In the Northern Hemisphere, October through April marks the most popular time of the year for growing cannabis indoors, largely thanks to professional indoor grow lights (especially high intensity discharge grow lights) providing heat to indoor spaces. Cannabis plants don’t thrive in excessive heat, so if you’re embarking on an indoor summer grow, air conditioning expenses can be sky high. In cold-weather months, however, grow-light heat is beneficial for plants — and not just because it keeps them warm. Some resourceful growers have been known to remove heat from their gardening space and transfer it to other parts of their home, thus offsetting overall heating costs.
Grow-room temperatures must be carefully managed. When grow rooms become too warm, plants grow too tall, produce thinner buds with less resin glands, and work harder to satisfy nutrient and water needs. Meanwhile, when grow rooms are too cold, plants grow slowly, if at all. And there are many seed strains that won’t germinate if temperatures are cooler than 66 to 68 degrees Fahrenheit.
What Is The Ideal Temperature Range For Growing Cannabis Indoors?
Ideal indoor cannabis-growing temperatures range from 74 to 85 F. The precise temperature within this range for your grow room depends on the cannabis strains being grown, whether or not carbon dioxide (CO2) is being added to grow-room air, leaf surface temperature, and relative humidity inside the grow room. More specifically, grow rooms without added CO2 should have a temperature range of 74 to 78 F. Grow rooms with added CO2 can operate at higher temperatures, up to 85 F.
Because heating an indoor grow space usually costs less than cooling it, cold-weather months save you money compared to growing indoors when it’s hot outside. During spring and summer months, hot ambient outdoor temperatures and indoor grow-light heat combine to create excessive and sometimes ludicrously high air conditioning and electricity costs.
I recall a particularly searing summer in the San Fernando Valley, when for days in a row, temperatures hovered around 100 F, with humidity at 10 to 15 percent. My air conditioners went into overdrive trying to keep the grow rooms below 80 F, but they ultimately failed. I had to use expensive humidifiers to add moisture to the air so the plants wouldn’t wither and die. Then, the power grid failed. As a consequence, my plants died and my electricity bill for that month was an eye-watering $373!
When growing indoors during cold-weather months, lights-off grow-room temps drop several degrees, mimicking natural conditions that are favorable to plant morphology and bud quality when growing indica and Kush strains.
Cooler lights-off, or nighttime, temperatures promote bud development and resin production, while limiting the stretch in plant height that occurs in the first two to three weeks of bloom phase. Night temperatures are optimum when they’re three to five degrees less than lights-on temps. This day/night differential is most helpful when you’re growing Kush strains whose genetic lineage comes from the mountainous regions of Central Asia.
Cooler night temperatures in a sealed indoor grow room often result in high relative humidity. In grow phase and in pre-flowering bloom phase, a humidity range of 55 to 75 percent is acceptable. However, in peak and late bloom phase, when thick indica or Kush buds are present, relative humidity above 54 percent creates conditions favorable to the dreaded gray mold that can destroy a crop overnight. Many growers install dehumidifiers in their grow rooms so the moist air is kept under control. Dehumidifiers add heat to the grow room, which could be of benefit if the grow room tends to get too cold.
Cooler weather months make it easier to maintain the optimum temperature of 68 F for hydroponic nutrient solutions. This temperature is hostile to harmful microbes and pests such as root aphids, while also encouraging nutrient intake. Water temps that are cooler than 68 F slow nutrient intake, while temperatures warmer than 75 F promote root rot, especially if the root zone and/or nutrient water aren’t sufficiently aerated.
Chilling Your Water, Warming Your Roots
Growers with hot grow rooms and hydroponic reservoirs are advised to run a water chiller, whereas growers with cold grow rooms might have to install a heater in their hydroponic nutrients reservoir to keep the temperature from dropping below 68 F. In either case, it’s advisable to add dissolved oxygen to your hydroponic reservoir water, where your plants’ roots intake oxygen.
Speaking of root-zone temps, growers whose plants are in pots on the ground floor of a building or a basement should be aware that cold easily transfers from the ground into the pots, creating roots that are too chilled to function properly. Savvy growers elevate their plants off the ground using construction blocks, while others place horticultural heating pads under the plants. The same is true for hydroponic nutrient reservoirs.
In places where cold-weather outdoor temps dip below 32°F for a period lasting five days or more, growers can benefit from such conditions because freezing and sub-freezing temperatures, along with snow and ice, induce dormancy in pests and diseases that harm cannabis plants, including aphids, spider mites, powdery mildew, gray mold, thrips and whiteflies.
A secure grow room has filtration and vector control that should prevent pests and diseases from entering the space, but these controls aren’t always perfect. If freezing temperatures are in action outdoors, then pests and diseases are suppressed, while some are eliminated altogether and therefore won’t vector into your grow room.
Using Your Grow Room To Fight Seasonal Affective Disorder
When it’s cold, wet, snowy, cloudy and gloomy outside, your grow room provides a quasi-summertime cultivation experience. Professional high intensity discharge and LED lights radiate energy that helps fight seasonal affective disorder, or SAD, a mood disorder induced usually around the same time each year, during winter’s long nights, short days, and lack of sun and warmth.
I spend lots of time in my grow rooms in winter because they’re warm, well-lit, filled with lush foliage and glistening flowers, are hygienic and placid. The plants intake the carbon dioxide that I breathe out. In turn, I breathe in the oxygen my plants expel. It’s a wonderful symbiosis. In bloom phase, with my grow room perfumed by a variety of terpenoids, it feels like my own private oasis.
By the end of November, most indoor growers have already germinated seeds or rooted clones for their cold-weather indoor season. Using autoflowering cannabis strains, they’ll be harvesting in early February. With photoperiod strains, the season could extend into March, which is still a cool-weather month in most places.
I don’t recommend growing tropical sativa strains indoors during cold-weather months. Those genetics prefer warm rooms and higher humidity, and you might have to spend additional money to heat the room. Indeed, growing indoors during warm months is the ideal time for growing tropical sativa strains.