Understanding Relative Humidity RH
Relative humidity is often overlooked when growing indoors. However, if you
want strong healthy plants, you need to consider the relative humidity of your
grow room. Here we explain what you need to know…..
What is humidity and why is it important?
Humidity is the amount of water vapour in the air compared to how much moisture the air
could possibly hold at that temperature. Humidity influences plant growth because it directly affects
the amount of water that your plants move through their stems and leaves. This keeps your plants
hydrated and transports vital nutrients.
What should the level of humidity be in my grow room?
For healthy plant growth, you should aim for a relative humidity of 50-70%. Humidity can be mesured using a hygrometer.
What are the consequences if the humidity in my grow room is too high?
If humidity in your grow room is too high, you have the perfect environment for spores to
germinate and fungi to grow. The most common type of mould you will encounter is Botrytis (Bud
Rot). Others include leaf moulds, such as Powdery Mildrew. There’s also a risk of stem and root
infections, although these are less common.
High humidity can be a sign that there’s not enough ventilation in your grow room so you may want
to rectify this.
What are the consequences if the humidity
in my grow room is too low?
If humidity is too low, it’ll cause the leaves of your plants to curl upwards at the margins, looking like
a tube/straw. This humidity decreases the amount of transpiration as less leaf surface is exposed to
dry air, which lowers the amount of water your plants will lose.
Generally, low humidity tends to be more common than high humidity. In fact, when growing indoors
under hot lights, most hydroponic growers tend to start off with very low humidity. This is problematic
because humidity needs to be relatively high during the vegetative growth stages.
Humidity and the Dark Cycle
Some indoor growers find that when the lights go out the relative humidity shoots up. This’ll lead to
mould problems like Botrytis on fruits or flowers if the night time humidity is too high. This can be
corrected by using a dehumidifier. Be sure that the dehumidifier does not remove too much water
during this time as it can over-dry the air, causing the plant to loose water through its leaf tips. This
water can collect on the leaves, creating the perfect micro environment for spore germination.
Ideally, you want your grow room to have a lower humidity during the day compared to the night.
The ideal figures to aim at during fruit/flower formation would be 50-60% in the light cycle and
60-70% during the dark cycle.
Humidity and Propagation
During the propagation stages, rooting, cuttings or germinating seedlings humidity is controlled and
kept high using a propagator, .It's recommended
that you spray the lid of your propagator daily to keep the humidity above 80%. This’ll
minimise water loss through the plant and concentrate its energy on producing new roots.
In summary, you’ll achieve the best results with your plants if you keep humidity above 50% and
below 70%, and make sure that the humidity is not lower in the night than day.
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