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How to regulate temperature and humidity in your indoor marijuana grow

December 30, 2019
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(hanohiki/Adobe Stock)
Cannabis, like all plants, prefers certain environmental conditions in order to thrive. One of the major benefits of cultivating it indoors is that growers have the ability to manipulate their environments to suit the needs of the plants. Temperature, humidity, light intensity, and airflow are all factors that will need to be monitored and regulated in order to keep cannabis healthy through its different phases. 

Optimal conditions for a cannabis garden

Cannabis is a resilient plant that can survive under a range of conditions. However, for the most part, it prefers to be kept under the following conditions in each growth stage for optimal health:

  • Seedlings/clones: 75-85°F (with lights on), ~70% relative humidity
  • Vegetative growth: 70-85°F, ~ 40-60% relative humidity
  • Flowering: 65-80°F, ~40-50% relative humidity

Tools to measure environmental conditions

To understanding how to regulate your indoor environment, equip yourself with these cheap and easy-to-use tools to take measurements. You’ll need:

  • Thermometer: A basic one will allow you to measure how warm or cool the environment is inside your garden.
  • Hygrometer: This measures humidity, or more specifically, water vapor content in the air.
  • Infrared thermometer, or IR thermometer (optional): IR thermometers use a detection device called a thermopile to measure surface temperatures. Although not necessary, these are helpful in finding out leaf temperatures, which will give you an extra layer of knowledge on how to properly regulate environmental conditions.
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How to control your environmental conditions

Now that you have the tools to take correct environmental measurements, you can begin to make actual changes. The two factors you need to control to dial in the environment are temperature and humidity

Inevitably, there will be fluctuations of these two in your garden. These fluctuations can occur both throughout a grow space as well as within pockets inside a given room. They can also occur at different points within a given day or throughout a season as conditions change in the environment outside your grow space.

Regulating Temperature

This can be achieved by manipulating four factors. 

Light. The largest impact on the temperature in your garden is the type of light you use and how much heat it generates. Different grow lights will give off different heat signatures and depending on the size of your indoor space, room temperature can vary. Hot lights such as metal halides (MH), high-intensity discharges (HIDs), and compact fluorescent lamps (CFLs) will produce much more heat in a small space than LEDs. However, even LEDs produce some heat. 

Additionally, lights can be raised or lowered to change temperature at the canopy level.

Airflow. Controlling airflow is the best way to move hot and cool air throughout your grow space. Airflow can remove warm air produced from lights out of the garden and bring fresh cool air into it when needed. Airflow can also help exchange air throughout your canopy, cooling leaves in the process. 

Fans and ducts help regulate airflow—fans push air where it is needed and ducting can direct air into and out of your grow space, as well as to specific areas. 

Inline fans pull cool air from outside of the garden inside, stationary and oscillating fans push fresh air throughout the garden, and ducted hoods can remove hot air produced by lights away from the canopy.

Air conditioners. In order to rapidly cool a grow environment that is too warm when light and airflow manipulation aren’t doing enough, you may need to bring in an air conditioner to cool the overall temperature of your grow space.

Heaters. Some gardens may actually require warm air, especially during times when lights are off and not generating heat. This is where a heater can come into play.

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Tips for Improving Air Circulation in Your Cannabis Grow Room

Regulating Humidity

Humidity is the measure of water vapor in the air. There are four things you can use to help control it.

  • Dehumidifiers: Dehumidifiers remove moisture from the air. These units are used much more frequently in indoor grow spaces because plants in a confined space tend to create moisture, and the space will usually feel warm and humid.
  • Humidifiers: A humidifier can produce a constant flow of water vapor for an indoor garden. They use cool water to create a mist that disperses throughout the space and increases moisture levels.
  • Airflow: As with regulating temperature, regulating airflow will allow you to move moisture in and out of your grow space and control humidity. Simply opening up a grow space, i.e., opening the door to your grow room or tent, can bring down the humidity level.
  • Water: In the absence of a humidifier, you can mist plants with a spray bottle to create extra moisture. 

Balancing the system

It can be a tricky getting the right balance of temperature and humidity because they affect each other—turning up your dehumidifier will lower the humidity of your grow space, but it will also increase the temperature of the area. This in turn may require you to turn on an AC unit—everything’s connected! 

Depending on how severe the seasons are in your area, you can schedule your light cycles depending on the temperature outside—in the summer, the heat outside your grow and the heat inside generated by the lights might be too much for your plants, so you may have to turn the lights on at night when it’s cooler outside. The same is true for winter—you may have to shift the light cycle to nighttime so that your plants don’t get too cold.

Advanced control: environmental monitors and controllers

For growers who have a little extra money to spend and want full control over their indoor garden, environmental controllers will allow you to automate the process. These devices are essential for monitoring and controlling your environment if you’re away from your garden for any amount of time. 

Controllers monitor conditions within your garden and are connected to devices necessary to manipulate those conditions. You can connect a controller to fans, dehumidifiers, humidifiers, heaters, or air conditioners, and set thresholds whereby each device will power on and off based on your ideal environmental settings. Some units run autonomously, making changes based on set parameters, while others allow you to control each element via an app on a phone, tablet, or computer. 

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How to Perfect Your Grow Room Dehumidifier Setup

Advanced methods—using VPD to monitor your environment

Your average homegrower doesn’t necessarily need to know this method for regulating your environment, but if you want to get really scientific about it, here goes.

Once you familiarize yourself with how to use a thermometer, hygrometer, and IR thermometer, you can go a step further and learn how to extrapolate data from them to properly correct environmental conditions in your garden. You can do this by understanding the Vapor pressure deficit (VPD) of your garden.

VPD is the best metric for measuring and mitigating indoor grow conditions because it provides a stable unit of measurement among the constant shifts in a garden’s temperature and humidity. 

Plants primarily absorb water in order to stay cool. The majority of that water comes in through the roots and out the leaves through tiny pores—called stomata—in a process called transpiration.

Inside of a healthy leaf, relative humidity is around 100%—this means that gasses within the leaf are 100% saturated with moisture, creating pressure (saturated vapor pressure, to be exact).

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How Controlling Vapor Pressure Deficit Can Improve Your Cannabis Grow

The stomata on a leaf’s surface act as valves for moisture, transpiring vapor quicker when hot and dry, and slower when humid. Plants like to have their stomata open so that photosynthesis can occur. When proper conditions within the plant are not met, plants can close their stomata in an effort to keep water in, thereby halting photosynthesis. 

To measure VPD, you need these readings:

  • Air temperature
  • Humidity 
  • Leaf temperature

Without an IR thermometer, you can still get a leaf temperature estimate by taking a temperature reading at the canopy level with a standard thermometer.

There are plenty of VPD calculators and charts available online that can help you figure out this metric for your garden. You will simply need to input the three metrics above into the calculator or use a chart to get an estimate.

Cannabis requires the following VPD in these phases to thrive:

  • Propagation and early veg: 0.4 – 0.8 kPa
  • Late veg and early flower: 0.8 – 1.2 kPa
  • Mid-flower to late flower: 1.2 – 1.6 kPa

Using VPD to measure your garden conditions will allow you to easily make environmental changes based on where your conditions sit on the chart. Making environmental changes based on temperature or humidity alone won’t necessarily look at the whole picture of plant health as using VPD does, even though it may be more complicated to determine.

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Patrick Bennett

Patrick lives with his wife and daughter in Denver, where he spends his time writing, photographing, and creating content for the cannabis community.

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