Written by Mike Steffes – Quest Dehumidifiers
Now that we’ve established the importance of transpiration and have some idea of how it happens, it should not be a stretch to accept that plant transpiration is driven by a vapor pressure difference of water: The partial pressure of water within the leaf vs. the partial pressure of water in the atmosphere outside the leaf.
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). 1 kPa is very close to 4 IN WC (inches of water-column). Plants can grow somewhat acceptably in a fairly wide range of VPD, loosely ranging from 0.40 kPa to 1.25 kPa.
VPD can be calculated fairly easily, the equation is listed at the end of this article. VPDs in the following examples were calculated using the equation. Don’t worry about how to get the numbers yet, just go through the examples and realize it’s not hard to come up with the numbers.
VPD during Vegetative Growth
Let’s say a grow room’s air temperature is 75°F when the lights are on. We’ll use the full 5°F 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 70°F (vapor pressure inside the leaf) is 2.53 kPa
In 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 75°F that requires a relative humidity of 57%.
This 75/57 combination provides a VPD value very close to the 0.80 target value.
Turning to night conditions. Let’s say we get the recommended 10°F drop and have a 65°F air temperature. There’s not as much leaf evaporation, so not as much leaf cooling. Let’s use 3°F for the leaf temperature reduction.
The SVP of 62°F is 1.92 kPa
The AVP of 65°F/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 75°F room at 57%RH. When the lights go off and the room drops to 65°F, we want the humidity to be at 52%RH.
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 production and minimize the chance of fungal infection.
Let’s say, again, that the room’s air temperature is 75°F when the lights are on. We’ll use the full 5°F temperature drop for leaf temperature. This time we’ll target 45%RH and then see what the VPD comes out to.
The SVP of 70°F is 2.53 kPa
The AVP of 75°F/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 10°F drop for a 65°F air temperature. Let’s once again use the 3°F leaf temperature reduction.
The SVP of 62°F (inside the leaf) is 1.92 kPa
The AVP of 65°F/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.
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 to jiggle the leaves.
The fan system is required because we know botrytis and other fungi are always waiting to pounce. Botrytis establishes itself best between 50 and 70°F, in still air having humidity above 55%RH. 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 75°F room at 57%RH, when cooled to 65°F goes to around 80%RH. That isn’t acceptable. You will need to remove water vapor from the room at lights out.
The humidification system is required because at night’s end when the lights come on and the temperature climbs back up to 75°F 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 quarts of water (rather than an ounce, or 5 gallons) up, into the air as quickly as possible.
You will also need some type of computer system capable of running a modern spreadsheet program. This is not rocket surgery, but you (or someone you know) will need to know how to use some basic features of a spreadsheet. This is useful to display the logged files from a data acquisition setup, as well as for calculating VPDs and other moisture quantities. Consider it the entry stakes to quantifying and visualizing the performance of your growing operation.
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.
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.
The VPD equation
Enter the formula on the next line into spreadsheet cell A10 (copy and paste it).
You will type-in 3 values into 3 other cells:
- Cell A5: The air temperature (A5 in the formula)
- Cell A6: The air %RH (A6 in the formula)
- Cell A7: The leaf temperature (A7 in the formula)
Cell A10 will then give you the total VPD for that grow room condition.
Room temperature= 80°F
Room %RH= 47%
Assumed leaf temperature= 75°F
VPD= 1.34 kPa (a little too dry for best growth)
Calculating Individual Vapor Pressures
For those interested in further exploring water vapor pressure.
Enter the formula on the next line into spreadsheet cell A20 (copy and paste it).
You will type-in 2 values into 2 other cells:
- Cell A16: The air temperature (A16 in the formula)
- Cell A17: The air %RH (A17 in the formula)
Cell A20 will then give you the water vapor pressure for that temperature and %RH combination.
Room air temperature= 80°F
Room air %RH= 47%
Water vapor pressure= 1.67 kPa
Leaf temperature= 75°F
%RH of the air inside the leaf = 100%
Water vapor pressure= 3.00 kPa
These 2 examples show the “long way” to calculate the VPD given in the VPD equation section above this one: Subtract the room condition from the leaf condition to come up with the room-to-leaf water vapor pressure deficit (3.00 – 1.67 = 1.33 kPa).