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What does heat do for weed?

For a lot of indoor growers, that is all you need to worry about. If it feels too hot or too cold for you in your grow area, it’s probably too hot or too cold for your cannabis plants as well.

Cannabis plants like about the same temperature as humans!

If your grow room feels warm or cold, humid or dry, that is a sign that you may want to look into changing the temperature or humidity of your grow area.

Generally, cannabis plants prefer temperatures in the 70-85 °F (20-30 °C) range during the day when lights are on. When grow lights are off (their “night”), cannabis plants are happy with slightly cooler temps.

Optimal Temps For Growing Cannabis

Vegetative Stage: Young growing cannabis plants in the vegetative stage like it a little warmer in the 70-85°F (20-30°C) range.

Flowering Stage: In the flowering stage (when cannabis plants start making buds), it’s best to keep temps slightly cooler, around 65-80°F (18-26°C). This isn’t for the plants themselves as much as to ensure the best bud quality. Slightly cooler temperatures in the second half of the flowering stage help produce the best bud color, trichome production, density, and smell. To really bring out colors, aim for a 10°F (8°C) difference between day and night.

Proper temperature brings out colors and can increase bud quality

7 Essential Concepts About Temperature Control

  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 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 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.

Why Temperature Matters to YOU As a Grower

Why Is temperature critical when growing cannabis? Can cannabis stand freezing temps? What happens if your grow room gets too hot?

Different afflictions can happen from high or low temps, so let me break that down for you.

Too Low (Cold) Temps

Colder temps will tend to slow down growth. Temps lower than 60°F (15°C) tend to upset plant growth, and freezing temperatures will shock or even kill a cannabis plant. Some plants continue growing in the cold without signs of stress, but they often don’t yield as well as they should be given the amount of light they receive.

Plants are also more susceptible to certain types of mold when it’s cool, especially if it’s damp, too. Mold sometimes occurs after frost melts and gets water all over your plants.

Colder temps and significant temperature fluctuations contribute to the over-purpling of leaves and can also reduce photosynthesis.

A plant that is grown in relatively cold temps can survive, but it will never grow as fast or as well as a plant living in a good environment. Indoor plants tend to be much more susceptible to cold than outdoor grown plants.

Too High (Hot) Temps

While cannabis plants don’t usually die from the heat, too hot temps will cause plants to grow much more slowly.

Please note temps above 80°F (26°C) in the flowering stage will slow down bud growth and may cause them to become airy and loose. The heat can also reduce the potency and smell of your buds. Keeping the grow room temperature under control is especially important in the flowering/budding stage!

In the heat, cannabis is also more susceptible to many problems, including spider mites, white powdery mildew (especially if it gets humid, too), root rot, nutrient burn (from increased water transpiration), increased stretching, wilting due to root oxygen deprivation, and reduced “smelliness” of buds (as terpenes can burn away at higher temps).

More About the Upper Limits of Temperature

  • With bright grow lights (such as LEDs or 600W HPS and brighter), cannabis plants of all stages can thrive at slightly higher temps up to a max of about 85°F (30°C) even with no CO2 enrichment, as long as there is low relative humidity and plenty of air movement.

  • Cannabis plants can thrive at even higher temperatures, up to an ultimate max of about 95°F (35°C), in an environment with low humidity, extremely bright grow lights, and enriched CO2 (above 1500 PPM) – is not your average grow!

  • However, it’s essential to pay attention to bud development. Just because the plants are okay doesn’t mean buds are going to be the highest quality possible.

Temperature & Humidity Adjustment Cheat Sheet

  • Dehumidifier – Raises Temperature (somewhat), Lowers Humidity

  • Space Heater – Raises Temperature, Lowers Humidity

  • Air Conditioner – Lowers Temperature, Lowers Humidity

  • Evaporative Cooler – Lowers Temperature, Raises Humidity

  • Humidifier – Raises Temperature (somewhat), Raises Humidity

What else is affected by the temperature in the grow room?

  • Bud Color – The temperature experienced by your plants influences the color of buds and leaves. The temperature either suppresses or brings out blues, purples, and pinks if your strain has the genetics for it.

  • Plant Growth Patterns – Temperature fluctuations can actually change how your plant grows! Warmer night temperatures tend to slow down upward growth. Cannabis generally prefers a cooler temperature at night. Maintaining slightly lower temps during the dark period will encourage your plant to grow as fast as possible.

  • Relative Humidity – Temperature has a significant effect on the relative humidity of your grow area.

  • Drying & Curing – Temperature is incredibly vital to the drying & curing process. A professional dry/cure is one of the most significant factors when it comes to producing top-shelf buds.

Tip: Use Temperature to Help Control Growth!

Cannabis generally prefers slightly cooler temperatures at night, and warmer night temperatures will tend to slow down upward growth. But… you can occasionally use this knowledge to your advantage to help control plant growth!

By artificially raising temperatures during the dark period, it will slow stretching plants that are growing too tall too quickly. That means warmer nights can be a helpful way to reduce the “stretch” if you’re growing in a situation where you don’t have a lot of vertical room. And never forget about super cropping, the ultimate way to break down the height of your plant at any time.

By keeping temps a bit cooler during the dark period, you will encourage your plants to grow as fast as possible.

Temperature & Humidity (VPD)

In this section, you will learn a little bit more about how the temperature in your grow room affects the humidity.

Before we talk about VPD, here are some humidity guidelines for growing cannabis

Although those are general guidelines, plants can thrive at different relative humidities depending on the temperature of the air.

Once the air becomes too saturated with water, it tends to form dew or films of water over leaves, which leads to mildew and bud rot. Plants also tend to grow more slowly in very high humidity.

Wet or humid conditions can lead to bud rot

Temperature and relative humidity (RH) are closely related to each other. Sometimes you can overcome a problem with one by fixing the other.

“Humidity” measures how much water is currently “being held” in the air. “Relative humidity” compares that amount to the maximum amount of water that air holds at that temperature.

For example, Warm air can “hold” more evaporated water than cool air.

For those interested in learning more about the science behind this, there is a term used by greenhouse growers known as VPD or “Vapour Pressure Deficit,” which roughly measures the temperature and relative humidity.

According to Wikipedia: “the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 kPa.”

When growing cannabis plants, relative humidity (RH) is just as important as the temperature of the air and is something you should pay attention to, especially if you notice that your grow room is dry or humid.

If your grow area is particularly dry or humid, you should pay attention to RH (Relative Humidity) in the grow room…

You need to control both temperature and RH at the same time to get the best results when growing cannabis.

  • If the air is too hot and dry (high VPD), plants will tend to have slow, stretched growth.

  • If the air is too cool and humid (low VPD), plants grow slowly and are prone to problems with mold or fungus.

Note: Pay attention to buds. The best VPD for water moving through the plant does not necessarily match the best temperature/humidity for bud growth, smell, and development. Additionally, each plant is different. As growers, we care more about bud quality than VPD, so make sure to pay attention to your plants before VPD numbers

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