A Deeper Dive into Humidity

Special Advertising Section - Special Report: Humidity and Climate Control

November 22, 2021

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Humidity control can be the bane of some cultivators’ existence. Rapid temperature drops in the night may cause humidity to jump upward, often into the danger zone. Daytime heat, or heat from lighting, causes a temperature increase which in turn causes the humidity to drop, and thirsty plants can wilt under that stress. But temperature is just one consideration in controlling humidity; there are other factors that come into play.

What some may not know, however, is that in addition to the obvious risks to crops, you and your staff could also face health risks. “The combination of warm temperatures and high humidity found in many indoor marijuana grow operations can fuel extensive mold growth,” stated Dr. John Martyny in a press release from National Jewish Health, regarding a study of the potential hazards of cannabis cultivation. “Airborne levels of mold spores that we found inside these structures may subject the occupants … to significant health hazards, especially allergies, asthma, hypersensitivity pneumonitis and other respiratory diseases.”

Control at Every Stage of Grow

While most growers agree that humidity is a concern, they are not in agreement on the optimal humidity levels at various growth stages of the cannabis plant. And while many growers admit to being unsure of their humidity-control practices, others say that their optimal room humidity levels absolutely work for them.

Source for data: Cannabis Business Times’ 2017 “Smart Humidity” special report

For clone rooms, 19 percent of research participants in Cannabis Business Times’ 2017 “Smart Humidity” special report said the ideal humidity level is 56%-60%. The same number of respondents said that, ideally, the humidity level should fall in the range of 76% to 80%. In all, the research showed that desired humidity levels in clone rooms vary from 40%-45% to above 80%.

For veg rooms, study participants’ responses varied almost as much, with the same number of respondents (10%) saying the ideal humidity level in these rooms is 40%-45% as those who said 71%-75%. The largest number of respondents (22%)—well shy of any kind of majority—indicated their ideal humidity level in veg rooms is 56%-60%, followed by 18% who said ideal humidity in these rooms is 61%-65%.

Source for data: Cannabis Business Times’ 2017 “Smart Humidity” special report

In the flowering stage, growers came closer to a consensus. Nearly three-fourths (73%) of respondents said the optimal humidity level in flowering rooms is in the range of 40% to 55%. Eighteen percent said 56% to 65%, while 3% of cultivators said their ideal flower room humidity level is in the range of 76% to 80%. No respondents said humidity above 80% was ideal.

The late flowering stage saw even more consensus among cultivators in the study. Nearly two-thirds (64%) indicated the optimal humidity level in the final stages of flowering is in the range of 40% to 50%. Within that range, more cultivators (39%) said 40%-45% was optimal than those who said 46%-50% was ideal (25%).

Just 4% said humidity levels during late flower are ideally above 70%, while only 1% said they prefer humidity above 80% in this stage.