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Ideal Temperatures for Marijuana Plants

Mar 10, 2020
Temperature Problems

Ideal Temperature for Marijuana Plants

Growing marijuana indoors allows cultivators to control everything about the environment, ranging from how much light their plants get to the level of CO2 found in the air they breathe. One of the most important factors contributing to plant health and growth is temperature. Temperature affects everything from how quickly seeds germinate to how well the finished product cures, so it should be regulated carefully throughout the growing, drying, and curing processes.

The Effects of Temperature on Growth

Unlike most animals, cannabis plants don’t create their own body heat. They’re reliant on external sources of heat to stay warm. That doesn’t mean they don’t have any natural mechanisms for controlling temperature, though.

Plants use a combination of external heat and evapotranspiration to regulate internal temperatures. That means the temperature reading on a thermometer won’t perfectly reflect the temperature of the plants themselves.

Most growers realize that plants left in an uninsulated and unheated room will almost certainly die if they are exposed to below-freezing temperatures at night, but they don’t always know that plants also suffer from excessively high heat, especially in combination with high humidity. More about that later. For now, growers just need to know that while marijuana plants can survive within a pretty large temperature range, they require a much narrower temperature range to thrive. Here’s why:

Temperature and Photosynthesis

When plants get too warm, they can’t photosynthesize efficiently. Their enzyme activity decreases, they produce fewer proteins, and some essential proteins start to break down. If the plant doesn’t get the opportunity to cool down eventually, it will die.

Excessively cold temperatures can also negatively impact plant growth, even if it isn’t cold enough in the grow room to kill the plants. While the plants will still photosynthesize and produce sugars, they won’t be able to move those sugars to where they need to go if they’re too cold. This begins to be a problem when the temperature falls beneath 68°F. Mature plants will store the sugars and move them when it warms up, but young plants can be stunted by prolonged periods of cold.

Temperature and Respiration

Marijuana plants also rely on exposure to the right indoor temperatures to facilitate optimal levels of respiration. The connection between respiration and temperature is a bit complicated, though.

When the temperature drops, respiration decreases, allowing plants to expend less energy. Periodic exposure to lower than normal temperatures can be a good thing and can help accelerate growth. Unfortunately, when the temperature increases again, the plant requires more energy to keep itself alive. It starts expending most of its energy on respiration, leaving little for growth.

Many growers intentionally stress their plants by exposing them periodically to lower-than-normal nighttime temps to increase THC content and improve flavor profiles. That’s fine, but make sure to not to overdo it and give the plant everything it needs to maintain an optimal growth schedule after acute stress.

Temperature and Disease

Many common diseases are exacerbated by exposure to high heat. They include root rot, white powdery mildew, and nutrient burn. Pests like spider mites also reproduce quickly in the heat, which can lead to further damage. Most of these problems are more likely to occur when plants are exposed to heat in conjunction with excessive humidity.

Ideal Temperature Ranges

While it’s just not realistic to offer one optimal temperature that applies to all strains, environments, and stages of growth, there are some guidelines growers can follow. They differ primarily based on what stage of growth their plants are in. This is part of the reason, along with differing light schedules, that many indoor growers segregate their plants by age or growth stage.

Clones and Seedlings

Young clones depend on transpiration to access water until they establish root systems that can pull it from the soil. They require a much higher level of humidity than established plants. In terms of temperature, they grow best within a range of 68 –77°F, which is close to the ideal temperature range for seedlings.

Since both seedlings and clones benefit from 24/7 light exposure for the first few weeks of their lives, there’s no need to alter the temperature to mimic daily highs and nightly lows. Instead, aim for somewhere in the middle of this temperature range and observe the young plants to see how they respond.

Vegetative Stage

The ideal temperature range for plants in the vegetative state is 68 – 77°F, the same as that prescribed for seedlings and clones. Don’t rejoice yet, though. It is still important to segregate plants in veg, which have lower humidity requirements, from recently cloned plants. While clones and plants that have only recently entered the vegetative state prefer humidity levels of 70% or more, older plants need a humidity level of 45 – 55%.

Plants in veg also require a different lighting schedule, and most growers tailor their temperature ranges to that schedule. Most experts recommend 18 hours of light followed by six hours of darkness for plants in veg. When the lights go off, growers should set the temperature in their veg rooms 10 – 15° cooler to encourage increased growth, but don’t let the temperature fall below around 60°F, especially while the plants are young.

Flowering Stage

Plants in the flowering stage thrive in slightly lower temperatures, typically under 75° unless they’re receiving CO2 supplementation. Even if they use CO2 generators, growers should still keep their temperatures below 82°F. Allowing the temperature to rise above this level slows bud growth and can cause the terpenes that give different strains of marijuana their unique taste and flavor profiles to lose their potency. Terpene production maxes out at around the sixth or seventh week of the flowering stage.

Encouraging optimal trichome production during the later weeks of the flowering stage requires setting the temperatures slightly lower at night, or whenever the lights are turned off. Flowering marijuana plants require a 12/12 split of light and darkness, so growers will have plenty of opportunities to stimulate trichome growth for maximum terpene production.

Temperature isn’t the only thing growers should carefully control during the flowering stage. They should also reduce the humidity levels in their grow rooms to between 35 –45% for the early flowering stage and lower it even further to 30% for the last few weeks before harvest. Lower humidity reduces the risk of serious problems like bud rot, mold, and powdery mildew.

Drying and Curing

Properly drying and curing bud is just as important when it comes to determining the strength, smell, and flavor of marijuana as how the plants are grown. Maintaining a relatively low temperature, typically around 64°, and a humidity level of around 45% in the drying room will ensure that the harvested buds won’t develop mold or dry too quickly and lose their taste and potency.

Growers should control airflow in their drying rooms as well as temperature and humidity. Try to create a smooth, light flow of air around the room, but don’t position fans so they blow directly at the drying plants. This can cause the buds to dry out too quickly.

Unique Concerns for Outdoor Growers

While most indoor growers struggle to prevent overheating, outdoor growers have the opposite problem. They need to find ways to keep their plants warm, especially at the beginning of the season when temperatures can still fall to well below the optimal range at night. In an ideal world, the temperature of outdoor plants would not fall beneath 60°F, even at night, but that’s just not a realistic expectation for growers in most climates.

Thankfully, strains bred for outdoor growers are usually hardier. They can endure temperatures as low as 50°F without suffering any noticeable damage, but prolonged exposure to cold temperatures can still slow growth and prohibit optimal photosynthesis. If the temperature falls beneath 40° at night, the plants will almost certainly be damaged and some may die.

There are a few things outdoor growers can do to create microclimates around their gardens that are favorable to marijuana growth. It may not be feasible to constrain temperature ranges to within just five or ten degrees of the target temp, but that doesn’t mean it’s not worth trying.

Greenhouses and Cold Frames

There’s no need for home growers to invest in professional-grade greenhouses. Instead, they can use PVC frames covered with polyethylene plastic to decrease heat loss at night without spending a small fortune. A second advantage of this strategy is that it also protects the plants from inclement weather events like severe rain and even hail.

Growers who only have a few plants may not need a full-sized greenhouse. They can take advantage of the same principles on a smaller scale by building cold frames. Cold frames are small boxes made out of lumber, usually 2×4’s, and glass. Since they’re so small, there’s really no reason for growers to skimp on the glazing and as a result, cold frames typically provide better insulation than plastic-clad greenhouses.

Patio Heaters

Growers who’ve opted to build full-sized greenhouses can go one step further by installing one or more gas patio heaters. These heaters are cheap, efficient, run on easily available fuel, and can keep plants warm and happy even on frigid nights. Plus, they produce carbon dioxide as a byproduct. This gas can be converted during photosynthesis into the glucose plants need to grow and thrive, so growers who install gas patio heaters can also expect slight increases in their yields.

Compost Piles

Don’t want to risk leaving a heater with an open flame in the greenhouse, or just prefer to stick to all-natural growing methods? Moving a medium- to large-sized compost pile into the greenhouse or even next to plants grown in an unprotected garden can also generate some extra heat.

The compost pile will have to be large enough to allow thermophilic bacteria to thrive if this technique is going to work. These bacteria break down the food scraps, plant matter, and dead leaves in a traditional compost pile into the nutrients plants need to grow. Hot compost piles should be around 1 cubic meter in size and should have around a 1:1 carbon to nitrogen ratio.

Hot compost produces more heat than most growers expect. At the height of decomposition, temperatures can peak at between 120 – 150 °F. Growers who add thermophilic compost piles to their greenhouses should make sure they’re prepared for that outburst of heat, which begins just one day to one week after finishing and watering the pile by adding adequate ventilation.

Choose the Right Seeds

Most outdoor growers in cool climates purchase seeds specifically intended for cold weather growing. Hybrids tend to be hardier than pure Indicas or pure Sativas, but there are some exceptions to the rule. Northern Lights, for example, is an Indica strain that is well-known for thriving in cooler temperatures.

As far as hybrid strains go, Silver Haze, Northern Critical, Blue Cheese, White Widow, and Somango are all popular with growers who live in colder climates. Outdoor growers certainly shouldn’t constrain themselves to growing only these strains, but they at least offer a good place to start.

Drying and Curing

Even those who plan to grow outdoors shouldn’t try to dry or cure their weed outside. Instead, they should construct a small drying room and follow the temperature and humidity recommendations for indoor growers above to the best of their ability. Unless they’re growing completely off-grid, providing some extra climate control shouldn’t pose a problem.

The Bottom Line

Growing good weed requires facing somewhat of a Goldilocks dilemma. Too hot, and the plants will experience heat stress. Too cold, and they could die.

Whether they’re growing indoors or outdoors, commercially or for home use, all marijuana cultivators should carefully monitor temperature and humidity levels from initial germination through final curing. Segregate the plants by growth phase and keep within the optimal limits recommended above. It will minimize the chances of trouble, maximize yields, and improve the overall quality of the finished product. Check out for a full catalogue of high-quality seeds!

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