Many of the problems associated with cannabis plants have to do with the relative humidity level. This environmental factor is an essential requirement for most plants, and needs to be properly monitored if we are going to cultivate top shelf cannabis. Achieving the correct level of moisture in the air can save you big headaches further down the line. What's more, your plants will be happier and healthier.
Any grower who has had mold on their cannabis plants will be able to reiterate the importance of keeping the proper humidity. Growing indoors, we have full control over the environment, making it our responsibility to get the humidity right. This article teaches you the different ways humidity can be maintained to ensure you have a successful growing experience.
Why is Humidity Important For Cannabis?
The intention for most growers is to produce high quality crops with decent yields. This involves creating the ideal conditions for plants, and that includes relative humidity. Although cannabis plants can perform in a variety of environments, they do have a preference.
Humidity is important for a cannabis plant to effectively regulate its water uptake and transpiration. When humidity levels are too high, a cannabis plant's roots do not function correctly. This is because high humidity hinders transpiration and encourages more water to be absorbed through the leaves rather than the roots.
The opposite happens in the case of low humidity. When leaves do not receive enough moisture, water uptake happens primarily via the roots. To conserve water, the pores in the plant's leaves begin to close, limiting carbon dioxide intake. This can lead to weak plants with pH problems, nutrient deficiencies, burns and stunted growth.
Optimum Humidity For Growing Cannabis
Humidity levels require our attention throughout the different stages of growth. Normally, vegetative plants benefit from a slightly higher relative humidity than those already well into the flowering phase. The humidity is often reduced throughout the grow cycle as plants develop buds.
The perfect conditions will depend on the strain you choose, however, it is more complicated than this. Most strains on the market today are a mix of Indica and Sativa genetics so it is not as easy as saying 'one size fits them all' Achieving the best results requires you to experiment to find out which humidity your chosen strains do best in.
Some genetics thrive with a higher humidity whereas others can underperform in the same conditions. Indicas, for example, are more prone to mold in high humidity because of their dense, compact structure. Sativas, on the other hand, tend to do well in warmer, humid conditions. The optimum level is usually between 55 - 65% relative humidity, but let's consider the whole plant cycle.
- Seedlings - Young cannabis plants do not yet have a proper root structure to uptake water so higher humidity levels are recommended. This also helps them from evaporating too much water and drying out too quickly. 65 - 70% relative humidity with a temperature of 20 - 25°C is ideal. The dark period should stay above 20°C if possible.
- Vegetative - The vegetation period is when roots begin to uptake more water, so the humidity can be reduced gradually each week until flowering. Humidity levels can vary here depending on which strain you are growing, however anything between 55 - 65% is acceptable. Temperatures can be increased slightly to encourage drinking. Up to 26 - 28°C during the day is fine. Night times should not drop below 18°C.
- Flowering - Cannabis plants often perform better with a lower humidity during flowering. There isn't a perfect range here, but humidity should be lowered to avoid mold problems. Usually, we set the relative humidity to be between 40 - 55%, keeping it in the lower range during late flowering. Temperatures can be between 20 - 26%.
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Seedling
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Vegetative
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Flowering
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Relative Humidity
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65 - 75%
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55 - 65%
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40 - 55%
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5 Tips on Maintaining the Correct Humidity
Regularly checking and maintaining humidity is easy but requires some investment on your part. There is plenty of equipment we can use for measuring or controlling moisture in the grow space, so let's discuss some of the options.
Ventilation
Installing inline and exhaust fans that have a higher capacity than is necessary for your grow room allows you to increase or decrease airflow. When the humidity is too high you can up the speed of your fans so more fresh air is pulled into the space, sucking hot (and humid) air out. If the humidity drops too much, fan speed can be reduced to help keep the warm air circulating.
Oscillating fans placed around the grow room prevent humid air from becoming stagnant. These can be turned down slightly if humidity drops too much, but it is not recommended to turn them off completely. There should always be air moving so mold spores have less of a chance to settle on the plants.
Hygrometer
To monitor the relative humidity we need to use a hygrometer (can be purchased with a thermometer). Keep in mind that a hygrometer will give us a general reading of the moisture content in the particular area we are measuring it in. So, the humidity may not be the same around the canopy in relation to the lower areas of the plant.
When using a hygrometer, the meter or probe (if it has one) should be placed as close to the top buds as possible. The reason we do this is because air is easily trapped around the dense, compact flowers, meaning the humidity is often higher in the canopy. Pockets of hot, humid air create the ideal conditions for bacteria and fungus to develop.
Humidifiers/Dehumidifiers
If you live in a very dry climate, installing humidifiers can be a great option during the vegetative phase to bump up relative humidity. Cool water can be added to the humidifier to help bring temperatures down. Just remember to clean and fill up your humidifier regularly (with filtered water if possible). One that can hold at least a few gallons is recommended so you don't have to refill it as often.
Alternatively, if you have problems mainly with high humidity (more common in grow rooms), then a dehumidifier can be a worthwhile investment. Make sure you check it daily to avoid spillage from overflows. It's amazing how much water can be taken from the air in a single day.
Temperature
Temperature has a big effect on relative humidity levels. The higher the temperature, the more moisture can be held in the air. So, a relative humidity of 65% with a temperature of 25°C contains more water vapor than a relative humidity of 65% with a temperature of 22°C.
Mold loves moist conditions, especially when combined with warm temperatures. When temperatures are above the comfort zone of 20 - 26%, there is a higher chance of mold establishing itself because the concentration of moisture in the air is higher.
Temperature can be lowered by:
- Increasing ventilation and airflow.
- Installing air conditioning.
- Set your light schedule to be on at night.
- Consider growing with LEDs.
You can try raising the temperature in the grow room if the relative humidity drops below the proper range. This can be done using heaters, mats or installing more lights (careful not to give plants too much light).
Gorilla Girl by JohnnyBlaz3 from GrowDiaries.
Irrigation
How and when your cannabis plants are watered affects relative humidity levels, as well as the temperature. A lower humidity can be encouraged by watering early right before the lights come on. Watering when the lights are on (it's warmer) causes water to be evaporated faster, which increases humidity.
If you are trying to raise the humidity you can try watering less but more frequently. Regularly adding moisture helps to prevent extended dry periods, which can also cause an imbalance in RH. Find out how much your plants are drinking each day and adjust accordingly.
Conclusion
Managing the moisture levels in your grow room is relatively easy if you have the right equipment. Don't forget though, it is up to you to keep track of the humidity so you can be sure that your plants are safe from mold infestations and other problems. It is recommended you study this area in depth as it will make a hugely positive impact on your growing efforts.
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External References
Floral Humidity in Flowering Plants: A Preliminary Survey. - Harrap Michael J. M., Hempel de Ibarra Natalie, Knowles Henry D., Whitney Heather M., Rands Sean A. (2020)
Transpiration: Water Use Efficiency. - Mortlock, Miranda. (2020).
This article was updated October 2020.
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