to fill the equivalent volume of the larger (single) bucket.



In this example only some of the buckets can catch water from the dripping tap. However, the buckets that are able to catch water can also release water into the buckets that aren’t catching water from the dripping tap. This means that some of the buckets have the role of ensuring that all of the buckets are filled.



If sufficient numbers of the buckets are able to catch water they will slowly fill all of the buckets (within the given timeframe). If only a few of the buckets can catch water then all of the buckets will only be partially filled.



The cumulative process of plants trapping light and storing light as chemical energy is similar to this. If you like, a plant’s leaves are a series of buckets that we wish to fill with chemical energy within a given timeframe; the leaves that receive the light distribute chemical energy throughout the plant for storage. This means that the more leaves that are able to trap light, the better the photosynthetic potential. Therefore: Light color + light intensity + surface area of plant exposed to light = chemical energy potential.



We’ll be covering a great deal more information on light later in the book so let’s leave that one there for now.



Carbon dioxide and rate of photosynthesis



Light provides the energy for photosynthetic pigments to convert carbon dioxide (CO2) and water into sugars and oxygen. As light intensity increases, up to a point where the machinery of photosynthesis – the chloroplast – can no longer convert the light into chemical energy, the amount of sugar increases and thus, more energy is available for plant growth and maintenance. However, the concentration of CO2 also influences photosynthesis in a dramatic way.



To simplify things somewhat, plants consume CO2 and release oxygen during the day as part of the process of photosynthesis (carbon dioxide + water → sugar + oxygen). At night they consume oxygen but don’t release oxygen. Instead they release CO2 as part of the process of respiration (sugar + oxygen → carbon dioxide + water). Therefore, plants require CO2 during the day for photosynthesis and oxygen at night for respiration.



During photosynthesis CO2 enters the plant through small openings in the leaves called stomata. It is then captured or ‘fixed’ by photosynthetic enzyme Rubisco and is then converted into carbohydrates. When atmospheric CO2 concentration goes up, more CO2 will enter the leaves of plants (photosynthetic/growth rates will increase) because of the increased CO2 gradient between the leaf and the air.



CO2 plays the most important role in the biomass production of plants because more than 90% of dry matter of living plants is derived from photosynthetic CO2 assimilation.[1] Plants use the carbon from CO2 and convert it into carbon compounds such as glucose, carbohydrates, lignin, and cellulose which is what becomes the biomass of the plant.



An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction, and so the rate of photosynthesis generally increases until limited by another factor or until the point at which critical mass is achieved and additional CO2 cannot be used by the plant. I.e. think of the plant as a growth factory. The factory can only operate so efficiently. The machinery of photosynthesis (the chloroplast) has its limits. There comes a point where more CO2 simply goes to waste.



Air Temperature and rate of photosynthesis



During photosynthesis CO2 enters the plant through small openings in the leaves called stomata where it is then captured or ‘fixed’ by photosynthetic enzymes and is then converted into carbohydrates. When air temperatures become excessively warm a plant may close its stomata to reduce water losses. When ambient conditions are excessively warm for a plant and it closes its stomata for too long in an effort to conserve water it has no way to move carbon dioxide and oxygen molecules resulting in less than optimal photosynthesis.



Additionally, photosynthesis takes place in two stages which are light-dependent reactions and the light-independent reactions of the Calvin cycle. The light-independent reactions of photosynthesis are dependent on temperature. They are reactions catalysed by photosynthetic enzymes.



Enzymes are protein molecules used by living organisms to carry out biochemical reactions. The proteins are folded into a very particular shape, and this allows them to bind efficiently to the molecules of interest. At lower than optimal temperatures, the enzymes that carry out photosynthesis do not work efficiently (enzymatic activity decreases), and this decreases the rate of photosynthesis.



As temperature increases the enzymes approach their optimum temperatures and the overall rate of photosynthesis increases until a threshold at which maximum photosynthesis occurs. As temperature rises above optimum, enzymatic activity decreases until such a point where temperatures become so high that the enzymes are denatured (destroyed).



Thus, at below or above optimum temperatures the rate of photosynthesis decreases.



Generally speaking, the optimum range for daytime temperatures for heavy flowering indoor crops tends to be between 24 – 30oC (75.2 – 86oF), dependent on genetics and other environmental factors, with 26 – 28oC (78.8 – 82.4oF) being ideal for most varieties. Following is a graph that demonstrates the rate of photosynthesis in relation to leaf temperatures.





Source: Chandra, S. Lata, H , Khan, I. A. and Elsohly, M. A. (2008) Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions





Heat and Photorespiration



Rubisco is a key enzyme in photosynthesis catalyzing carbon dioxide fixation. Rubisco is ubiquitous for photosynthetic organisms and is regarded as the most abundant protein on earth



The simple view of photosynthesis is that it involves carbon dioxide (CO2) uptake and release of oxygen (O2). Unfortunately it is not quite this simple. Even during photosynthetic CO2 uptake (‘fixation’), some CO2 is simultaneously released by the process of photorespiration. Photorespiration is a consequence of the high oxygen content of the air, which leads to a competing oxidation reaction at the same site as carbon fixation, resulting in loss of carbon and energy from the plant. This is largely seen as a wasteful process where the rate of carbon fixation is reduced.



Any factor that reduces the availability of CO2 or increases the availability of O2 to rubisco will increase the levels of photorespiration.



One factor that increases photorespiration is high air temperatures.



The decrease in photosynthesis rate, or rise in photorespiration, as temperature increases is due to an increase in the affinity of rubisco and oxygen. Rubisco combines more with oxygen relative to carbon dioxide as temperature rises, which slows the rate of photosynthesis. In other words, rubisco acts mainly as a carboxylase (combining with carbon dioxide) at lower temperatures but acts more as an oxygenase (combining with oxygen) at higher temperatures.



Studies have consistently shown that the rate of photorespiration is decreased with cooler air temperatures.



In understanding the impact that temperature has on the rate of photorespiration you can perhaps see that there is a very narrow range in what would be considered optimum temperatures to promote optimum growth. Cool temperatures will decrease the rate of photorespiration; however, cool temperatures also decrease the rate of photosynthesis. As temperature increases the rate of photosynthesis increases but so too does the rate of photorespiration. At warmer than optimum temperatures the rate of photorespiration increases to a point where optimum rates of photosynthesis are compromised as the plant acts more to oxygenase, resulting in lower CO2 fixation and growth rates.



In understanding this it is also important to understand that different genetic variants of the same species of plant will have an optimum temperature which promotes optimum growth. For this reason, while we can generalize somewhat and say optimum growth is likely to occur between the temperature range of 26 – 28oC (78.8 – 82.4oF) it is important to understand that the strain you are working with may perform best at slightly higher or lower temperatures than those which are typically stated as optimum in literature. For example, some tropical varieties, which evolved in warmer climates, tend to perform best at 29 – 30oC (84.2 – 86oF) while some cooler climate, Northern Hemisphere varieties tend to perform best at 26 – 27oC (78.8 – 80.6oF). Therefore, it is recommended that you cautiously experiment with growroom temperatures to identity what is the sweet spot for promoting the highest level of growth with the particular genetics you are working with.



Heat Stress in Plants



High leaf temperatures reduce plant growth and limit crop yields. In outdoor grown crops, estimates range up to a 17% decrease in yield for each degree Celsius increase in average growing season temperature.[2] Even moderate heat stress can reduce the photosynthetic rate to near zero in plants.[3]



Damage to leaves (plant tissue necrosis) can be caused by reactive oxygen species (ROS). For example, rubisco can make hydrogen peroxide (H2O2) as a result of oxygenase side reactions. H2O2 production by rubisco was recently shown to increase substantially with temperature. Overproduction of reactive oxygen species (hydrogen peroxide, H2O2; superoxide, O⋅-2; hydroxyl radical, OH⋅ and singlet oxygen, 1O2) can cause oxidative damage to plant macromolecules and cell structures, leading to inhibition of plant growth, or even to plant death. [4]



Visual heat injury symptoms in plants include scalding and scorching of leaves and stems, sunburn on fruit and flowers, leaf drop, rapid leaf death and a reduction in growth.





Daytime versus Night Temperatures – Thermoperiod DIF



While not strictly relevant to photosynthesis per se, one cannot discuss optimum daytime (lights on) temperatures without also talking about optimum night (lights off) temperatures.



That is…



An often overlooked environmental factor that can greatly impact yields is the DIF, or the day/night temperature differential which is also referred to as the thermoperiod or thermoperiod DIF. DIF is the difference in the highest daytime (lights on) temperature and the lowest nighttime (lights off) temperature which is calculated by subtracting the nighttime temperature from the daytime temperature. So, if our daytime temperature was 28oC (82.4oF) and our nighttime temperature was 18oC (64.4oF), DIF would be 10oC (50oF). In this case because we have a higher day than night temperature we would have a positive DIF of 10oC (50oF) or DIF = + 10oC (+50oF). If we were to reverse this situation and have a warmer night temperature than day we would have a negative DIF or DIF = – 10oC (- 50oF). If night and day temperatures were the same this would be expressed as equal DIF.



Generally speaking, optimum growth rates with heavy flowering indoor crops will be achieved when daytime temperatures are about 6-10oC (42.8-50oF) above nighttime temperatures (positive DIF). This allows the plant to photosynthesize (build up) and respire (break down) during an optimum warmer daytime temperature and to curtail the rate of respiration during a cooler night. Temperatures higher than needed at night cause increased respiration, sometimes above the rate of photosynthesis.



Thus, optimum range for daytime temperatures for heavy flowering indoor crops, as previously noted, tends to be between 24 – 30oC (75.2 – 86oF), dependent on genetics and other environmental factors, with 26 – 28oC (78.8 – 82.4oF) being ideal for most varieties, while optimum nighttime temperatures should be 6-10oC (42.8-50oF) lower at 18 – 22o (64.4 – 71.6oF).



There is one proviso to this, which we’ll cover later when discussing reducing plant stem elongation (stretch) through running an equal or negative DIF during the ‘stretch’ phase of the crop cycle.



Relative Humidity and Photosynthesis



Excessive relative humidity (RH) prevents plants from properly taking in CO2 and moving nutrients and water, resulting in a reduction in photosynthesis.



Relative humidity refers to the amount of water vapor in the air relative to the maximum amount of water vapor that the air can hold at a certain temperature. If the relative humidity level is 75 percent this means that every kilogram of the air in the respective space contains 75 percent of the maximum amount of water that it can hold for a given temperature.



Relative humidity levels affect when and how plants open the stomata on the undersides of their leaves. Plants use stomata to transpire, or “breathe.” Transpiration is the evaporation of water from the surface of leaf cells in actively growing plants. The process of transpiration provides the plant with evaporative cooling, nutrients, carbon dioxide entry and water.



Land plants can transpire passively by evaporation because the difference between the humidity of the gas in the stomata and the surrounding air causes the water in the stomata to diffuse outward.



A hydrated leaf would have a RH near 100%. Any reduction in water in the atmosphere below this creates a gradient for water to move from the leaf to the atmosphere. The lower the RH, the less moist the atmosphere and the greater the driving force for transpiration. When RH is too high, the atmosphere contains more moisture, reducing the driving force for transpiration.



A reduction in transpiration reduces CO2 intake, resulting in less than optimal photosynthesis.



Low Humidity Also Reduces Photosynthesis



If humidity is very low the rate of transpiration becomes too high. As a result, the plant closes its stomatal openings to minimize water loss and wilting. Unfortunately, this also means photosynthesis is slowed and subsequently, so too is plant growth.



Optimum relative humidity levels that ensure high rates of photosynthesis are typically expressed at between 45 – 75%. However, it is important to note that higher levels of humidity can promote leaf and flower fungal infections (e.g. botrytis and powdery mildew) and, therefore, the lower end of the humidity range (45 – 50% RH) is recommended once flowers begin forming.



References

[1] Zelitch I (1975). Improving the efficiency of photosynthesis. Science, 188: 626-633.

[2] Lobell D.B. & Asner G.P. (2003) Climate and management contributions to recent trends in U.S. agricultural yields. Science 299, 1032

[3] Sharkey, T.D. Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene. Plant, Cell and Environment (2005) 28, 269–277

[4] Hossain MA, Bhattacharjee S, Armin SM, Qian P, Xin W, Li H-Y, Burritt DJ, Fujita M, Tran LSP (2015) Hydrogen peroxide-priming modulates abiotic oxidative stress tolerance: insights from ROS detoxification and scavenging. Front Plant Sci 6:420

I think the key in this informed article posted by @Rexer (thks, Rex) is this passage:

"Generally speaking, the optimum range for daytime temperatures for heavy flowering indoor crops tends to be between 24 – 30oC (75.2 – 86oF), dependent on genetics and other environmental factors, with 26 – 28oC (78.8 – 82.4oF) being ideal for most varieties."

There you go. I range of temp recommended for flowering based on science.

For LEDs the higher temperature levels in the ranges is better. For HID/HPS the lower temperature levels of the ranges is better.

@Bill284 - You can resume smoking that doober now. Sorry to interrupt the "morning process".

 

[Bill284]

I think the key in this informed article posted by @Rexer (thks, Rex) is this passage:

"Generally speaking, the optimum range for daytime temperatures for heavy flowering indoor crops tends to be between 24 – 30oC (75.2 – 86oF), dependent on genetics and other environmental factors, with 26 – 28oC (78.8 – 82.4oF) being ideal for most varieties."

There you go. I range of temp recommended for flowering based on science.

For LEDs the higher temperature levels in the ranges is better. For HID/HPS the lower temperature levels of the ranges is better.

@Bill284 - You can resume smoking that doober now. Sorry to interrupt the "morning process".

Thank you sir I put it down to read.
I'm just enjoying everyone's opinion on the matter.
Once I got my rooms dialed in I never worried about it again.
All this back and forth is very interesting though.

Stay safe everyone
Bill

 

[Rexer]

I didn't get a chance to read this one yet. But it's right on topic, and is from a trusted source.

Vapour Pressure Deficit (VPD) for Indoor Growers​

By manicbotanix

Relative Humidity versus VPD



Relative Humidity



Relative humidity refers to the amount of water vapor in the air relative to the maximum amount of water vapor that the air can hold at a certain temperature. If the relative humidity level is 75 percent this means that every kilogram of the air in the respective space contains 75 percent of the maximum amount of water that it can hold for a given temperature.



Relative humidity levels affect when and how plants open the stomata on the undersides of their leaves. Plants use stomata to transpire, or “breathe.” Transpiration is the evaporation of water from the surface of leaf cells in actively growing plants. The process of transpiration provides the plant with evaporative cooling, nutrients, carbon dioxide entry and water. Land plants can transpire passively by evaporation because the difference between the humidity of the gas in the stomata and the surrounding air causes the water in the stomata to diffuse outward.



A hydrated leaf would have a RH near 100%. Any reduction in water in the atmosphere below this creates a gradient for water to move from the leaf to the atmosphere. The lower the RH, the less moist the atmosphere and the greater the driving force for transpiration.



To generalize, plant growth is improved at a higher RH; however, excessive humidity can result in lower rates of transpiration which limits the transport of nutrients. One study showed that under high humidity (95% RH) P, Ca and S uptake were reduced by 9.6%, 8.7% and 27% respectively in tomato plants.[1] In another study, Shamshiri et al. (2014) showed that maintaining an optimum humidity caused the hourly water uptake rate to increase by 35 to 50%. They also noted that increases in water uptake led to a higher crop yield.[2] This information perhaps offers some insight into the important role humidity plays in plant growth.



Further, excessive humidity increases the risk of disease outbreaks. Therefore, while higher humidity is preferred by plants there comes a point at which excessive humidity begins impacting on plant health.



If high humidity conditions exist at the same time as high temperatures, the plant gets stressed as it can’t evaporate enough water from its foliage to cool its tissue and it will overheat. Cell damage, wilting and reduced growth can occur when hot plants can’t effectively cool themselves via transpiration due to high humidity.



Low Relative Humidity Also Reduces Growth



In general, plants don’t benefit from overly dry atmospheres, as dry atmospheres rapidly suck the moisture from the foliage and this can lead to a reduction in photosynthesis, fruit size and growth. When humidity is too low the rate of evaporation from the leaves can exceed the supply of water into the roots. This causes the stomata to close, and photosynthesis to slow or stop. Once the stomata close, the leaves are at risk of high temperature injury since evaporative cooling is reduced due to the lack of water to evaporate.



Optimum relative humidity levels that ensure high rates of growth are typically expressed at between 65 – 75% in developed plants. However, it is important to note that higher levels of humidity can promote leaf and flower fungal infections (e.g. botrytis and powdery mildew) and, therefore, low humidity at about 50 -55% RH is recommended during the latter stages of flowering.



Differing Relative Humidity Optimums for Different Points of the Crop Cycle



As higher and lower humidity ranges can encourage certain behaviours in plants, growers can use humidity to manipulate the environment based on the plant’s growth phase.



For example, during the early vegetative phase of a plant’s lifecycle, it has a small, undeveloped root system and as a result it lacks the ability to uptake high levels of water and nutrients. This is the case during cloning or transplant shock when clones are first placed into the growing system. Growers therefore may aim to maintain high humidity at about 85% to prevent excessive water loss from the leaves to reduce plant stress.



As the plants move into the late vegetation phase and into early flowering, reducing humidity to about 65 – 75% will help maintain optimal rates of nutrient/water uptake and transpiration.



However, it is important to note that running a lower humidity (approx. 50%) during the stretch phase of the crop cycle (the first 2 – 4 weeks of the 12/12 light cycle, genetic dependent) can help in reducing stretch. Higher humidity causes more lavish growth with longer shoots and sometimes larger leaves. When the humidity is increased, plants stretch the main stem, shoots and petioles more. The cell elongation and hence the stretching of the entire crop depends on the cell pressure (Turgor pressure). In the event of low humidity, evaporation is high and the cell pressure of the crops is relatively low. This will slightly slow down the cell elongation reducing plant stretch. Under conditions with little evaporation (high humidity), the cell pressure is generally higher and more stretching can occur.



When plants move into the mid to late flowering stage, growers can look to decrease humidity further to about 50 – 55%. By encouraging leaf evapotranspiration with low humidity, plants can stay cooler through higher rates of transpiration and avoid flower fungal diseases.



Further, relative humidity optimums, in reality, when discussing the fine science, come down to vapour pressure deficit (VPD) which is determined by air pressure, canopy temperature, ambient air temperature and relative humidity.



Vapour Pressure Deficit (VPD): Wrapping Temperature, Humidity and Transpiration Potential into a Single Value



I’m going to keep this one as simple as possible because vapour pressure deficit (VPD) is more information than many beginner (newbie) indoor growers need to know. I.e. VPD is a handy a tool for environments where air temperature fluctuations occur during the course of the day (lights on hours). However, where environments have a stable temperature at any fixed air temperature and pressure, there is an excellent inverse relationship between RH and VPD. For this reason, many growers simply use RH values for the same purposes as VPD with good results.



This said, talking about VPD helps us to wrap up on temperature and humidity and the all-important roles they play in plant growth. Further, and perhaps more importantly, talking about VPD enables us to look more closely at temperature and humidity optimums and how ambient air temperature influences what would be considered optimum RH. You’ll find, for example, a table in the following material that cross references temperature and humidity optimums. This table makes for a very handy reference guide to ensure that your grow room is running within optimal temperature and humidity parameters at all times.



What is VPD?



VPD essentially measures evaporation potential in plants. That is, VPD measures the ability of the plant to release water from the stomata into the surrounding atmosphere through transpiration. In turn, because the rate of transpiration greatly influences the ability of the plant to draw in water and nutrients through the roots, VPD is the driving force for water and nutrient movement between roots and leaves.



Therefore, a key parameter for controlling plant water/nutrient uptake in the growing environment, which, in turn, affects growth and yield, is the air water ‘vapour pressure deficit’ (VPD).



All gasses in the air exert a certain “pressure.” The more water vapour in the air the greater the vapour pressure.



Vapour pressure deficit comes down to the difference between the vapour pressure inside the leaf compared to the vapour pressure of the air. In high RH conditions there is a greater vapour pressure being exerted on the leaf surface than in low RH conditions. From a plant’s perspective, high vapour pressure can be thought of as an unseen force in the air pushing on the plants. This pressure is exerted onto the leaves by the high concentration of water vapour in the air making it harder for the plant to push back by losing water into the air by transpiration. This is why plants grown in a high RH environment transpire less. By comparison, in environments with lower RH, only a small amount of vapour pressure is exerted on the plants leaves, making it easy for them to release water into the air. Therefore, vapour pressure greatly influences the amount of water vapour a plant can transpire into the surrounding atmosphere.



VPD is the difference between saturation vapour pressure (the maximum saturation pressure possible by water vapour at a given temperature) and the actual vapour pressure or, in layman’s terms, the difference between the amount of moisture in the air and the amount of moisture the air can hold when saturated. It is directly related to transpiration and affects the quality and yield of plants. The water vapour pressure increases exponentially with an increase in air temperature. Estimation of plant evapotranspiration or water loss to the atmosphere depends on VPD.



While we are now familiar with the role RH plays in plant growth it’s not necessarily the best measurement to understand vapour pressure. This comes down to the point that we covered earlier about relative humidity being the amount of water vapour in the air relative to the maximum amount of water vapour that the air can hold at a certain temperature. If the relative humidity level is 75 percent this means that every kilogram of the air in the respective space contains 75 percent of the maximum amount of water that it can hold for a given temperature. So, for example, cold air holds less water vapour than warm air; the water-holding capacity of air doubles with every 10oC increase in temperature. Therefore, air at 28oC can hold twice the amount of water vapour when compared to air at 18oC. This means the vapour pressure of the air at any given RH value can vary considerably depending on temperature. As a result, humidity alone cannot be used as a good indicator of the vapour pressure on plants.



Vapour pressure deficit (VPD) combines the effects of both humidity and temperature into a single value; it’s basically a measure of the drying capacity of the air, which drives transpiration. According to Zolnier et al. (2000), VPD is capable of more accurately reflecting how the plant feels by taking into account both the measurements of temperature and RH.[1] Therefore, VPD is an ideal means in which to establish the optimum relative humidity in the grow room at a given temperature.



VPD Optimums



Where cannabis is concerned the optimum VPD range is typically expressed between 0.8–1.1 (kPa). However, it is a bit more complex than this.



VPD can be measured in pounds per square inch (psi), millibars (mb) or kilopascal (kPa), with kPa or mb being commonly expressed units of measurement which tend to be used interchangeably between authors. This can make things a little tricky when you need to compare various data/information where different units of measurement are used. However, to convert kPa to mb one simply needs to multiply the kPa value by 10 to establish mb. E.g. 0.8 (kPa) x 10 = 8 (8 mb). Or to convert mb to kPa one simply divides by 10. E.g. 8 (mb) divided by 10 = 0.8 (0.8 kPa).



Stating an optimum VPD is tricky as there is no one size fits all answer. Much like RH optimums, VPD ideals are influenced, in part, by the point of the crop cycle where younger less established plants do better under low VPD (high humidity) while more established plants and flowering/fruiting plants do better under higher VPDs (lower humidity) because factors such as fungal pathogens need to be taken into consideration.



Optimal VPD can also change depending on lighting conditions and other factors. For example, optimal VPD during the day is usually lower than optimal VPD during the night. In general, it’s better to have a rise in VPD (lower humidity) during the night relative to the VPD that is maintained during the day. This helps to prevent fungal pathogens taking hold in the crop.



However, while plants have different needs during the different stages of growth, generally speaking it is commonly asserted that 0.85 kPa is about optimum VPD, with most plants growing well at VPDs of between 0.5 and 1.0 kPa. See following table.



The table indicates VPD values in kPa at various temperatures and humidity levels.



The darker grey shaded area, approximately 0.5 – 1.2 kPa being about ideal for many crops. The mid grey areas indicate an acceptable but marginal VPD range and the light grey areas are either too high or too low. VPD measures evaporation potential. Therefore, VPD values run in the opposite way to RH values, so where RH is high, VPD is low (low VPD = high humidity = low evaporation potential). A high VPD means the air has a high capacity to hold water (high VPD = low humidity = high evaporation potential).



It is important to note that VPD can provide a better indication of the evaporation potential than RH alone. For example, when looking at the table, as the temperature climbs from 15 to 35˚C (59 – 95˚F) at a constant 75% RH, the VPD will range from a bit on the low side (0.43) to too high (1.40) with VPD being about optimal at 26oC (78.8˚F) and 0.84 kPa. Since this digression is much less noticeable when the crop temperature only varies over a few degrees, it allows many growers to produce fairly good results using RH measurements corrleated to air temperatures. So, for example, if the day (lights on) temperature in the grow room was consistently between 26 – 28oC (78.8 – 82.4oF) with a constant of 75% humidity, VPD would be between 0.84 and 0.95 mb which is a fairly ideal VPD to promote optimum growth.



When looking at this table, to keep things simple, the main thing to take from it is that by correlating your grow room temperature to optimum RH on the table it gives you a very good picture of where your RH sweet spot is, relative to temperature. So, for example, at 28oC (82.4 oF) the table shows that optimum RH/VPD is at 70 -85% RH. Therefore, if you maintain a constant day/lights on temperature at 28oC (82.4oF), as long as you maintain RH at between 70 – 85% your plants are in the best environment possible to maximise growth. Also, as a tip, I tend to find 70 – 75% RH is about ideal for most genetics where ambient air temperature is 28oC (82.4 oF). Going above this range isn’t necessary as no growth benefits are obtained; however, high humidity increases the risk of fungal diseases and, therefore, it is wise to stay at the lower humidity end of the optimum VPD range.



Just keep in mind also that during flower you pretty much have to throw optimum VPD out the window because it is critical to run lower humidity (50 – 60% RH) than VPD optimums allow. That is, while you may achieve higher rates of growth in flower maintaining VPD optimums, the higher humidity in these VPD optimums may also result in fungal disease outbreaks in the crop. Therefore, VPD is somewhat of a compromise.



Measuring VPD



To get a reasonably accurate VPD measurement you measure leaf surface temperature (LST), ambient air temperature and relative humidity. These three values are then calculated to provide a VPD value.



One problem with VPD is it’s difficult to determine with complete accuracy because you need to calculate the averages of air and leaf temperatures along with humidity throughout the crop canopy. This is quite hard to do because these values will vary as you measure from the top of the canopy to the lower regions of the canopy. However, to get a reasonably accurate VPD use the following steps.



Measuring Air Temperature and Humidity



The most practical approach for getting a reasonably accurate VPD value is to take measurements of air temperature and humidity (using a thermometer and humidistat) just below the top of the canopy. For our purposes, it’s not necessary to measure the actual average canopy temperature and humidity 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.



Let’s say ambient air temperature and RH based on these readings are:



Air temp = 27oC

Relative Humidity = 65%



Measuring Leaf Surface Temperature (LST)



Using an infrared (IR) thermometer you take LST (leaf surface temperature) readings of several leaves just below the canopy surface (at the same locational height where RH and temp readings were taken) and calculate an average LST value from these readings.



Let’s say LST is 25oC



This gives us our three values of:



Air temp = 27oC

Relative Humidity = 65%

LST = 25oC



Calculating VPD from Air Temp, LST and Humidity Readings



We now have our three necessary readings; LST, ambient air temperature and relative humidity.



VPD can be calculated using an equation; however, this equation and the principles that underpin it are enough to scare the vast majority of indoor growers off going anywhere near VPD. For example, when calculating VPD this is a commonly used equation:



VPD=exp (6.41+0.0727T-3 10-4T2+1.18 10-6T3-3.86 10-9T4)

(1-RH/100)



Should we go there? Let’s not! Instead, there are several very good online calculators that run the equations for you.



One such VPD calculator is available at Vapor Pressure Deficit VPD Calculator - Dimlux Lighting - The Best Grow Lights or Google “VPD calculator Dimlux Lighting”. Another good VPD calculator can be found here ANTHEIA Grow Smart - VPD.



Theses calculators allow you to factor in leaf surface temperature (LST), ambient air temperature and relative humidity to get a VPD reading. Using our example RH, air temp and LST values we are able to establish our VPD is 0.85 kPa. You then compare this reading against the VPD chart to see where you are at. Where cannabis is concerned the optimum VPD range is between 0.8–1.1 kPa so our VPD of 0.85 kPa is looking good.



Controlling the Grow Room Environment to Optimise VPD



Maintaining VPD within optimal parameters can be tricky, but in closed environments such as indoor grow rooms it is relatively easy to do using dehumidifiers and humidifiers. That is, growers can reduce the humidity using a dehumidifier, while growers can use humidifiers to increase humidity. Ideally you will want to use an AC unit that heats and cools to keep your temperature at exactly the value you want it to be and you can then use a humidifier or dehumidifier to control the exact point where you want your VPD to be by controlling the value of your relative humidity at the fixed temperature provided by the AC unit.



How to measure leaf surface temperature with an infrared (IR) thermometer can be found HERE



Refs:

[1] Zolnier S., Gates R.S., Buxton J., and Mach C., 2000. Psychro- metric and ventilation constraints for vapor pressure deficit control. Computers and Electronics in Agriculture, 26(3), 343-359.

 

[ppm Charlie]

Thank you sir I put it down to read.
I'm just enjoying everyone's opinion on the matter.
Once I got my rooms dialed in I never worried about it again.
All this back and forth is very interesting.

Stay safe everyone
Bill

Well here's another "opinion" - A chart showing what a number of polled growers (mostly professional, I assume) were doing a few years ago regarding RH humidity.

How important RH control was to them? What RH they used during different stages of veg and flower.

Smoke em you got em @Bill284 . Don't fall asleep in the armchair this time though @Krissi1982 is manifesting and dispensing great knowledge through her wise advice most recently.

As you see - RH levels and RH concerns vary considerably in the clone/seedling and veg stage cases - in flower and late flower more tightly packed together regarding opinions of proper flowering RH.

 

[013]

nugget scored…. Thank you Rexer

Optimal VPD can also change depending on lighting conditions and other factors. For example, optimal VPD during the day is usually lower than optimal VPD during the night. In general, it’s better to have a rise in VPD (lower humidity) during the night relative to the VPD that is maintained during the day. This helps to prevent fungal pathogens taking hold in the crop.

that whole article the bomb… lotsa good stuff in there

bookmarked

 

[ppm Charlie]

nugget scored…. Thank you Rexer

Optimal VPD can also change depending on lighting conditions and other factors. For example, optimal VPD during the day is usually lower than optimal VPD during the night. In general, it’s better to have a rise in VPD (lower humidity) during the night relative to the VPD that is maintained during the day. This helps to prevent fungal pathogens taking hold in the crop.

that whole article the bomb… lotsa good stuff in there

bookmarked

So TRUE!

 

[Reave]

This chart has no information on their "assumptions". If you look at the charts I posted - people make assumptions based on the difference between the leaf surface temp and room temp. I see no info here on what assumptions they made or what lighting they used. Kind of a useless chart then if you do not know how they came up with there RH vs Temp chart.

It’s off leaf temp. Which is what they all should be unless otherwise stated. Since room temp would just be a ballpark increase. Type of light? Light is measured in par and irrelevant to this chart you could be in range at 1000ppfd or at 500. after 500 you should be using c02.

Edit: Link to my journal where I’m dialing in with a heat gun to 1Kpa about.
Post in thread 'Reave's Perpetual Grow'
Reave's Perpetual Grow

 

[ppm Charlie]

It’s off leaf temp. Which is what they all should be unless otherwise stated. Since room temp would just be a ballpark increase. Type of light? Light is measured in par and irrelevant to this chart you could be in range at 1000ppfd or at 500. after 500 you should be using c02.

Edit: Link to my journal where I’m dialing in with a heat gun to 1Kpa about.
Post in thread 'Reave's Perpetual Grow'
Reave's Perpetual Grow

While the leaf surface temp and the room temp may be similar, they are not usually equal (during "lights on"). Under HID/HPS the room temp is close the value of the leaf surface temp (leaf temp is 1 to 2 deg F less). For LEDs, the leaf surface temp can be as much as 5 - 8 deg F cooler and siginificantly less than the room temp. That is why with LEDs you need a higher room temp to allow the leaf surface temp to rise to achieve the maximum rate of photosynthesis.

 

[Reave]

Exactly. You will not know the leaf temp unless you measure it. Room temp is a guess at what the leaf temp might be. Just buy the 15$ Infrared thermometer.

 

[ppm Charlie]

Exactly. You will not know the leaf temp unless you measure it. Room temp is a guess at what the leaf temp might be. Just buy the 15$ Infrared thermometer.

Yes, it's on next b-day's list - I want more than a $15 one. Get to get all you can on your b-day.

 

[Reave]

Yes, it's on next b-day's list - I want more than a $15 one. Get to get all you can on your b-day.

Yeh for sure, if you can afford quality stuff that is always a good idea.

 

[Delps8]

The folks who make the Pulse have good info re. VPD. I use their PulseOne sensor and it makes things really easy. The PulseOne uses the "offset" - the difference between the leaf surface temperature ("LST") (use an IR gun thermometer to take readings in multiple places on canopy) and the temperature in the tent.

You can download an Excel document that gives VPD info for multiple stages of growth. It's nicely done but the dropdown for the growth stage doesn't work correctly in Excel on the Mac.

Re. tent temps - I try to go by the recommendations in the Chandra paper — temps at least 77° and a PPFD of 900 µmols.

 

[ppm Charlie]

The folks who make the Pulse have good info re. VPD. I use their PulseOne sensor and it makes things really easy. The PulseOne uses the "offset" - the difference between the leaf surface temperature ("LST") (use an IR gun thermometer to take readings in multiple places on canopy) and the temperature in the tent.

You can download an Excel document that gives VPD info for multiple stages of growth. It's nicely done but the dropdown for the growth stage doesn't work correctly in Excel on the Mac.

Re. tent temps - I try to go by the recommendations in the Chandra paper — temps at least 77° and a PPFD of 900 µmols.

One of these days I hope to upgrade to a more intuitive system like yours. Some of the VPD charts are specialized and "product-centric". Pulse's VPD is one I looked at and found value in. Nice setup...wish money grew on trees...expensive upfront costs but a fair return on value over time.

 

[ppm Charlie]

Here's the straight "poop" regarding VPD and the LED vs HID/HPS on the room temperature issue. The VPD of HID/HPS is not the same as the VPD for LEDs (unless the room temp for LED lighting is elevated to compensate for the lack of HID/HPS infrared heating).

Providing Sufficient Airflow for Plant Growth Environments

A look at what effects water and air movement in plants.

www.cannabissciencetech.com

 

[ppm Charlie]

Here's the straight "poop" regarding VPD and the LED vs HID/HPS on the room temperature issue. The VPD of HID/HPS is not the same as the VPD for LEDs (unless the room temp for LED lighting is elevated to compensate for the lack of HID/HPS infrared heating).

Providing Sufficient Airflow for Plant Growth Environments

A look at what effects water and air movement in plants.

www.cannabissciencetech.com

@Bill284 - This series of articles helps much in the understanding of the optimum conditions for 420 growth and health.
I was thinking that since you will eventually build a new "outside-the-house" grow, that such tested information may help you in the design of your future grow space for near perfect repeatable and buffered "dialed-in easily every time" environmental conditions for the particular size and configuration design of the space.

In fact, this series of articles should be helpful to all those trying to understand the intricacies of getting the most "bang" for the 420 "buck" (when it comes to total money spent on a crop from seed to harvest). All new growers are "wise" to learn the short series of lessons presented here.

Personally, after reading this particular article, I am going to reevaluate the airflow conditions in my veg and flowering tents. Once my most recent harvest is completed, airflow configuration changes are much contemplated by me.

 

[Melville Hobbes]

Here's the straight "poop" regarding VPD and the LED vs HID/HPS on the room temperature issue. The VPD of HID/HPS is not the same as the VPD for LEDs (unless the room temp for LED lighting is elevated to compensate for the lack of HID/HPS infrared heating).

Providing Sufficient Airflow for Plant Growth Environments

A look at what effects water and air movement in plants.

www.cannabissciencetech.com

A lot of LEDs have infrared diodes now. I don't have any leaf temp readings under them other than what the back of my hand says, but it definitely feels warmer than my older lights without IR diodes.

 

[Nunyabiz]

Here is what I do for VPD for the past 6 years.
First two weeks I am at 78-82⁰ and 75-80% humidity.
Weeks 3-8 veg I'm at 78-84⁰ and 65-70% humidity.
First week in flower I go to 77-80⁰ and 60-65% humidity.
Second week I go to 76-78⁰ and then each week until harvest I drop the humidity by 5% while keeping the temp between 75-77⁰ until harvest.

Last week or so I'm in the 30s for humidity.