Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is missing but im going to skip alot of the more in depth stuff to keep this simple.
WHAT IS VPD:
stands for Vapor Pressure Deficit
. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit
is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor pressure) and the air’s actual vapor pressure
. A high VPD
value raises a plant’s transpiration rate and increases nutrient
movement through the the plant’s xylem, while a low VPD
value slows the movement of nutrients
through the plant.
View attachment 910696
WHY SHOULD WE CARE:
control is related to the following:
- Increasing or decreasing metabolic rate
- Improvement in yield quality
- Determining plant stresses
- Pathogenesis (more on this later)
- Carbon dioxide injection (more on this later)
Growers should care about VPD
because it impacts yield quality, overall plant vigor, and nutrient
utilization. Managing VPD
lets me get away with using fewer nutrients
, which improves my bottom line. I’m also seeing increased trichome
production in the plants, which naturally follows better health.
You reach expert growing level when you learn to manage humidity
. Everybody spends their time managing temperature, nutrients
, and whatever else, but the last little thing you learn to manage is humidity
, and it is significantly more finicky.
HOW TO STEER PLANT GROWTH:
In order to stay on the same page, I should point out that relative humidity
) andVPD are inversely related. This means that when relative humidity
) is high, VPD
is low, and vice versa.
When growers want to know how to steer plant growth, they are interested in maximizing growth. So let’s start with how the plants react to changes in VPD
- The bulk flow of water changes within a plant’s xylem as VPDchanges.
- If you have a high VPD, meaning that the RH is low, the plant will increase its transpiration rate and start pulling water faster from the substratein an effort to stay cool and moist.
- If the VPD is too high, the plant will become stressed, leading to inefficiencies.
- In the same vein, if the VPD is too low, meaning that the RH is high, the transpiration rate will decrease, slowing the flow of water through the plant.
- Nutrients follow the flow of water through the xylem and into its various tissues. Nutrientslike calcium primarily move with the bulk flow of water through the arteries of the plant.
- Therefore, as VPD rises (and the bulk flow of water increases), nutrient uptake will also rise.
- If VPD falls (and the bulk flow of water decreases), nutrient uptake will also fall.
- If you’re injecting carbon dioxide, you want the plants’ stomata to stay dilated for as long as possible. Ideally, the stomata would be fully dilated at all times to maximize carbon dioxide use. Plants open and close their stomatato regulate moisture loss.
- If you have a high VPD, or low RH, your plants are going to close their stomata to reduce water loss. When the stomata close, you’re not getting adequate gas exchange, and you’re not making the most of your carbon dioxide.
- If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide.
control is about balancing gas exchange. There is a “Goldilocks” zone where the plant is getting everything.
If your VPD
is too low, then your plants aren’t going to acquire enough nutrients
, slowing growth; if your VPD
is too high, you’re going to stress the plant and the stomata
will close, rendering your extra carbon dioxide
ineffective. Like everything else with growing, there’s a Goldilocks zone. One VPD
is too high, one VPD
is too low, and one VPD
is just right. There are charts of a VPD
curve with a three way graph of humidity
, temperature, and growth. There’s a sweet spot along the center of the chart.
DO NOT USE THIS CHART ITS REFERENCE ONLY
chart for a hypothetical plant, image courtesy of Argus Controls
. The far left side of the graph is too wet for the plant,
uptake is inhibited. The right of the graph is too dry for the plant and stomata
close, inhibiting CO2
VPD IN DIFFERNT STAGES OF GROWTH:
Ideal KPA (kilopascals) ranges for different stages of growth.
Early flower 1-1.4
Late flower 1.3-1.5
As a matter of fact, most growers use some form of VPD
control already, without even knowing it. When you put clones
under a dome, you’re keeping the RH
high and the VPD
low. This, in turn, slows transpiration to a crawl, greatly reducing the stress on the cuttings
, which need time to form roots. Typically, most growers will keep their vegetativehumidity
a little bit higher as well, which reduces stress.
Domes are a form of VPD
Most growers are concerned about the flowering
cycle because that’s where the magic happens. You want to keep your VPD
relatively high (low RH
) during the flowering
cycle. If you assume an average flowering
cycle of 8 weeks, start with a moderate VPD
) during the first 3-4 weeks of your flowering
cycle, then increase your VPD
(lower your RH
) towards the end of flowering
. This reduces pathogenesis
One thing you can do when a plant is stressed, say from moving from one room to another, is to raise the humidity
. This lowers the transpirational stress and eases their transition into whatever phase or room you have set up. Additionally, HID
lights can be stressful for plants, and VPD
control gives you the ability to reduce their stress. If you have a dry environment and bright lights towards the top, you’ll see canopy
leaves fold in like a taco. Plants do this to reduce light capture and reduce their internal temperature. If you see this happening, you need to ease up on the plants and reduce their stress.
CONCERNS WHEN USING VPD:
is a big issue, which we’ve touched on briefly. The biggest drawback to running a low VPD
) is that you can run into a lot of problems with pathogens if your rooms aren’t clean. As a result, many growers reduce their humidity
as much as possible. Some growers brag that their humidity
is as low as 20%, which is really bad for the plants and slows their growth.
Homogenizing a room’s environment is a struggle. In my experience, there are always new micro-environments forming in your room due to the nature of working with living organisms. Keeping on top of it all takes a lot of effort.
Good ventilation/circulation is necessary for VPD
Accurate sensor readings are also a problem I keep running into. Keeping the environment at your desired setpoint of temperature and humidity
can be tricky. Having the right equipment and the right room layout can make a big difference.
WHAT EQUIPMENT DO YOU NEED TO EFFECTIVELY CONTROL VPD:
You’re going to need a humidifier
for starters. You want to be able to inject humidity
into the room without causing any problems such as being too close to one plant. If you have your humidifiers spraying plants directly with vapor, you will end up with undesirable microclimates
which could favor pathogenesis
. Personally, I think that ultrasonic humidifiers work best.
You are going to need a way to measure the leaf temperatures in order to accurately calculate VPD. This is where the online charts cause many growers problems and botrytisis becomes of real concern when not taking leaf temps into account. A simple $15 Infrared Temp gun will do the job quite well.
If you’re going to manage VPD
, you’ll also want a controller
that integrates your humidification and dehumidification systems. You want your controllers set up in such a way that when the lights are off, the humidification setpoints for the dehumidifiers are different if possible. An RH of 10-15% lower at night is ideal but not required.
Paying attention to RH after the lights go out is a big concern. As temperature drop the RH increases (ergo relative humidity) Slowing the temperature drop will aid in the dehumidifiers ability to keep the humidity in range. I would recommend checking humidity from 20-40 mins after lights out to ensure RH is not spiking.
Temperature are also important to control using a temp controller that controls both heating and cooling is ideal. This could be done by controlling fans, heaters, ac etc.
If you don't have all the fancy stuff listed you can still use VPD to to make adjustments to your setup that will improve the VPD for your stage of growth.
It’s important to note that plants are their own internal humidifiers, depending on how many plants are in a room and what stage of growth they’re at. Small plants have less surface area and transpire less. Small plants in a big room will require humidity
injection to keep the humidity
up, whereas plants at full size don’t need as much humidity
injection because they’re already transpiring at an increased rate. When you hit the final stages of growth, you may have to run dehumidifiers
to take water vapor out.
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.
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.
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.
HOW TO CALCULATE YOUR OWN VPD:
If you don't like math your in luck here is a formula you can put into a spreadsheet to do it for you. I use excel personally.
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).
Ok well that the long/short version and I hope this helps. If ya have any questions I will do my best to answer