Relative Humidity versus VPD
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. 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. 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. Therefore, VPD is an ideal means in which to establish the optimum relative humidity in the grow room at a given temperature.
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.
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)
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 https://www.dimluxlighting.com/knowledge/vapor-pressure-deficit-vpd-calculator/ or Google “VPD calculator Dimlux Lighting”. Another good VPD calculator can be found here https://www.antheiagrow.com/en/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
 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.