Speed & Slow Transpiration (Humidity & Vapour Pressure Deficit)
Why? The ideal humidity depends on your temperature. 50% RH at 25°C is VERY different to 50% RH at 28°C.
To really understand how your humidity affects transpiration, here's what you need to know.
Humidity vs. Relative Humidity (RH)
Temperature needs no introduction, but humidity and relative humidity are a little trickier.
Humidity = the amount of water vapour in the air (not visible, unlike mist)
The thing is, the warmer the air, the more water it can hold. So what your humidity DOESN'T tell you is how full of water your air is.
Enter Relative Humidity.
As the name implies, it tells you how humid your air is, relative the temperature.
Relative Humidity (RH) = what % of water vapour is being held (of the total amount the air could hold at that temperature).
Basically - it tells you how saturated your air is, as a %.
| Example |
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At 50% RH, the air is holding 50% of the water vapour it could possibly hold at that temperature. |
Remember, the warmer the air, the more water it can hold.
SO, 50% RH air at 25oC is holding less water than 50% RH air at 28oC.
But at both temperatures, the air is 50% saturated.
RH and Transpiration
One of the most important things that RH influences is transpiration (water loss through leaves).
- A low RH (not much water vapour) increases the rate of transpiration
- A high RH (plenty of water vapour) decreases the rate of transpiration
The rate of transpiration is VERY important. The more plants transpire, the more water and nutrients they both absorb and transport.
But a balance is needed.
- Too much transpiration causes undue stress
- Too little transpiration leads to poor growth
So, to get the rate of transpiration right, your grow room RH also needs to be right.
To work out the right RH, you need to know what your Vapour Pressure Deficit is...
What's Vapour Pressure?
Water vapour creates pressure.
In the air
Water vapour in the air puts pressure on plants. The more water vapour, the more pressure.
Think of water vapour as a weight bearing down on plants. The more water vapour, the harder it is for plants to ‘push back’ via transpiration.
In leaves
Water vapour inside healthy leaves is 100% RH.
The RH in your grow room is never that high! So plants will always be able to 'push back' and transpire.
The question is, how much?
The bigger the pressure gap between leaf and air vapour pressure, the more transpiration.
The gap is known as Vapour Pressure Deficit (VPD).
Vapour Pressure Deficit (VPD)
VPD = the pressure difference between 100% RH and the acual RH air (at the same temperature).
VPD essentially combines temperature and RH to tell you how well plants will transpire.
As a measure of pressure, VPD is typically calculated in millibars (mb).
Earlier, we said: 50% RH at 25°C is VERY different to 50% RH at 28°C.
We meant it. Look:
- Temperature 24°C and RH 50% = typical VPD of 15mb
- Temperature 28°C and RH 50% = typical VPD of 19mb
Just a 4°C temperature rise increased the VPD by almost 30% - quite a big change!
So, if you want to know how your humidity affects transpiration, go by your VPD.
| Top Tip |
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Think of VPD as a measure of the air's power to pull water from plants. |
Measuring Vapour Pressure Deficit (VPD)
To calculate your VPD, you'll need to be able to:
- measure your leaf temperature
- measure your grow room temperature & humidity
You can't go wrong with a good old Accuread Temperature and Humidity Meter.
Once you're all equipped, follow these 3 steps:
1. Find leaf vapour pressure
Find out your leaf temperature.
Use the tables below to find out your leaf VPD at 100% RH.
2. Find air vapour pressure
Find out your air temperature and humidty.
Then use the tables below to find out your air's VPD.
3. Calculate VPD
leaf VP - air VP = VPD
| Example |
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leaf VP 29.8mb |
| Vapour Pressure (mb) | ||||||
|---|---|---|---|---|---|---|
| Relative Humidity (%) | ||||||
| 100 | 95 | 90 | 85 | 80 | 75 | |
| 15oC | 17.0 | 16.2 | 15.3 | 14.5 | 13.6 | 12.8 |
| 16oC | 18.2 | 17.3 | 16.4 | 15.4 | 14.5 | 13.6 |
| 17oC | 19.4 | 18.4 | 17.4 | 16.5 | 15.5 | 14.5 |
| 18oC | 20.6 | 19.6 | 18.6 | 17.5 | 16.5 | 15.5 |
| 19oC | 22.0 | 20.9 | 19.8 | 18.7 | 17.6 | 16.5 |
| 20oC | 23.4 | 22.2 | 21.0 | 19.9 | 18.7 | 17.5 |
| 21oC | 24.8 | 23.6 | 22.4 | 21.1 | 19.9 | 18.6 |
| 22oC | 26.4 | 25.1 | 23.8 | 22.5 | 21.1 | 19.8 |
| 23oC | 28.1 | 26.7 | 25.3 | 23.9 | 22.5 | 21.1 |
| 24oC | 29.8 | 28.3 | 26.8 | 25.3 | 23.9 | 22.4 |
| 25oC | 31.7 | 30.1 | 28.5 | 26.9 | 25.3 | 23.7 |
| 26oC | 33.6 | 31.9 | 30.2 | 28.5 | 26.9 | 25.2 |
| 27oC | 35.6 | 33.8 | 32.1 | 30.3 | 28.5 | 26.7 |
| 28oC | 37.8 | 35.9 | 34.0 | 32.1 | 30.2 | 28.3 |
| 29oC | 40.0 | 38.0 | 36.0 | 34.0 | 32.0 | 30.0 |
| 30oC | 42.4 | 40.3 | 38.2 | 36.0 | 33.9 | 31.8 |
| Vapour Pressure (mb) | |||||
|---|---|---|---|---|---|
| Relative Humidity (%) | |||||
| 70 | 65 | 60 | 55 | 50 | |
| 15oC | 11.9 | 11.1 | 10.2 | 9.4 | 8.5 |
| 16oC | 12.7 | 11.8 | 10.9 | 10.0 | 9.1 |
| 17oC | 13.6 | 12.6 | 11.6 | 10.6 | 9.7 |
| 18oC | 14.4 | 13.4 | 12.4 | 11.3 | 10.3 |
| 19oC | 15.4 | 14.3 | 13.2 | 12.1 | 11.0 |
| 20oC | 16.4 | 15.2 | 14.0 | 12.8 | 11.7 |
| 21oC | 17.4 | 16.2 | 14.9 | 13.7 | 12.4 |
| 22oC | 18.5 | 17.2 | 15.9 | 14.5 | 13.2 |
| 23oC | 19.6 | 18.2 | 16.8 | 15.4 | 14.0 |
| 24oC | 20.9 | 19.4 | 17.9 | 16.4 | 14.9 |
| 25oC | 22.2 | 20.6 | 19.0 | 17.4 | 15.8 |
| 26oC | 23.5 | 21.8 | 20.2 | 18.5 | 16.8 |
| 27oC | 24.9 | 23.2 | 21.4 | 19.6 | 17.8 |
| 28oC | 26.4 | 24.5 | 22.7 | 20.8 | 18.9 |
| 29oC | 28.0 | 26.0 | 24.0 | 22.0 | 20.0 |
| 30oC | 29.7 | 27.6 | 25.4 | 23.3 | 21.2 |
Environmental Guidelines
So, now you know how to calculte your VPD, here's what you should be aiming for.
| VPD | |
| Low Transpiration | 4 - 8 mb |
| Moderate Transpiration | 8 - 12mb |
| High Transpiration | 12 - 16mb |
Not much help? Let us break that down a bit... (you're welcome!)
Make sure you check out the tables to find out what RH and temperature give you the target VPD
Low Transpiration (Propagation)
Sometimes, you want to slow transpiration
- limited foliage
- small root system
Newly rooted cuttings, germinated seeds and young plants all need a low VPD.
To get a low VPD (small vapour pressure gap) , you want a high humidity, close to that in leaves.
A a nice, humid, propagator is just the ticket!
| Typical Low VPD of 4 - 8 | ||
|---|---|---|
| Best RH | Acceptable RH | |
| 19 - 20oC |
70% | 65% - 85% |
| 21-22oC | 75% | 70-90% |
| 23-24oC | 80% | 75-95% |
| 25-26oC | 85% | 80-95% |
| 27-28oC | 90% | 85-95% |
Moderate Transpiration (Veg Growth)
For optimum growth, you want plants to transpire at a good rate with a VPD of 8 - 12.
- Vegging plants growth
- Small-medium sized plants with healthy roots
Your RH shouldn't be as high as it was in propagation, but you still might want to use a:
NOTE: It's easier to keep your temperature and humidity low than it is to aim for a higher temperature and humidity.
| Typical Low VPD of 8 - 12 | ||
|---|---|---|
| Best RH | Acceptable RH | |
| 19 - 20oC |
60% | 55% - 65% |
| 21 - 22oC | 62.5% | 57.5% - 67.5% |
| 23 - 24oC | 65% | 60% - 70% |
| 25 - 26oC | 70% | 65% - 75% |
| 27- 28oC | 75% | 70% - 80% |
| 29 - 30oC | 80% | 75% - 85% |
High Transpiration (Flowering)
You may need to drop your humidity (and VPD) during flowering, to keep fungal diseases at bay!
When your VPD is low, plants will create lots of foliage and a healthy root system. They also absorb more water.
This is great during Bloom - a time when you want to reduce your humidity to prevent fungal diseases!
NOTE: You'll want to keep temperatures down too. This'll prevent too much transpiration stress with high VPD.
| Typical Low VPD of 12 - 16 | ||
|---|---|---|
| Best RH | Acceptable RH | |
| 19 - 20oC |
40% | 35% - 45% |
| 21 - 22oC | 45% | 40% - 50% |
| 23 - 24oC | 50% | 45% - 55% |
| 25 - 26oC | 55% | 50% - 60% |
| 27 - 28oC | 60% | 55% - 65% |
| 29 - 30oC | 65% | 60% - 70% |
