Syncing Temperature and Humidity for Better Yields in your Marijuana Garden
In his 20 years of growing cannabis, and in his current role as chief cultivation officer of Green Dragon Co-op, Glenn Standridge has heard more than his share of wives’ tales about how to avoid yield-crushing plant diseases like powdery mildew.
While there aren’t any secrets or myths to stop these diseases, Standridge said there is a nearly foolproof way to prevent infection—controlling your environment.
If done correctly, managing environmental conditions and eradicating disease can allow you to not only keep all of your crop, but also improve growth and yields. Doing this, first, requires an understanding of how temperature and humidity work together.
The relationship between temperature & humidity
To put it simply, if temperature increases, relative humidity (RH) decreases. Temperature plays a role in how much water the air can hold at any given time, and variations in temperature cause humidity levels to fluctuate. Maintaining a stable relationship between these two factors creates a predictable environment that allows you to control your plants’ ability to transpire, and ultimately helps you produce bigger, better buds.
Vapor pressure deficit (VPD), the industry’s latest buzzword, is a concept that describes the relationship between temperature and humidity. But Standridge said what really matters isn’t the term; it’s more important to understand how moisture and pressure affect a plant’s ability to breathe and transpire.
A plant that’s been warmed up by grow room lighting and kept in an environment with a consistent temperature will maintain its transpiration and growth rate. However, a cold plant in a warm environment is a different story.
When the plant is cold and the room is warm, condensation is likely to form because the plant's surface is cooler than the air around it. Moisture droplets held in the warm air will settle and condense on the plants, creating wet surfaces and a prime environment for bacteria and fungi to grow—including two of the most feared plant diseases within the cannabis industry: botrytis cinerea (bud rot) and powdery mildew.
Air conditioning isn’t enough
Taking moisture out of the air is an important first step to prevent condensation. Standridge said when people come to him experiencing problems with molds and mildews in their own grow operations, he can quickly spot the problem—dehumidification hasn’t been taken seriously.
Standridge said often times people have one, small dehumidifier in a space that requires several commercial units capable of removing large quantities of water from the air. Also, they try to get around the rest of their dehumidification problems by using more air conditioning.
While it’s tempting to think that simply adding air conditioning can keep temperatures balanced and air dry, that’s not the case. Air conditioning units don’t keep air dry enough, and what’s more is that these cooling units can create their own fluctuation in temperature (when sized incorrectly), which directly impacts the humidity of grow rooms.
Rather than creating stable environments, over or undersized air conditioning systems create short, cyclical environments that are too warm, and then too cold. This can spell major problems for cannabis plants. Tomasini said one solution is to create longer running cycles to minimize the peaks and valleys of temperature fluctuation.
Find balance in your environment
Standridge said being able to fully monitor and control his environment has been a game changer when it comes to growing premium cannabis. Understanding how his lighting, dehumidification and air conditioning work in tandem, has allowed him to minimize fluctuations in both temperature and humidity.
“That’s really helped us to see what's happening," Standridge said. “So much of the time you’re not in the room … so you don’t know what’s happening with the plants because you’re not standing next to them.”
By monitoring his own environment closely, Standridge noticed the likeliest time for condensation and humidity spikes to occur was around the times he turned his grow room lighting off and on. To account for this, Standridge found it effective to run his dehumidification system while the lights are off, but then power down his dehumidifiers right before the lights turned back on.
Plants will continue transpiring after the lights are turned off, and without heat from the lamps the room will begin cooling. Running dehumidification units during this time removes the moisture from the air before it becomes a problem and keeps humidity levels consistent.
Shutting the dehumidifier off before the lights are turned back on allows the plants to warm up in a dry room, Standridge said. This way plants don’t create condensation.
What a stable environment means for your yield
Creating dry, stable environments encourages plants to transpire quicker than when high levels of humidity add pressure to the plant canopy. This increase in transpiration allows plants to grow faster, which in turn means they’ll take in more nutrients and water than they would in a more humid environment. While you’ll have to feed and water your plants more often than when humidity is high, Standridge said feeding and watering more is a good thing.
“If I can get my plant to uptake that extra amount, I’m going to have more growth,” Standridge said. “If I have more growth during this same time, I’m going to have double the [flower] you do.”
According to Standridge, needing to feed and water your plants more also gives you peace of mind when it comes to powdery mildew. He said that if you need to water more, it shows the plant is transpiring.
To make sure you’re able to keep all of the extra crop healthy, Standridge said keeping humidity lower than what people advertise is a safer bet than ideal VPD situations. He also said that when you add moisture, you also add a risk for plant disease.
“Once plants get past their teenage stage, usually you can’t get it too dry,” Standridge said. “Even if you turn on every dehumidifier you have, your plants are really working and they’ll just start to grow more.”