Speed & Slow Transpiration (Humidity & Vapour Pressure Deficit)

Speed & Slow Transpiration (Humidity & Vapour Pressure Deficit)

Alex
Alex
16 Jun 2014

Some things in life aren't black and white. Sorry folks, your humidity is one of them!

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

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.

Humidity Humidity

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...

Transpiration Transpiration

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. 

Vapour Vapour

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

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

(a Dimlux Maxi Controller & Dimlux Temperature Camera are best. But any handheld infrared thermometer will do)

  • measure your grow room temperature & humidity

You can't go wrong with a good old Accuread Temperature and Humidity Meter.

Accrued Temperature Accrued Temperature

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

leaf VP 29.8mb
air VP 20.6mb
VPD = 9.2mb

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
17o 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
23o 28.1 26.7  25.3  23.9  22.5  21.1
24o 29.8 28.3 26.8 25.3 23.9 22.4
25o 31.7 30.1 28.5 26.9 25.3 23.7
26o 33.6 31.9 30.2 28.5 26.9 25.2
27o 35.6 33.8 32.1 30.3 28.5 26.7
28o 37.8 35.9 34.0  32.1 30.2 28.3
29o 40.0 38.0 36.0 34.0  32.0 30.0
30o 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
17o 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
23o 19.6 18.2 16.8 15.4 14.0
24o 20.9 19.4 17.9 16.4 14.9
25o 22.2 20.6 19.0 17.4 15.8
26o 23.5 21.8 20.2 18.5 16.8
27o 24.9 23.2 21.4 19.6 17.8
28o 26.4 24.5 22.7 20.8 18.9
29o 28.0 26.0 24.0 22.0 20.0
30o 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%

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Alex
Alex
About the author

Alex is one of our Directors! It's his job to sniff out the best products and keep operations in order. His toe stays firmly dipped in marketing, too! 

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