by Nebula Haze
Table of Contents
Cannabis plants like a temperature similar to humans, or a little warmer – not too dry, not too humid.
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. More about temps in the vegetative stage.
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 helps 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. More about temps in the flowering stage.
Proper temperature brings out colors and can increase bud quality
7 Essential Concepts About Temperature Control
- 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.
- 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.
- 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%.
- 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.
- 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.
- Get Extra Help – Here are 3 supplements that help cannabis with heat stress, and here are some tips for growers dealing with the cold.
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.
Learn more details below…
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 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.
This plant is drooping after experiencing a cold night
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).
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) – this is not your average grow! Learn more about CO2 here: https://www.growweedeasy.com/co2-marijuana-yields
- 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
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. Learn how to grow pink or purple buds.
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.
Cool night air can trigger purpling in some plants
Cannabis generally prefers slightly cooler temperature 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 supercropping, 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.
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.
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.
Learn more about humidity and growing cannabis: https://www.growweedeasy.com/humidity-growing-cannabis
Clones prefer higher humidity than plants with developed roots
Preference: Comfortable Room Temperature or Slightly Warmer 70-85°F (20-30°C) – High humidity preferred (60-80%)
Clones do not have a root system yet, and so they need to get all their water via transpiration of the leaves until their roots form. Therefore, clones prefer humid conditions with plenty of water evaporated in the air. That’s why many people use a humidity dome or “cloner” to artificially raise the humidity for clones. However, like anything in life, there can be too much of a good thing. You shouldn’t let the humidity get higher than 80% if possible because it can encourage the growth of mold or mildew, and may cause plants to root slower.
Clones seem to root and grow the fastest when the temperature is slightly warm, about 70-85°F (20-30°C), just like a cannabis plant in the vegetative stage.
Learn more about cloning cannabis plants here: https://www.growweedeasy.com/complete-guide-cloning-marijuana
Did you know you can clone cannabis in a cup of water?
Preference: Comfortable Room Temperature or Slightly Warmer 70-85°F (20-30°C) – Avoid very low humidity
Young seedlings tend to grow faster with mid-to-high humidity and warm temperatures.
However, a cannabis plant’s ability to withstand drier and colder/warmer conditions tends to increase as it gets older. Outdoor-grown plants tend to be much more robust about temperature fluctuations than indoor plants.
It’s good to aim for a temperature of 70-85°F (20-30°C), during the vegetative stage. It’s not always possible, but try to give plants slightly cooler temps during the dark period than during the light period as long as you stay within the recommended range.
Maintaining slightly cooler night temperatures (in the specified range) tends to promote the fastest and healthiest vegetative growing for cannabis plants.
Two happy vegetative plants getting a comfortable 75°F (24°C)
Vegetative cannabis plants like higher temps when you supplement the grow room with CO2, but there are a few critical things you must know about CO2.
For CO2 enrichment to be successful, you must…
- Seal the environment and maintain at least 1500 PPM of CO2 in the air
- Provide a lot of light (more light than your plants would typically be able to use)
- Keep temperatures between 85°F (30°C) and 95°F (35°C).
- Avoid high temperatures after buds start forming, so they develop properly
Learn more about CO2 here: https://www.growweedeasy.com/co2-marijuana-yields
Preference: Avoid hot temps! 65-80°F (18-26°C) – Mid-to-Low humidity – Slightly cooler at night than during the day
In general, you should strive to keep temps under 80°F (26°C) throughout the flowering stage, and especially towards the crucial second half when buds are in the midst of development.
The temperature in the flowering stage is too important to ignore. That’s because keeping temps low during the flowering stage will help your plants produce the most potent, dense, smelly, and trichome-covered buds as possible.
Unfortunately, higher temperatures during the flowering stage can cause plant problems and slowed bud growth. It can also cause terpenes and potency to evaporate into the air.
What that means is if your buds stay too hot during the flowering stage, the higher temperature may accidentally be burning away some of your terpenes and cannabinoids (reducing the final taste/smell/potency of your buds after harvest).
Read more about how to improve the taste and smell of your buds here: https://www.growweedeasy.com/better-taste-better-smell
It is especially important to make sure buds are not exposed to too-hot temps after week 6 or 7 of flowering, as this is when the terpene content in the buds starts to ramp up.
Give plants colder temperatures at night, especially towards the end of the flowering stage
Note: Slightly chilly is okay, but never expose your plants to freezing temps!
Too-cold air can cause buds to grow airy and loose
Bonus! In addition to improving taste and smell, terpenes can affect the color of plants and cannabis buds. For example, terpenoids are what gives tomatoes their red color. The same process may be at work with certain cannabis strains, too. By maintaining lower night temperatures in the last few weeks before your cannabis harvest, you will help bring out colors in your cannabis (purple, pink, blue, etc.). Of course, this only works if your strain has the genes to produce colored cannabis buds. Most strains will only grow green buds, but if your plant can produce other colors, cooler night temps can help bring them out!
While starting with good genetics and properly caring for plants is key to growing top-shelf quality buds, your job isn’t over when you cut down your plants at harvest. I’d say that almost 50% of the final bud quality is determined after you cut down your plants, but how you choose to dry & cure your buds.
Buds that have been professionally dried and cured are more potent, denser, smoother to smoke, look better, and have that coveted “sticky-icky” feeling.
The main thing you must do as a grower to professionally dry and cure your buds is to maintain the proper temperature and humidity during the process. If you follow the right method, you never have to worry about mold or overdrying, and you will get beautifully cured buds every time.
Learn more about how to professionally dry & cure your buds here: https://www.growweedeasy.com/how-to-cure-buds
Many cannabis growers worry about how their indoor grow lights will affect the temperature of their grow room. Heat production from grow lights is a valid concern, and this section will break everything down for you.
CFLs & Other Fluorescent Lighting
CFL grow lights can be purchased from the grocery store and can be kept mere inches from your plants, so they are great for growing in very short/tight spaces where ultimate stealth is the biggest goal.
CFL grow lights have earned a reputation for being very low heat. And, indeed, you can successfully grow a cannabis plant under just a few CFLs, and they won’t give off much heat.
However, if you want to get bigger yields, you’ll have to keep adding more and more CFLs, and the heat they give off will start building up.
Other fluorescent lighting uses very similar technology as CFLs, but the bulbs come in different shapes, some of which can be more suitable for growing. While not as stealthy or small as CFLs, other fluorescent lighting can be a cheap and effective way to nurture young cannabis plants.
HID Grow Lights – Metal Halide (MH), High Pressure Sodium (HPS) & LECs/CMH
MH/HPS/LEC grow lights are known for using a lot of electricity and producing a lot of heat. And while that’s true for the bigger models of HID lights, I think a lot of growers overestimate how much heat will be produced by the smaller HID models.
For example, a lot of people recommend that new growers get CFLs instead of HIDs so that there’s less heat in the grow room. Yet it’s essential to keep in mind that 250W of HPS light will produce about the same amount of heat as 250W of CFLs, except that you’ll get more light and better yields with HIDs. Another advantage of HIDs over smaller lights like CFLs is good models of HIDs come pre-built to accommodate cooling via an exhaust system.
That’s why it’s easier to cool an equivalent amount of HIDs compared to CFLs.
However, HIDs do need more height than growing with CFLs, so if the height is limited, than HIDs may not be the right choice for you.
But, if you do have the height to accommodate an MH/HPS light, I highly recommend going for that over CFL grow lights. In my opinion, CFL grow lights a best in very short/tight spaces where ultimate stealth is the biggest goal.
Learn About LEC/CMH grow lights (highly recommended as it seems to beat other HID lights for yields, smell, and trichome production)
LED Grow Lights
LEDs are known for needing just a little electricity and for producing less heat than HID grow lights like MH or HPS lights.
But is all the marketing hype true?
It’s actually a lot more complicated than just a “yes” or “no” answer. The truth is, some LED grow lights work incredible, and some are a ripoff. It’s helpful to educate yourself about LED grow lights to make sure you get exactly what you’re looking for.
In addition to measuring the temperature, it’s also important to know the relative humidity (RH) of your grow room. So you might as well get a thermometer that measures both temperature and humidity at the same time.
But it’s not necessarily that easy. The quest to find the perfect thermometer/humidity monitor can be tough.
While you can find cheap ones at the local store, you may notice that a lot of the cheaper models are a few degrees “off”. A few degrees probably won’t have a massive effect on your results, though. Some growers will buy 2 or 3 different thermometers and use the average to determine what’s really going on in the grow room. I have several that I’ve picked up over the years, and they all sit in my grow room now.
Here’s a popular thermometer/hygrometer that has all standard features and costs $12
That model works pretty well, but next is the temperature and humidity monitor that I use primarily. The humidity number is more prominent on the screen because it was initially made for incubating chicken eggs (humidity is critical to egg hatching rates). I’ve found that many thermometers slowly die from the heat of a grow light, but this particular one seems to keep on trucking despite the extreme environment. I bought my first one over 5 years ago, and it still works great.
This thermometer ($25) seems more accurate than many others I’ve tried and is resistant to heat from grow lights
It can be tough on the quest to find the perfect thermometer/humidity monitor, especially if you don’t want to spend a pretty penny. I like this one. It only costs $8, and it comes with a probe. I have the probe hanging in my grow tent near the plants, with the screen outside the tent. This setup makes it easy to see the temperature and humidity exactly where the plants are without having to open the tent.
I like that this thermometer/hygrometer ($8) has a probe you can hang in the grow space.
Chart – optimal temperature for growing cannabis plants
Vegetative Stage: 70-85°F (20-30°C)
Flowering Stage: 65-80°F (18-26°C)
If your temperature is too high or low….
Both hot and cold in your grow room? For those who struggle with both hot and cold temps where you live, you may want to consider getting an All-in-One Heater & Air Conditioner.
Too Hot: What to Do
Exhaust System: Make sure you have a sound exhaust system to pull out hot air and increase air circulation to your plants. If you don’t have an exhaust fan pulling hot air from your grow lights outside, fixing that is your first step. Remember! An exhaust system is important not only keeps temps down; it improves the air circulation your plants need to thrive. Learn how to set up a proper exhaust system here: https://www.growweedeasy.com/hps-grow-lights-setup
Tips for controlling the heat in the grow space using an exhaust system
Air Circulation: If there is a lack of proper air circulation in the grow room, you will tend to get hot spots that can damage your plants. It’s always a great idea to install fans in the grow room to help circulate the air inside the tent. Remember, don’t point fans directly at plants. Leaves can be rustling slightly, but stems should not be waving around due to the direct breeze of the fan. I like to point fans somewhat over or under the canopy, so it doesn’t aim directly at plants. Sometimes I point the fan at a wall to move the air around without blowing on the plants.
Change the Light Schedule: If you’re in a situation where the temperature is climbing too high at specific parts of the day, then you can change your light schedule so that your grow lights are off during the hottest part of the day. For example, in the vegetative stage, your plants need 18+ hours of light each day. If it gets too hot in the middle of the day, you could set your timer to turn your lights off for 6 hours during that part of the day. This strategy will not fix a major heat problem but can help alleviate symptoms of heat stress during a short hot spell or heatwave.
Consider Getting Different or Smaller Grow Lights: Learn about different grow setups.
AC or Evaporative Cooler: If you have good air exchange and have tried all the steps above, but the temperature in the grow area is still too hot, you will need to use a piece of equipment that can lower the temperature of the air.
- Get a Heat-Resistant Strain: Some strains can naturally handle heat without a problem.
Air Conditioner – (some are portable, some ACs fit in your window) – in addition to cooling the air, an AC will also tend to lower the relative humidity of the air. To be effective, and AC must be able to exhaust hot air outside the house, just like hot grow lights. If the air outside the grow tent is already too warm, you will need to get an AC to pre-cool the air before it enters the grow tent. No matter how strong your exhaust system, it won’t be able to bring the temperature lower than your ambient room temperature. If you need to lower the ambient room temperature, you’re going to need a way to cool the air, and air conditioners are hands-down the most effective way to cool your air.
Swamp cooler – (also known as an “evaporative cooler”) Ideal for those where it’s both hot & dry as the swamp cooler will bring down temps while also adding additional humidity to the air. Works best when humidity is too low, and the temperature is too high. Repeat: these do NOT work well if your humidity is above 30% RH! Read how one user used a swamp cooler to control his temps & humidity.
Too Cold: What To Do
If it’s too cold in your grow room, sometimes you have additional options besides just getting a heater, such as getting a stronger/hotter grow light or using extra insulation to contain the heat of the grow lights you do have.
If your plant is not insulated from the cold (for example, if it’s growing in the middle of your garage, basement, attic, etc.), then the first step is to make sure your plant is adequately insulated. It’s cheaper and easier to insulate a small space, so create a dedicated grow box or get a grow tent that will act as the outward bounds of your grow area.
Then you can buy rolls of insulating materials for cheap at your local home improvement store and use these to insulate your grow area further. Insulation will help keep the cold air out, and the hot air inside.
Reflectix material is both reflective and insulating. Line the inside of your grow space to help contain heat.
With a properly insulated grow box, your grow lights will likely provide enough heat to keep your cannabis plants warm unless you live in a freezing cold area, or perhaps if you’re in the middle of winter or a cold spell.
If you must get a heater to keep your plants warm, avoid letting it blow hot air directly on your plants, and especially avoid ever having it in the tent with your plants. You want a heater that radiates heat out evenly, instead of pushing out a blast of hot air like a space heater. Never allow hot air to blow directly on any part of the plant. Most heaters can turn on and off depending on the temperature, so they turn off when unneeded.
The following model of heater is an excellent example of the type of heater you want for your grow room – it puts out gentle heat and has a thermostat you can use to have it automatically turn on and off as needed. The heat radiates out instead of being pushed out (which you don’t want in your grow room). It’s almost entirely silent. Plus, it comes with a timer, so you can have it preset to turn on just before lights go off for the night (or whatever you need for your space).
This heater gently radiates heat instead of blow hot air
Keep roots warm and up off the cold floor
If you’re growing in a basement or garage, it’s common for the floor to be cold. If growing plants in containers, it can help to put them on a piece or some other barrier to prevent them from sitting directly on the floor.
Some people are growing in a garage or attic that is okay during the day but is getting too cold at night when their grow lights go off.
It can help to switch your timer’s on/off cycle, so your grow light is on at night and off in the day.
Switching the light schedules helps equalize day and night temperatures. Keeping grow lights on at night will keep plants warmer when it’s the coldest.
Change your timer so grow lights are on at night
However, to get most cannabis plants to start making buds, you need to give them at least 12 hours of complete darkness every night, which means grow lights need to be off for 12 hours/day. For some growers in a wintry climate, a 12-hour dark period (even during the day) will send the temperature plummeting too low.
But did you know there’s a type of cannabis that will make buds even if you leave your grow lights on for 24 hours a day?
Auto-flowering strains let you keep your grow lights on for 24 hours/day, which helps keep plants warm in cool growing areas!
They are described as “auto-flowering” because these strains will automatically start making buds and be ready to harvest in about 3 months, no matter what light schedule they get! This lets you leave your grow lights on all day, every day, so your plants stay warm!
Be Prepared For When the Electricity Goes Out!
One challenge for growers in wintry climates is the impending chance of a power outage during the winter months. Indoor growers rely on electricity to keep plants alive, especially in the winter, so try to prepare ahead of time!
And while you’re here…
Strain Makes a Difference!
Some strains are much hardier than others when it comes to the cold. If possible, it helps to pick a cold-resistant strain. Learn where you can get seeds.
One advantage of cold temperatures is it tends to bring out colors like purple and pink