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Humidity is and remains an important issue when growing indoors in the grow box. Basically, many plants like humid air, but it must not become too much. The plants release moisture into the air through their leaves and this becomes increasingly difficult the more humid the air already is.
If the humidity is too high, the absorption of nutrients slows down and growth and flowering are impaired. Furthermore, in the worst case, mildew or, especially in the flowering phase, grey mould can develop on the flowers.
Measure and control humidity
So what can be done? As the plants constantly release moisture into the air, the humidity in the grow tent increases. For the reasons mentioned above, the humidity should never be too high. The best way to check the humidity in the grow room is with a hygrometer and, ideally, you should permanently record the humidity and temperature so that you can also assess the development of these values. An inexpensive solution for measuring humidity and temperature is the GrowPRO Thermometer & Hygrometer. To record the data, we recommend the SensorPush, which allows you to view the course of the values in an app on your mobile phone.
What is measured?
Humidity is measured in RH (relative humidity) from 0 to 100%. This is a percentage that indicates how much of the maximum potential to hold water as a gas in the air at a certain temperature has just been exhausted. It is therefore the ratio between the absolute humidity and the maximum humidity.
What role does humidity play in indoor growing?
Gases in the air each exert a certain vapour pressure. Simply put, this increases as the humidity rises. The higher it is, the more pressure is exerted on the plants, so to speak. Therefore, the plants are increasingly unable to withstand this pressure and to release moisture into the air through their leaves. Especially in the flowering phase, low humidity is very important in order to achieve the best and highest possible yields.
How high can the humidity be for indoor growing?
What value the humidity in the grow box or grow room should not exceed depends primarily on the temperature. In cooler rooms, the humidity should also be lower. As a rule of thumb for the beginning
- Growing: RH 70-80%
- Growth: RH 50-80
- Flowering: RH 40-60
Below we show in a table which humidity is optimal in which growth phase and at which temperature. Note: Especially when using an activated carbon filter, the humidity should always be below RH 70%, otherwise it will lose its effectiveness.
Why do cuttings need such high humidity when growing?
Cuttings still have to develop their roots before they can absorb water. Until this happens, the cuttings must be able to absorb water from the air through their leaves. This requires high humidity during the growing phase. The best way to achieve this is to grow them in a greenhouse. This should be ventilated to prevent mould.
Further development in the growth and flowering phase
In the growth phase and later in the flowering phase, the humidity should be lower. In the growth phase, the plants can still benefit from being able to absorb water through the developing roots as well as the leaves due to a relatively high humidity. In contrast, the humidity should be significantly lower in the flowering phase. This causes the plant to produce more resin and reduces the risk of mould on dense leaves or flowers.
How do I lower the humidity in the grow box?
To lower the humidity, these are the best two options:
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Remove more air. What is the capacity of your fan? Can you turn it up or get a stronger tube fan? Keep in mind that more exhaust air means lower temperatures in the grow room, more noise and more energy costs. Basically, an adjustable exhaust air system is a good solution to be able to react flexibly to the humidity. Multi-stage fans, transformers or climate controllers are also recommended.
More information on choosing a fan with the right power can be found in this article: Exhaust air systems for indoor grow - A dehumidifier. This pulls the moisture out of the air and works quickly and effectively. The water extracted is completely neutral and can therefore be optimally reused. The dehumidifier should collect as much liquid within 24 hours as the plants have absorbed within the same period.
And how do I increase the humidity for indoor growing?
Conversely, the humidity in the grow box can also be increased by removing less exhaust air or by using a humidifier.
Humidity in the day/night cycle
The day/night cycle has a considerable influence on the air humidity and should be counteracted accordingly with the measures mentioned to control the air humidity. Ideally, you measure the temperature and humidity at the top and bottom of the tent with a SensorPush thermometer / hygrometer, where you can later look at curves to see how the values behave when the light goes on and off. Here you can see what happens when the light goes on: the lighting warms the air and since warm air can absorb more moisture, the relative humidity drops.
In this NDL grow in the flowering phase, the humidity is a little too low. The effect of the temperature (when the light comes on) on the relative humidity, which continues to drop, is clearly visible.
What influence does the temperature have on the optimal humidity?
The following table shows at which temperature which humidity is ideal. In the growing phase and the early growth phase you should be in the blue range, in the late growth phase or the early flowering phase you should be in the green range, and in the mid to late flowering phase you should be in the yellow range.
(Source: just4growers)
Example: At a temperature of 20 degrees, the humidity should be between RH 62.5% and 75% in the growing phase.
Conclusion
It is therefore important to measure and control the relative humidity continuously so that it is never too high or too low. The optimal value depends especially on the temperature in the grow room. This ensures optimal growth and flowering of the plants and minimises the risk of mould or fungal infestation.
