As this is more an advanced Environmental Control guide
Vapor pressure deficit, or VPD, is a measurement describing the vapor pressure differential from inside a leaf, compared to the room's air pressure. It rolls air temperature, leaf temperature, and relative humidity into an easy to manage number. Growers experienced in VPD management understand optimal control of a grow room environment is about finding the sweet spot between humidity and temperature.
As plants grow, they transpire water vapor through the stomata (special cells located on the surface of the leaf) out into the atmosphere, thus boosting relative humidity levels in their environment. A basic component to the growth cycle for any plant is the movement of water and nutrients drawn up by the roots, through the body of the plant, and eventually transpired out by the canopy.
This upward movement of nutrients and water is nature's way of seeking balance. There is naturally a differential, or VPD, between the pressure of the room's environment and the vapor pressure within the interior of the plant. Any gases trapped inside the plant are 100 percent saturated with water, and eventually, these saturated gas molecules are drawn out into an external environment.
Getting an extreme VPD under control will also control water and nutrient use. With a high VPD and low relative humidity, cannabis will naturally transpire water much more quickly. A dry environment will simply pull more transpiration out of the plant itself, drawn up through the roots. If the pressure imbalance goes on too long, and the atmosphere is too dry, plant's close off their stomata as a final protective mechanism to reduce moisture loss.
In a relatively humid environment, the VPD is low. The vapor pressure internally and externally even out. Finding a perfect balance of humidity and temperature, guarantees water and nutrient movement, but prevents aggressive waste seen in low relative humidity environments. Plants are generally less stressed, and more receptive to CO2 in the grow room.
The Benefits of VPD
VPD management is an advanced growing technique, however once mastered can be especially beneficial for large-scale warehousing operations relying on massive AC units for temperature control. Air conditioning is a perpetually dehumidifying process, and can quickly throw the VPD out of balance if not counteracted with some sort of humidifying process.
Even greenhouses, which are generally humid environments can benefit from VPD monitoring. It can help growers maintain a perfect relative humidity throughout the growing season, and through the phases of development.
Once conquered, VPD monitoring enhances the usefulness of other elements of a grow room as well. A low VPD prevents cannabis plants from closing their stomata in an attempt to protect its water reserves; this naturally allows increased CO2 absorption rates.
Understanding the VPD at any given time also puts a grower closer in touch with the stress level of their crop. Remember, a high VPD generally means a stressed-out plant, as it struggles to manage water transpiration, uptake, and low relative humidity. A happy plant applies nutrients more effectively, including any additional CO2, and will produce a better overall harvest.
In brief
Stomata Opening
As VPD increases, stomata get smaller.
CO2 uptake
As VPD increases and stomata get smaller, CO2 uptake gets reduced.
Transpiration
As VPD increases, the plant transpires (evaporates from leaves) faster due to the larger difference in vapor pressures between the leaf and the air.
Nutrient intake at the roots
As VPD increases, and transpiration increases, the roots pull in more nutrients. The plant is like one connected system of plumbing!
Plant stress
As VPD increases, there are more forces acting on the plant – from the leaves to the roots – and the plant experiences more stress.
Air and pressure
Air consists of several gasses, including Nitrogen (almost 80%), Oxygen (around 20%), a range of trace gasses, including carbon dioxide, (around 0.04%), and water vapor. The molecules of these constituents vibrate randomly depending on the air temperature (the higher the temperature, the more vigorous the vibrations) and this vibration causes a force (pressure) on the surroundings.
The vibrations of the molecules of each constituent contribute to the overall air pressure and we can define these contributions by the individual constituents as partial pressures. Thus, the total air pressure (as measured with a barometer) is the sum of a number of partial pressures that are determined by the amount (number of molecules) of each constituent and the air temperature.
The amount of water vapor in the air can vary: Heating the air typically reduces the humidity (think of dry desert heat), and humid and hot summer air can make conditions rather unbearable (makes it difficult for us to lose heat by perspiration). Such hot-and-dry or hot-and-humid conditions can cause problems for plants, too—when the air is dry, plants lose more water (until the water uptake by the roots can no longer keep up with the transpiration rate from the leaves). And when the humidity is high, plants find it more difficult to transpire water from their leaves, thus reducing their own cooling capacity. Transpiration requires heat that’s drawn from the plant during this process.
GETTING REAL CONTROL OVER VPD
The ideal ranges for VPD for cannabis cannot be too high or too low. Plants are essentially water pumps, and when the VPD is too high, then the plant will lose too much water. If the VPD is too low, then the plant will choke. Keeping the vapor pressure deficit in an ideal range allows plants to cycle water and nutrients effectively.
For successful VPD management, it is imperative to chart temperatures and humidity during the night and day cycles. Environmental control is the only way to address VPD. There are several options to manage the environment.
[*]Moving lights. Sometimes the heat given off by lights can upset the balance in an environment. Switching lights, or even raising them above the crops, can address issues with high VPD. HPS lights, in particular, can upset VPD. LEDs paired with heatsinks are excellent for controlling temperature.
[*]Using domes. By creating a smaller closed environment around a plant, farmers can gain better control over VPD. Doing so is an ideal solution in a large environment.
[*]Limiting fans and air conditioning. Air movement can exacerbate the effects of poor VPD control. Controlling airflow is crucial to managing VPD.
[*]Measure often. Once you have enough experience with any crop, you'll learn what VPD works for your plants at what points in time. Practicing diligence with data will speed up this process.
The easiest way is to find your left temp/room temp and humidity and use a chart to keep within range at which ever stage you are
VPD Cal: https://www.dimluxlighting.com/knowledg ... alculator/
Chart Example:
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