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Understanding Relative Humidity

Understanding Relative Humidity (RH)

Relative humidity (RH) 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 RH and why is it important?

RH is the amount of water vapor in the air compared to how much moisture the air could possibly hold at that temperature. Plant growth is influenced by RH because it directly affects the amount of water your plants move through their stems and leaves - essential for keeping plants hydrated and transporting vital nutrients.

What should the level of RH be in my grow room?

For healthy plant growth, you should aim for RH of 50-70% (RH can be measured using a hygrometer).

What are the consequences if the RH in my grow room is too high?

When RH levels are too high in your grow room, you have the perfect environment for spores to germinate and fungi to grow. The most common type of mold you will encounter is Botrytis (bud rot). Others include leaf molds, such as Powdery Mildew. There is also a risk of stem infections, although these are less common.

Hint: a lack of adequate ventilation can often be a cause for excessive RH. Please refer to our ventilation article for information on how to solve such problems. If ventilation is not an issue, a quick and easy way to lower humidity is to use a dehumidifier. An air conditioning unit during the summer that expels cool, dry air, will also help reduce RH. 

What are the consequences if the RH in my grow room is too low?

When your grow room is suffering from low amounts of RH, the leaves of your plants begin to curl upwards at the margins; looking like a tube/straw. The low RH 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 RH tends to be more common than high RH. In fact, when growing indoors under hot lights, most hydroponic growers tend to start off with very low RH. This is problematic because RH needs to be relatively high during the vegetative growth stages.

Example of a low humidity problem:

When you start out, your plants will be small with not much vegetation. The grow light(s) will also heat and dry the air in the grow room; giving a RH level of 35-50%.This will cause the small plants to take up and release increased amounts of water to balance water vapor in the air. This stresses the plants considerably by taking away the energy it needs to produce new roots, shoots and leaves. But if you increase the RH levels up to 60-70% during these vital, early stages, you will ensure quicker establishment of roots, vigorous growth and compact internodes. Ultimately, your plants will grow faster and healthier.

Once the plants have grown in size and have produced more shoots and leaves, they will take up more water into their roots and lose more water vapor from their leaves. As there is more water vapor being released into the air by the plant, the humidifier can now be switched off.

If the RH in your grow room falls back below 40%, you will experience a lot of problems -  one of them being over-fertilization. High temperatures and low RH will cause your plants to take up and release more water. Whilst your plants are taking up more water, they will take up less nutrient - causing the nutrient to continually increase in strength. It is this increase in nutrient strength - coupled with an increase in water uptake and release - that causes over-fertilization of your plants and other nutrient related problems.

The link between RH and temperature is the cause of many problems during the summer months, but despite this, it is still one of the first things that is overlooked.

Humidity and the Dark Cycle

Some indoor growers find that when the lights go out, RH shoots up. This leads to mold 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 lose water through its leaf tips. This water can then 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) RH is controlled and kept high using a propagator - such as the Stewart Heat and Grow Electric propagator. We recommended that you spray the lid of your propagator daily to keep the humidity above 80%. This will minimize water loss through the plant and concentrate its energy on producing new roots.

In summary, you will achieve the best results with your plants if you keep humidity above 50% and below 70%, and ensure RH is not lower in the night than day.


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