What is the Optimal Humidity for a Cannabis Grow?
Humidity is key for cannabis plant growth, and for each stage of your grow there is an optimal grow room humidity level.
Ideal humidity levels vary based on strain, season, geographic location and whether you grow in or outdoors. It’s worth spending time dialing in levels because humidity control enables you to boost plant growth and limit conditions where mold and mildew thrive.
In addition to plant growth and disease prevention, optimizing humidity allows you to control nutrient intake. Plants take in moisture and nutrients through their leaves and roots, so by controlling humidity you control this intake too.
But before you start playing around with levels of moisture in the air, it’s important to understand what humidity is. Relative humidity (RH) is the ratio of the current absolute humidity to the highest possible absolute humidity, according to How Stuff Works. Humidifiers add moisture to the air and dehumidifiers remove moisture; both help you control relative humidity so you can maintain an ideal level for your plants.
Depending on your conditions, you’ll have to adjust levels to reach your optimal grow room humidity. The primary RH control method for indoor growers is dehumidification because it keeps RH levels consistent. Hobbyists may not need much dehumidification, but on the other end of the spectrum–and everywhere inbetween–commercial growers benefit from high-capacity units like Quest’s 506
Once you’ve determined ideal humidity levels in your specific environment, and the best grow room dehumidifier for your setup, they will have a beneficial impact on your grow and play a role in increasing yield, Tomasini said.
According to Seth Lee, Operations Manager at GreenLight Cannabis Company, many growers are afraid of high humidity because they know it can lead to disease, mold and mildew. Because of this worry, growers tend to overcorrect and keep humidity too low.
While it’s true humidity can encourage the spread of fungal problems, your plants need humidity to flourish in the early stages. Maintaining good air circulation and ventilation, proper spacing between plants, and keeping a close eye on your plants are easy ways to reduce common problems associated with high humidity.
Plants have different needs at each stage of the growing cycle, so to keep it simple, we’ve organized the grow stages into four parts and each one is defined below. Tomasini said this, generally, is what optimal grow room humidity looks like at each stage:
This stage lasts around a week and occurs when clones are cut from the mother plants. Because clones don’t have roots, their only method of obtaining moisture is through their leaves, so they thrive in high humidity of 80 to 100 percent. Once roots are formed, humidity can be lowered to 60 to 65 percent. With high humidity, there are no concerns of mold at this point because it can easily be washed or wiped off.
The vegetative stage lasts about a month and occurs when the plant is gaining mass and building its immune system and structure. During this stage, you can decipher between male and female plants, and by now the plant has developed a strong root system, which will assist in water and nutrient transfer. Humidity should be around 50 to 70 percent.
Flowering is the final stage of growth and lasts 8 to 12 weeks. Many growers will follow vapor pressure deficit (VPD) guidelines at this point to keep the plants as comfortable as possible. You will begin to see the bud production; growth and humidity should be between 50 and 60 percent.
- End of flowering:
By now the plant is likely very large, dense and taking in a lot of water. The value of the plant is at its prime because harvest is near, so most growers go on the defensive at this point and reduce humidity to 40 percent. Lower humidity levels will minimize the development and spread of mold and diseases, which is critical to protecting the health of the buds.
Though optimal humidity alone doesn’t ensure profitable crops, humidity control is a key piece to the puzzle of a successful grow.
“Too often, the environment is overlooked, when in reality it’s actually just as, if not more important than the other concerns, and often the reason people fail,” Tomasini said.