Welcome to the cannabis school for new growers section. In this section you can ask and get answers to questions about growing cannabis. Whether you are are new to growing cannabis or thinking about getting into the hobby, there are no silly questions so ask away. To become part of our online cannabis growing community click here to register.

VPD - Environmental Control

A place for new grower to ask questions and learn about growing cannabis
Post Reply
User avatar
ncrypta.core
Known User
Posts: 1252
Joined: Wed Mar 24, 2021 10:42 pm
Has thanked: 217 times
Been thanked: 1704 times
Contact:
Status: Offline

VPD - Environmental Control

Post by ncrypta.core »

Please read this first:viewtopic.php?f=6&t=14522
As this is more an advanced Environmental Control guide


Image
The Science Behind VPD

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.


Image

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.

Image

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:
Image
These users thanked the author ncrypta.core for the post (total 3):
Nom (Sun Jul 25, 2021 5:09 pm) • Keeno (Sun Jul 25, 2021 5:12 pm) • Growmie (Fri Oct 01, 2021 5:56 pm)
Where the salts go, the water will follow

User avatar
ncrypta.core
Known User
Posts: 1252
Joined: Wed Mar 24, 2021 10:42 pm
Has thanked: 217 times
Been thanked: 1704 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by ncrypta.core »

How Do You Calculate VPD?

To calculate Air VPD you just need two things: temperature & humidity, but there are a couple of steps.

How to calculate air VPD:
Figure out the SVP
SVP = 610.78 x e^(T / (T +238.3) x 17.2694))

T is in degrees Celsius

The result, SVP, is in pascals (divide by 1000 to get kPa)

is a mathematical constant called Euler's Number, approximately equal to 2.71828.

Calculate the VPD
SVP x (1 – RH/100) = VPD

However, since the inside of a plant is water, the plants feel a different VPD than just the normal Air VPD. If you want to figure out what’s going on with the plant, you need to consider the cooling effect of evaporating water as it exits the leaves’ stomata. To calculate this, you need to know the difference between the air temperature at canopy level, and the temperature of the leaves. An IR thermometer is useful for this.

How to calculate leaf VPD:
Figure out the air SVP (ASVP)
Same formula as getting the SVP for air VPD

Figure out the leaf SVP (LSVP)
It is the same formula as ASVP, but you use the leaf temperature in your calculation (typically 1-3 °C or 2-5 °F cooler)

Leaf VPD = LSVP – (ASVP x RH/100)
These users thanked the author ncrypta.core for the post (total 2):
Keeno (Sun Jul 25, 2021 5:12 pm) • Stickylemonz (Fri Apr 29, 2022 8:03 pm)

User avatar
ncrypta.core
Known User
Posts: 1252
Joined: Wed Mar 24, 2021 10:42 pm
Has thanked: 217 times
Been thanked: 1704 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by ncrypta.core »

Ideal VPD for Clones
Clones are baby plants, they can’t handle a lot of stress because they are still just trying to form roots. Target a higher humidity and VPD closer to the lower end of the general range.

The ideal VPD for clones is closer to 0.8 kPa.

Ideal VPD for Vegetative Stage

In veg (vegetative) stage the plants are bigger and more robust. You can reduce the humidity in your environment to increase VPD. This will increase water and nutrient uptake, but you don’t want to increase VPD too much. This will cause the stomata of the plants to close, causing them to absorb less CO2. CO2 is particularly important in the vegetative stage, because that’s the main ingredient that plants use to grow large.

The ideal VPD for the vegetative stage is close to the middle of the general range, around 1.0 kPa.

Ideal VPD for Flower Stage
In flower stage the plants are robust, but the flowers are sensitive to various issues. You need to avoid excess humidity.

The ideal VPD for the flower stage is closer to the top end of the range, 1.2kPa – 1.5kPa.

User avatar
Keeno
Site Admin
Posts: 25512
Joined: Sat Oct 07, 2017 10:11 pm
Has thanked: 10550 times
Been thanked: 17044 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by Keeno »

Brilliant write up @ncrypta.core. Thank you for sharing. I'm sure it will help many over the years.

This deserves to be a sticky.
These users thanked the author Keeno for the post (total 2):
Nom (Sun Jul 25, 2021 5:18 pm) • ncrypta.core (Sun Jul 25, 2021 5:29 pm)

User avatar
ncrypta.core
Known User
Posts: 1252
Joined: Wed Mar 24, 2021 10:42 pm
Has thanked: 217 times
Been thanked: 1704 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by ncrypta.core »

Keeno wrote:
Sun Jul 25, 2021 5:13 pm
Brilliant write up @ncrypta.core. Thank you for sharing. I'm sure it will help many over the years.
No problem man was thinking of doing a thread about this and once noticed your post, I snapped into action as this is like a follow-on to your post.
These users thanked the author ncrypta.core for the post:
Keeno (Sun Jul 25, 2021 7:18 pm)

User avatar
GMO
GR420 Leg End
Posts: 10529
Joined: Sat Jun 02, 2018 2:44 pm
Has thanked: 5717 times
Been thanked: 3805 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by GMO »

pardon my ignorance. whats short vegetative phase got to do with VPD or what is the context of the acronym SVP and LSVP?
“If you can't explain it to a six year old, you don't understand it yourself.” -Albert Einstein

User avatar
ncrypta.core
Known User
Posts: 1252
Joined: Wed Mar 24, 2021 10:42 pm
Has thanked: 217 times
Been thanked: 1704 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by ncrypta.core »

saturation vapor pressure

Air can only hold a certain amount of water vapor at a given temperature before it starts condensing back to liquid water (in forms such as dew or rain). The maximum amount of water vapor that air can hold at a certain temperature is called “saturation vapor pressure” or SVP.
These users thanked the author ncrypta.core for the post:
GMO (Sun Jul 25, 2021 7:50 pm)

User avatar
Keeno
Site Admin
Posts: 25512
Joined: Sat Oct 07, 2017 10:11 pm
Has thanked: 10550 times
Been thanked: 17044 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by Keeno »

ncrypta.core wrote:
Sun Jul 25, 2021 5:29 pm
Keeno wrote:
Sun Jul 25, 2021 5:13 pm
Brilliant write up @ncrypta.core. Thank you for sharing. I'm sure it will help many over the years.
No problem man was thinking of doing a thread about this and once noticed your post, I snapped into action as this is like a follow-on to your post.
Really helps the place mate so thank you. As forums go we are still very young and still gathering info hence my articles now I have a few days spare. Again thank you for taking the time to share the knowledge.
These users thanked the author Keeno for the post:
ncrypta.core (Sun Jul 25, 2021 7:51 pm)

User avatar
ncrypta.core
Known User
Posts: 1252
Joined: Wed Mar 24, 2021 10:42 pm
Has thanked: 217 times
Been thanked: 1704 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by ncrypta.core »

GMO wrote:
Sun Jul 25, 2021 6:01 pm
pardon my ignorance. whats short vegetative phase got to do VPD
VPD is a environmental controller in a way as it’s how much environmental pressure your putting on plant like biometric pressure etc. So at different times off it life it need different pressure Applied so it can grow at optimal rate. Also helps the plant cool itself

Ref:
https://youtu.be/a6GUPfepLIc

https://youtu.be/DLUCdBJkJ6k
These users thanked the author ncrypta.core for the post:
GMO (Sun Jul 25, 2021 7:57 pm)

User avatar
GMO
GR420 Leg End
Posts: 10529
Joined: Sat Jun 02, 2018 2:44 pm
Has thanked: 5717 times
Been thanked: 3805 times
Contact:
Status: Offline

Re: VPD - Environmental Control

Post by GMO »

ncrypta.core wrote:
Sun Jul 25, 2021 7:14 pm
saturation vapor pressure

Air can only hold a certain amount of water vapor at a given temperature before it starts condensing back to liquid water (in forms such as dew or rain). The maximum amount of water vapor that air can hold at a certain temperature is called “saturation vapor pressure” or SVP.
Sweet; so what are AVSP and LVSP?

i only knew this as saturation point or 100% relative humidity or dew point

its also confusing once i googled VSP cos it seems to be based on a closed system? and the air strictly held above a liquid in the afore mentioned closed container?

Post Reply

Return to “Canna School for Newbies!”