What Is The Optimal plant Temperature?

Getting the temperature right in your marijuana grow tent is extremely important as a grower because it affects your plant's ability to photosynthesize and grow.

Photosynthesis is comprised of a variety of processes that are either light-dependent or light-independent. Temperature affects the light-independent process of photosynthesis, either increasing or decreasing the rate of photosynthesis.

The light-independent reactions of photosynthesis take place in the stroma—fluid surrounding the grana (stacks of thylakoids) in chloroplasts. These reactions are catalyzed by specific enzymes which work best at certain temperatures. Hence, plants growing in ideal temperatures will photosynthesize faster than plants grown in temperatures that are too high/low.

In general cannabis plants grow best at temperatures between 20–30°C (70–85°F) during the day. Growers tend to agree that the ideal temperature is around 25°C (or roughly 75°F). At night, cannabis plants like slightly cooler temperatures of roughly 17-20°C (62-68°F).

1. Venting is your friend – Hot air should vent out of the grow space if heat is a problem. Vent air to the outdoors if you want to prevent the hot air from being recirculated around the grow room.


2. Fans only push air around – It’s a common misconception that fans bring down the temperature, but inline duct fans don’t cool the air. They provide a breeze and help level out the temperature within an area. If it’s hot in your grow tent but cool in your room, then a fan will help equalize the temperatures. But if your entire room is too hot overall, then fans won’t bring the temperature down.


3. ACs and Evaporative Coolers bring the temperature down – Besides exhausting your heat outdoors, the only way to bring the temperature down is to use an AC or Evaporative Cooler. Note: An AC will work in any climate, but evaporative coolers need dry air and only work when the humidity is under 30%.


4. Strain makes a difference – Heat and cold bother certain plants more than others. The strain has a significant effect on a plant’s heat or cold resistance. Get a list of heat-resistant strains.


5. Choose the right light schedule – Too hot during the day or too cold at night? Switch your timer’s on/off cycle, so your full spectrum LED grow light is on at night and off in the day. Switching the time your grow lights are turned on will help equalize day and night temperatures. It may even reduce your electricity bill as some people get charged less for electricity used at night. Keeping grow lights off during the day will help with heat, while your grow lights being on at night will keep plants warmer when it’s the coldest.


6. Get Extra Help – Here are 3 supplements that help cannabis with heat stress, and here are some tips for growers dealing with the cold.

The best grow room temperature during the vegetative stage of growth is 70-78 degrees F. when the lights are on during the “daytime” and no more than 10-15 degrees cooler at “night” with a relative humidity of 45-55%. With these settings, your plants will best be able to convert light into energy for growth. This is the time when the plant puts on leaves and branches and expands it’s root system throughout your growing medium. If it gets too cold or hot, growth stops and you eventually risk losing your plants altogether.

The best grow room temperature during the flowering stage of growth is 68-75 degrees during the day and no more than 10-15 degrees cooler at night. If you’re supplementing with CO2, daytime temps can be as high as 75-82 or so. During flowering, you should lower your relative humidity to 35-45% and even lower (30%) for the last couple of weeks before harvest. This will help you avoid issues with mold, bud rot and PM (Powdery Mildew) that can arise in higher humidity.

The drying room is a place that must be carefully monitored. Keep in mind that your plants will be giving off a large amount of moisture into the room as they dry. It’s important to pull wet air out and keep air circulating in the room without actually having fans blowing right on your hanging branches, which can dry them out prematurely resulting in a harsh taste and burn. Also, growers in dry places such as Colorado struggle to extend their drying time with humidifiers, while farmers in more humid climates such as Northern California use dehumidifiers to pull water from the air in order to avoid mold growing on their buds.

The ideal temperature for a drying room is between 65 – 74 degrees Fahrenheit and humidity between 45 – 55 percent in a dark well-ventilated room. You may need a Best Humidifier For Grow Room. Cannabinoids, terpenes, and flavonoids can evaporate and be released at temperatures above 80 degrees, diminishing the scent, flavor and potency of your buds. Within 6 – 10 days your branches should snap instead of bending and the buds should feel popcorn dry on the outside. This is the time to cut the individual buds from the branches and put them into glass jars to begin the curing process. Cure your buds in a cool (68-72 degree F.) and dark place.

If the temperature drops below 60*F (15*C) during the dark period, plants will grow more slowly and yields will not be as abundant. This won’t be readily apparent if you aren’t particularly familiar with the garden’s normal output. A few nights of cool temperatures won’t significantly damage your crop, but if it continues to occur throughout the flowering period, it can definitely be cause for concern. A CO2 generator or electric heater can heat the room adequately.

If the floor can have a steady temperature at around 80*F (27*C), the roots will be warmed and the stems and leaves will withstand influxes of colder air. A heating mat is ideal if you’ve only got a few plants to worry about. Larger gardens might require the use of a recirculating hot water heater to ensure optimal temperatures.

Most outdoor varieties can endure temperatures as low as 50*F (10*C) without any problem. That being said, fifty degrees is still not an ideal temperature because it tends to slow down tissue growth and photosynthesis later in the day. Anything below 40*F (4*C) can result in damage to the tissue. Gas patio heaters can keep gardens warm on frigid nights. Maintaining a temperature of 60 degrees will promote plant growth substantially.

These outdoor plants can also benefit from a polyethylene plastic covering that keeps things nice and toasty while also protecting the garden from the elements. Heaters can increase protection even more.

High temperatures are usually less of a concern than cold temperatures, as cannabis plants rarely die from too much heat. However, temperatures over 30°C will lead your plants to grow more slowly as the enzymes involved in photosynthesis aren’t able to work properly.

Apart from stunting growth, heat also attracts pests and diseases. Spider mites and white powdery mildew, for example, tend to thrive in hot conditions.

Hot conditions will also cause water to evaporate from your plant’s soil more quickly. Not only will you need to water your plants more regularly, but they may also struggle to take up the nutrients in the soil. This can lead to nutrient burn, which causes damage to your plant’s roots and foliage.

Also, remember that high temperatures combined with high humidity can expose your plants to a whole variety of additional issues. Plants growing in hot, humid conditions without enough air circulation can develop bud rot or mold, which causes a lot of stress on the plant and essentially destroys the buds you’ve been working so hard to grow all season.

Cold temperatures can be just as detrimental to cannabis plants as high temperatures. Unlike hot temperatures, however, extremely cold temperatures can shock plants or even kill them entirely.

As a general rule of thumb, cannabis growers should avoid growing in temperatures below 15°C (59°F). At these temperatures, your cannabis plant will struggle to grow and will eventually die.

Remember, cold temperatures can reduce a plant’s rate of photosynthesis just like hot temperatures. The enzymes responsible for the light-independent reactions of this process are not as active at low temperatures, meaning your plant will grow at much slower rates.

Lower temperatures can also create ideal environments for mold. Some molds prefer cold, damp environments, and may form on your plant’s roots, leaves, or buds should temperatures drop below an acceptable range.

Again, note that most cannabis plants prefer slightly colder temperatures at night (roughly 18°C). Some plants may also develop beautiful purple or blue hues when grown at lower temperatures, which can make for great-looking buds come harvest time.

Most plants can withstand high temperatures if they have a large root system that can bring up an adequate amount of water to keep the plant comfortable during transpiration. During vegetative growth, temperatures that extend into the 80’s and 90’s (27-37*C) can produce elongated stems. During flowering, it can produce airy buds. This is the same for both indoor and outdoor marijuana plants.

For indoor plants, you can lower the temperature with ventilation, air conditioning, or by using air- or water-cooled lights that eliminate heat. Just to be clear, it isn’t the temperature in the aisles of a room that you need to worry about; it’s the temperature directly under the lights at the plant canopy level on which you need to focus your attention.