Enroll Now

News and Best Practices

Taking the Temperature of Your Grow

August 31, 2016

In grow rooms, we’re always checking the temperature. We need to keep the grow space at that sweet spot to ensure our plants not only survive, but flourish, too. If it gets too hot, we risk causing molds to spread. If it gets too cold, our plants may not grow as big.

Why is temperature so critical? A successful grower will understand that heat dictates how everything performs in a grow, so becoming intimate with the concepts of heat and temperature will give you some insight into a successful operation.

Fahrenheit and Celsius

We tend to use the terms “temperature” and “heat” interchangeably, but they’re not the same thing. Temperature does not measure heat. Rather, it measures the radiation coming from the heat rather than directly measuring the heat itself. Chemists use some fancy math to calculate heat from a temperature reading.

In the modern world, we typically measure temperature according to two scales. Fahrenheit is the scale we use in America, where water boils at 212°F and freezes at 32°F. The other common system is Celsius. This one’s used internationally by scientists because, well, it just makes more sense than Fahrenheit. In Celsius, water freezes at 0°C and boils at 100°C.

If you’re scrolling through forums or chatting with growers of other countries, keep these two scales in mind. If you’re given advice from a grower in Spain, they’re going to give you temperature values in Celsius. By using a simple bit of math, you can convert Celsius to Fahrenheit (and vice versa):

Fahrenheit to Celsius: (°F − 32) ÷ 1.8 = °C

Celsius to Fahrenheit: (°C × 1.8) + 32 = °F

Or you can just search “temperature conversion” on the web and punch the numbers into a handy conversion calculator.

Heat = energy

So that brings us to the thing generating all that radiation detected by our thermometers: heat. “Heat” is a bit of an abstract concept, but scientists define it as energy that can pass between systems without the need for work or matter transfer. “Energy” is simply the capacity to do work, while “work” is defined as motion that initiates change. In other words, we can think of heat as this free-floating, spontaneous energy that can affect anything and everything it touches.

Hopefully, you can now see why heat is so essential to a successful grow. With enough heat, we can accelerate the actions of enzymes in our plant’s cells, causing them to grow, multiply and die faster than they would in a colder environment. Just as heat can accelerate our plants’ cells, the same thing goes for the microorganisms living in the soil. Too much heat, however, will cause the cells to malfunction and die.

With less heat, we can control how quickly our microfauna multiply. We can also slow the growth of our plants, which may be necessary in a confined space. Because heat facilitates evaporation of water from our plants, growing in a colder environment will cause the plants to draw up less water; in turn, causing them to draw in fewer nutrients. With greater heat, there’s more evaporation, so the plants pull in more water through their roots and more nutrients in the process.

Ideal temperatures

Professional growers have general rules of thumb regarding temperature and grows. Depending on the cycle your plants are in, you want to maintain these optimal temperatures. These are only for when the lights are on. During the dark stages when the lights are off, you can let your plants sit at a cooler temperature (and they prefer this).

  • Vegetative: 70-85°F (20-30°C)
  • Flowering: 65-80°F (18-26°C)

What happens if your grow stays too cold? If your plants are kept below 60°F (15°C) for too long, you risk stressing them out. Stressed plants are more susceptible to disease, and they won’t grow as large. Additionally, some molds love cold, damp rooms, so failure to monitor your warmth could lead to nasty pest outbreaks.

On the other extreme, you don’t want the plants to be too hot, either. “Hot” in this case are temperatures above the 80°F (26°C) mark. Heat can also stress the plants. The dreaded powdery mildew infestation—the bane of indoor growers everywhere—becomes more likely in poorly ventilated grows that stay above the mark.

And there’s one other reason not to be above the mark: terpenes. Many terpenes will evaporate if they’re sitting in a room that’s too hot for too long. With the loss of terpenes, you’re losing the characteristic scent and flavor of your strains. Terpenes are also responsible for the kind of elevated effect we experience from particular strains. So, if your buds are losing terpenes, they may not initiate the same kind of high they’re famous for.

Maintain the “Goldilocks Zone”

We don’t want it too hot or too cold. We want it just right. How can we manage that when we’ve got tons of watts blasting hot white light all over our plants?

First, invest in lots of thermometers; you don’t want just one in the back corner of the grow. You want several, if not dozens, placed all over the grow. Temperatures are never consistent in a grow room. The heat levels near the floor will always be cooler than the plant canopies. So, ensure you’ve got thermometers at the canopy level, at the ground level and somewhere in between.

Note where the air currents are strongest in the room: near windows, vents, fans and doorways. Thermometers placed near these areas will give colder readings than those in less ventilated areas.

Since lights are the main culprits behind a grow’s heat, be especially mindful of the temperatures at the plant canopies. If it’s too hot, just move the lights up (or, with supercropping, you can move the plants outward). If it’s too cold, lower the lights.

If the entire room is too hot or too cold for whatever reason, consider buying electric coolers or heaters. These can run anywhere between $25 to $250, depending what you need and how heavy-duty they are. Remember that fans in a poorly ventilated grow may actually increase heat due to friction in their motors, even as they blow somewhat cooler air over the plants.


By Randy Robinson
Cannabis Cultivation Today articles are for informational purposes only and should not be considered legal guidance or advice on grow practices. You should contact an attorney or a qualified cultivation consultant for specific compliance and cultivation advice.


Subscribe to Cannabis Cultivation Today

* indicates required
Sign Up for Cannabis Cultivation Today

© 2019 CAN Performance Group, LLC. All rights reserved.