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.
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, Transpiration and Water Uptake 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 novice 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. However, if the air temperatures change significantly over the course of the day, VPD can provide a more accurate indication of evaporation potential since it combines the effects of both temperature and humidity into a single value.
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.
In simple terms, vapour pressure deficit comes down to the difference between the vapor pressure inside the leaf compared to the vapor 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 vapor 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 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 vapor 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 vapor pressure. This comes down to the point that we covered earlier about relative humidity being 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. So, for example, cold air holds less water vapour than warm air; the water-holding capacity of air doubles with every 10oC (200F) increase in temperature. Therefore, air at 28oC (82.4°F) can hold twice the amount of water vapour when compared to air at 18oC (64.4°F). 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 stress 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 growroom at a given temperature.
VPD can be measured in pounds per square inch (psi), millibars (mb) or kilopascal (kPa), with kPa and mb being the most 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 mb. Or to convert mb to kPa one simply divides by 10. E.g. 8 (mb) divided by 10 = 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 thrive under higher VPDs.
Optimal VPD can also change depending on lighting conditions and other factors. For example, optimal VPD during the day is usually higher than optimal VPD during the night. In general, it’s better to have a drop in VPD during the night relative to the VPD that is maintained during the day. Declines in canopy carbon dioxide exchange rates can be correlated with increases in the VPD during this time. Further, if you’re enriching carbon dioxide – which puts further transpiration stress on the plants – the optimal VPD is also likely to be lower than if you didn’t use any CO2 enrichment.
However, while plants have different needs during the different stages of growth, generally speaking it is commonly asserted that 0.85 kPa (8.5 mb) is about optimum VPD, with most plants growing well at VPDs of between 0.5 and 1.0 kPa (5 – 10 mb). See following table.
Table source: Argus Controls found at https://www.arguscontrols.com/resources/VPD_Application_Note.pdf
The table indicates VPD values in millibars (mb) at various temperatures and humidity levels. The green shaded area, approximately 5.0 to 12.0 mb (0.5 – 1.2 kPa) being ideal for the crop. The yellow areas indicate an acceptable but marginal VPD range and the red 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. A high VPD means the air has a high capacity to hold water (high VPD = low humidity = high evaporation potential), while a low VPD means the air has a low capacity to hold water because it is near saturation (low VPD = high humidity = low evaporation potential).
Note that VPD can potentially provide a better indication of the evaporation potential than RH. For example, when looking at the table, as the temperature climbs from 15 to 35 ˚C at a constant 75% RH, the VPD will range from a bit on the low side (4.2) to too high (14) with VPD being about optimal at 26 oC and 8.4 mb (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 just RH measurements. So, for example, if the day (lights on) temperature variable in the growroom was consistently between 25 – 28 oC with a constant of 75% humidity, VPD would be between 8 and 9.5 mb (0.8 – 0.95 kPa) which is a fairly ideal VPD across the daytime temperature spectrum.
The most accurate VPD measurement consists of air pressure, air temperature, leaf (canopy) temperature, and relative humidity being calculated to arrive at VPD. You can perhaps tell by the number of elements that need to be equated, that establishing VPD with 100 per cent accuracy is somewhat harder than simply working with ambient air temperatures and relative humidity.
One problem with VPD is it’s difficult to determine with complete accuracy because you need to know the leaf/canopy temperature. This is quite a complex issue because leaf temperature can vary from leaf to leaf; it is the combination of ambient air temperature, shading, RH and the plant transpiration rate that determine the canopy temperature. So, for example, the optimal value for air temperature to promote optimum growth might be expressed at e.g. 28oC but if you were to measure the leaf/canopy temperature using an infrared sensor at the top of the plant canopy, where ambient air temperature was 28oC, leaf temperature would normally be about 1-3oC cooler at between 25 – 27oC; taking this one step further, deep inside the canopy where high levels of shading occur this may translate to as much as 5 – 6oC cooler at 22 – 23oC. Therefore, VPD allows growers to take canopy temperature into account which is an advantage because it is the canopy temperature, rather than the surrounding air temperature that is more important in plant growth.  However, taking a correct canopy temperature is difficult because the question then becomes how do I get an accurate canopy temperature reading? 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 just below the crop canopy is generally adequate for providing a good indication of actual leaf conditions.
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 using 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, a nice little VPD calculator can also be found on the University of Arizona website @ https://cals.arizona.edu/vpdcalc/. This particular calculator only takes into consideration air temperature and humidity; however, by monitoring humidity and air temperature just below the top of the plant canopy this calculator should give you a reasonably accurate VPD measurement. As a tip, what I tend to do when using this calculator is, I subtract about 2oC from my ambient air temperature taken at the top of the plant canopy. While not strictly a perfect way of monitoring VPD it does give me a reasonably accurate idea. So, for example, if my ambient air temperature at canopy top under the lights is 28oC and my RH is 75%, I would enter 26oC in the ‘Air Temperature Celsius’ field and 75% in the ‘Relative Humidity’ field which would tell me upon computation that this equates to 0.841 kPa (8.4 mb).
Given 0.85 kPa (8.5 mb) is about the ideal VPD, this reading tells me that my growroom atmosphere is within about ideal parameters to promote optimum growth (at least where temperature and RH is concerned).
Controlling the Growroom Environment to Optimise VPD
Maintaining VPD within optimal parameters can be tricky, but in closed environments such as indoor growrooms 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 to keep your temperature at exactly the value you want it to be and you can then use a humidifier/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.
What VPD Does and Doesn’t do
Lastly, on the subject of VPD is that there seems to be some misunderstanding among some in the grow community about what VPD does and doesn’t do. Therefore, so as not to give the reader the impression that VPD is the be all and end all of ensuring optimum yields, some clarification is required.
VPD does not measure plant stress
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 acclimatisation 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 substrate salinity, 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.
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:
- Transpiration losses caused by the leaf (stomata) responses to the immediate surrounding environment. Contributing factors include: VPD/humidity, temperature, solar radiation, wind speed, and CO2 levels.
- Water availability and water uptake. This is affected by soil/substrate water availability, salinity (osmotic pressure) and root system structure and health.
- Transport mechanisms between the “root and the shoot” including the structure and health of the vascular system.
Let’s leave VPD there.
 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.
 Jones J.B., 2013. Instructions for Growing Tomatoes in the Garden and Green-House. GroSystems, Anderson, SC, USA.