Harnessing the Power of Humidity

Harnessing the Power of Humidity

18 Mar 2013

Believe it or not, your humidity can have a huge impact on your plants.

The right humidity depends on your temperature and stage of growth. Here’s what to aim for.

What's humidity?

Humidity is the amount of water vapour that air holds.

Humidity is usually measured as RH (relative humidity).

Your relative humidity is a percentage of the maximum amount of water vapour that the air will hold at that temperature.


e.g. If your humidity is 70% RH (relative humidity), the air is holding 70% of the water vapour it can possibly hold at that temperature.  

Your humidity and temperature are directly related. The warmer the air, the more water vapour it can hold.

In short, this means the RH reading at one temperature should be different to the RH you aim for at another temperature.


e.g. At 100% humidity the air cannot hold any more water vapour - unless the temperature rises.

The Big Humidity Myth

A lot of growers think that plants do not like high levels of humidity.

This just isn’t true!

For most of their lives, plants love high humidity. But all too often, growers keep their humidity low to avoid potential pathogens that aren't really an issue until flowering.

This “play it safe” attitude is understandable - nobody wants a fungal or bacterial problem. But through careful humidity management you can avoid these problems while improving your grow rates.

The Science

Your plants’ ability to stay hydrated and absorb the nutrients needed for growth depends on how efficient water transportation is within their systems.

The humidity of your grow room can disrupt this process. Why? Plants give off water vapour through their leaves (transpiration). The humidity of your grow room can speed this up or slow it down.

Fact 1:

Water tends to move from high humidity areas to low humidity areas (osmosis)

Like how pasta cooks - water moves into the pasta because there's less water in ithe pasta than is in the pan.

Humidity Humidity

Fact 2:

The bigger the difference between higher and lower humidity, the faster the rate of transpiration.

Like how ice melts faster in hot water than in cold water - the bigger the difference in temperature, the faster the melting. 

Humidity Humidity

Fact 3:

The humidity inside plants has been found to be around 100%.

You can't get higher than 100%!

So, to slow the rate of transpiration, keep the humidity in your grow room high (during propagation and vegetative growth) - remember, the RH in leaves is 100%, so high means high.

This is especially important before fruits and flowers begin to form.

Humidity Humidity

Target Humidity

The warmer your air, the higher your RH needs to be. 

That's why it's easiest to manage your RH at a lower temperature - you don't have to humidify as much, and you're extracting less of it out.

Here's what the temperature and humidity you should be aim for .

Humidity & Temperature Rate
40%  15o
41% 15o
42% 15oC
43% 15oC
44% 15oC
45% 15oC - 16o
46% 15oC - 16o
47% 15oC - 16o
48% 15oC - 16o
49% 15oC - 16o
50% 15oC - 18oC  
51% 15oC - 18o
52% 15oC - 18o
53% 15oC - 18o
54% 15oC - 18o
55% 15oC - 19o
56% 17oC - 19o
57% 17oC - 19o
58% 17oC - 19o
59% 17oC - 19o
60% 17oC - 22oC
61% 19oC - 22oC
62% 19oC - 22oC
63% 19oC - 22oC
64% 19oC - 22oC
65% 19oC - 24oC
66% 22oC - 24oC
67% 22oC - 24oC
68% 22oC - 24oC
69% 22oC - 24oC
70% 22oC - 26oC
71% 22oC - 26oC
72% 22oC - 26oC
73% 22oC - 26oC
74% 22oC - 26oC
75% 25oC - 29oC
76% 25oC - 29oC
77% 25oC - 29oC
78% 25oC - 29oC
79% 25oC - 29o
80% 28oC - 33oC

Cuttings & Seedlings


A high humidity is ideal when plants are young.

At this stage, you don't want them to lose much water - they don't have any roots for water uptake.

Aim for: 70 - 80% RH (depending on temperature - see table)

A humidity like this encourages plants to devote most of their energy to growing roots and foliage, rather than trying to correct their humidity.

  • roots form quicker
  • health improves
  • growth is much more vigorous.

Use a propagator to create a humid propagation environment.

All propagators that we sell have ventilation panels which can be opened and closed to change your humidity.

Veg growth


During veg, your humidity should be lower.

Aim for: 50% - 80% RH (depending on temperature - see table)

Now plants have some roots, you want them to transpire.  As water is lost through leaves, nutrients and water are drawn up through roots. 

Plants start growing leaves and shoots at a rapid rate. This puts less transpiration stress on each leaf, so you can afford to have a lower humidity. 

Don't let your humidity drop too low, though. If plants lose too much water, they draw more up through roots. Over a long period of time, this can cause overfertilisation.



During flowering, buds start to form.

At this point, you want to lower your humidity to reduce the risk of rot.

Aim for: 40 - 60% RH (depending on temperature - see table)

Keep a close eye on it with a temperature and humidity meter

To stop your humidity from rising too high, make sure your ventilation is up to scratch.

Failing that, you can get a dehumidifier.  



During lights off, your temperature drops. This means your air can hold less water. To counter this, you may have to adjust your humidity.

Featured Products

About the author

Paul is our Merton shop manager. If you haven't met him yet, he's as friendly as they get. 

GroWell Hydroponics Merton

Call GroWell Merton:
020 8648 6327