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Ideal Temperature For Marijuana Plants

Ideal Temperature For Marijuana Plants

The temperature in your grow room is very important for the photosynthesis of your marijuana plants. For example, low temperatures reduces evaporation through the leaves. The result is that the suction force, which takes up nutrients via the roots, becomes smaller.

The nutrients that are necessary, but aren’t absorbed, remain in the growing medium and eventually disintegrates in the root environment. A high acidity in the growing medium reduces the working of the roots which makes the plant absorbs less water and nutrients, and the growth can even come to a halt.

So what posed as a small external imperfection at first, can have serious consequences in an entirely different part of the plant. Therefore it’s very important to create a good climate in your grow room. Read this article and you know everything there is to know about temperature & marijuana.

And please share your knowledge or ask questions in the comments below.

Effects on the plant

A plant’s temperature is vital to their health, but unfortunately, unlike animals and humans, plants cannot create their own heat. Instead, your marijuana plant will be entirely dependent on its environment.

A plant’s temperature develops from a combination of external light, external temperature, and the amount of evaporation. A plant’s exact temperature is not something you can read on a thermometer, but it is a definite measure of health.

One example of how temperature can affect the overall health of your plants is the process of photosynthesis. To a certain degree, photosynthesis is not affected by temperature – it can safely occur at 60ºF (15ºC) or 85ºF (30ºC). Regardless of the temperature, your plant will still be able to produce enough sugar.

How does photosynthesis works cannabis?

How does photosynthesis works?

Temperature becomes a factor when your plant needs to send those sugars to the places they’re needed. Sugar doesn’t move as well when it’s less than 68ºF (20ºC). In fact, the sugars will get stuck, and your plant will suffer.

When this happens in mature plants and only lasts a few days, it’s not that big of a problem. Once the temperature is resolved, the backed-up sugars will go where they should be. However, in immature plants, this situation will stunt the plant’s growth.

Tip: make sure to download my free Grow Bible  and learn how a marijuana plant grows

Another temperature- sensitive function is respiration. Respiration decreases as the temperature drops. This should be a good thing because it lowers the amount of energy a plant uses. However, it also creates a dangerous crutch.

When a heavier crop develops because of a lowered temperature, total crop respiration increases . Because of this, when the temperature increases, more energy is needed to keep the plant alive. This leaves little room for the plant to grow.

Maintaining the perfect temperature for your plants can be tricky. Lucky for growers, there’s already plenty of research around the ideal temperatures for your plants. So keep on reading…

Grow room

The ideal temperature in your grow room depends on several factors. The location of your room in the building is an important one; in a basement it’s a lot cooler than in an attic with a flat roof.

Aside from that, the size of your room, the airflow, the number of lights and the extraction rates play an important role. Keep this in mind when building your grow room.

Image powered by Bergmanslab.com

temperature cuttings and seedlings

Temperature between 68ºF and 77ºF for cuttings and seedlings

When the light is turned on, an ideal temperature for the cuttings and seedlings is between 68ºF and 77ºF (20ºC and 25ºC). As the plants get older they can evaporate a little bit more and the temperature may increase to a maximum of 82ºF (28ºC).

When the lights are off, the temperature should lie between 59ºF and 72ºF (15ºC and 22ºC). Another important rule is that the temperature differences between day and night cannot be too high, a maximum difference of 18ºF (10ºC). So when it’s 82ºF (28ºC) during the day, it cannot go below 64ºF (18ºF) at night.

Relation to humidity

First, here’s a bit of information on how humidity works. The air our plants ‘breathe’ contains water vapor. The amount of water vapor in the air can vary. This is the humidity.

In a grow room, the humidity is always a bit higher because the plant’s leaves evaporate water. Marijuana plants only use 10% of the water they absorb for growing and evaporate the other 90%.

However, the air can only hold a certain amount as well. When this number is reached, condensation begins. You’ll notice condensation as little droplets of water in the colder areas of your grow room or on your plant.

Temperature plays a huge role in humidity because it affects the amount of water vapor that the air can hold. Warm air can handle much more water vapor than cold air.

Humidity while growing

Humidity while growing

How do you know you are getting close to having too much moisture? The relative humidity (RH) is a measurement of the percentage of moisture already in the air. For example, an RH reading of 70% indicates that more water vapor can be absorbed into the air. However, 70% at 77 degrees is different than 70% at 68 degrees, because warmer air can hold more moisture.

Specifically, at the higher temperature, 2 pounds of air could have as much as .45 ounces of water vapor in it. On the other hand, 2 pounds of 68-degree air only holds .32 ounces. So, if you allow the air to cool down, the air can hold less water vapor, leading to condensation while also raising the relative humidity.

Example: 77 Degree air cooled to 73 degrees — RH rise from 70% to 80%

This process leads to what is commonly known as the dew point. The dew point occurs when the air can no longer hold any more water vapor, and it condenses into droplets called dew.

In the example above, we know that the dew point of 77 degrees with 70% RH is 66 degrees. Therefore, we must keep the temperature above 66 degrees to prevent excessive moisture.

Ideal humidity for marijuana seedlings

Ideal humidity for marijuana seedlings

Maintaining proper moisture in your grow room is important because a properly ventilated grow room always experiences regular drops in temperature. The heat from the lamps as well as the naturally humid air is regularly sucked out through fans and other ventilation methods. Regardless, you must make sure the humidity levels are optimal for your plants.

We discuss ways to increase humidity here.

How to measure temperatues

You measure the temperature in your marijuana grow room with a thermometer. There are analog and digital thermometers, and they’re for sale everywhere for a few bucks. I always use a digital thermo/hygrometer (something like this one) with a built-in memory, so I can see what the maximum and minimum temperature was.

It’s also a good idea to get one with a temperature sensor on a wire, so you can hang the display outside your grow room, and can see the temperature when the lights are off.

Measuring temperature and humidity

Measuring temperature and humidity

Always measure the temperature in the shade, and at various places in your growing room. Provide good air flow by placing several fans. The lamps emit radiant heat which does not affect the air temperature.

Therefore it will always be a few degrees warmer right under the lamp than a shaded spot. That is not really an issue, but make sure there’s enough distance between the lamp and the plant, so the tips of the plants won’t literally burn. Radiant heat does affect the leaf temperature.

How to lower temperatures

Heat often becomes a problem in your grow room. This has to do with the fact that the lamps we use produce a lot of heat. Fluorescent lighting is not really a big deal, but HPS lamps can heat up your room to soaring temperatures of 122ºF (50ºC), which is fatal to your marijuana plants.

First of all, the dimension of the room is important. For a 600 watt HPS lamp always use a minimum space of 3 ft x 3 ft x 6.5 ft (100cm x 100cm x 200 cm) .

For the extraction, use the following simple formula; number of watts divided by two = extractor in cubic feet (meters). So 2 x 600watt HPS is requires an extractor fan of  22000 cubic ft. (600m3). The extractor fan blows the hot air outside, and sucks in fresh air inside.

You can put a carbon filter on it, so it doesn’t blow marijuana smelling air out. You can also put ballasts etc. outside your grow room because they generate a lot of heat.

carbon filter in growroom

Lower your temparature by using a extractor with filter

Perhaps the easiest way to keep the temperature low is by running at night. Your lamps will turn on for a few hours after the sun has set, and turn off a couple of hours after sun rise. This way you’ll have your lights on at the coolest period of the day.

You can imagine that if it’s hot weather outside, you’ll also suck this hot air into your grow room with your extractor. So, the room temperature never gets below the hot temperature outside. There are professional growers who use an air conditioning unit so they can run it during the day and at night.

How to increase temperatures

When the lights are on, it’s not necessary to increase the temperature. The lamps themselves will take care of that. However, it is important to distribute the air over your room evenly so that you’ll get the same temperature everywhere. Use swivel fans for this, and aim them between the lamps and the plant.

increase temperature while growing cannabis

Increase the temperature while growing by using swivel fans

Fresh air from outside should also be well distributed over the growing area, so that there won’t be any cold spots. Especially in winter when temperature can get below freezing.

When the lights are off, it can get cold in your grow room. Luckily there are plenty of things you can do to increase the temperature. A simple space heater with thermostat is usually sufficient enough to heat your space. However, they do consume a lot of electricity. A radiator with a thermostat works fine too. You can also turn off the extractor fan (that provides the fresh air) on the moment the lamps turn off. This can be done by a so-called fan controller with thermostat, or with a timer.

Seedlings and clones

Clones do not have their root system yet, so they depend on transpiration for their water. Therefore, they require high amounts of humidity, at least until they have fully formed roots. Many growers use a humidity dome to create an ideal amount of humidity for clones.

Image powered by Bergmanslab.com

humidity for growing clones

Clones like a high humidity

The ideal temperature for clones is between 68-77°F (20-25°C) with high humidity. At these temperatures, they should quickly form roots and become more self-sufficient. These temperatures are similar for seedlings.

Vegetative stage

During the vegetative stage, young plants prefer a high humidity (70% or more) and temperatures between 68-77°F (20-25°C). However, as the plant gets older, a slightly lower humidity is okay. This is more of an issue for indoor grown plants, as outdoor grown plants are able to withstand more temperature fluctuations.

Tip: make sure to download my free Grow Bible for more information about the marijuana plant life cycle

The ideal temperature during the vegetative stage is between 68-77°F (20-25°C) with moderate humidity while also providing slightly cooler temperatures during dark periods.

The cooler ‘night’ periods are perfect for encouraging growth. Don’t let temperatures drop below 59ºF (15ºC). Once the plant matures, it should be able to withstand cooler temperatures during the day and drier air as well.

Flowering stage

Once your plant reaches the flowering stage, it can thrive at a comfortable room temperate with low humidity. Unless you are adding additional CO2 to your grow room, a temperature under 82°F (28°C) is ideal. These lower temperatures encourage potent, trichome rich buds that you can smell and enjoy before you burn them.

Don’t go over 82°F (28°C) because higher temperatures cause terpenes to evaporate and they also slow bud growth. If your buds are too hot during this time, you may be literally burning away the good stuff as they grow, leaving very little taste or smell by the time of harvest.

Be especially careful to keep it comfortable for your plant after week 6 or 7. This is when terpene production is at its max, and you risk evaporating them due to high heat.

For optimal trichome production, make sure it is slightly cooler during the dark periods. The change in temperature may trigger increased terpene content as it optimizes plant processes. Just don’t make it too cold.

Humidity during flowering stage

Creat the ideal temperature during flowering stage

If you aren’t familiar with terpenes, they involve much more than taste and smell. They also impact the color of both the buds and the entire plant. Terpenes are responsible for the tomatoes red color, and they possibly may do something similar in certain cannabis strains. With the proper dark-period temperatures, you can bring out interesting colors in your plants (such as blue, pink and purple).

This phenomenon depends on the strain, of course. Most marijuana plants only grow green buds, but if you want to see what your plant can do, keep it cool for it ‘night’ temperatures and find out.

Drying and curing

Harvesting is not the last step to top-quality bud. Even with the best genetics, you’ll need to properly dry and cure your buds. Perhaps half of what determines great marijuana is how it has been dried or cured.

Professional dried and cured marijuana is more potent, looks better and produces a smoother smoke.
It will have that ‘sticky-icky’ feeling that marijuana lovers crave.

temperature and humidity while drying

Maintain the right temperature and humidity while drying

If you want the best results from your harvest, focus on maintaining the correct temperature and humidity throughout the entire process. This will help prevent mold and over-drying as well as make it easier to produce the best weed possible.

Keep temperatures around 64°F (18°C) and the humidity at 45%. The values on the picture above are not correct. I made this pictures just after harvesting so temperature and humidity were still high.

Create a smooth air flow bud don’t blow directly on your buds. This will cause them to dry too quickly.

Too low

Marijuana grown indoors functions best at moderate temperatures between 68 and 77°F (20-25°C) during the light period and a drop of no more than 18°F (10°C) to 60°F (15°C) during the dark period. CO2-enriched plants will produce more at a marginally higher temperature of just under 82°F (28°C).

What if the temperature in your marijuana grow room is too cold? 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.

CO2 system setup

CO2 system setup. Over $1000,- investment

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.

Too high

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.

Tip: make sure to download my free Grow Bible for more information about the influence of temperature on marijuana plants

Marijuana plants won’t usually die from heat, but they can cause the plant to grow slower. High heat (above 80 degrees) while its flowering will not only slow down the bud growth, but it will also reduce the smell and potency. If you care about growing buds with plenty of cannabinoids, you need to be sure to keep the temperature under control during the flowering stage.

Heat also causes other problems for cannabis. When it is too hot, your plant is more likely to suffer from spider mites, root rot, white powdery milder, nutrient burn, wilting and increased stretching. These situations are even more likely if it is also humid, or if there is increased water transpiration.

Outdoor

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.

Result of wrong temperature

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.

Cooltubes

For a quick fix, just keep the lights farther away from the plants. If you want something more substantial, try installing air-cooled lights with reflectors that will reduce the heat near the light. Water-cooled lights are actually more effective at diminishing light-generated heat.

air-cooled lights

Air-cooled lights, quick fix

For the most part, HPS lights should maintain a distance of around 3 to 4 inches per 100 watt from the tops of the plants. Air-cooled lights make the acceptable distance range between 2 and 3 inches per 100 watt.

Water-cooled lights make the acceptable distance about 2 inches or even less per 100 watt from the plant tops. With light movers, you can move the lights closer or farther away depending on your preference.

Check this table with distances for all grow lights. Including LED and CFL.

Heatstress

Don’t stress out your plants. If your plant gets too warm, photosynthesis is impacted, enzymes activity decreases, and fewer proteins are produced. Some proteins even break down. If this continues long enough, your plant can die.

This rarely happens to outdoor grown plants because we are referring to temperatures greater than 105 degrees. However, this can occur in indoor setups if you are not paying attention.

Heatstress cannabis

Heatstress on plants – leaves curl up inwards

You’ll recognize heat stress on the top leaves of your plant. They will start to turn yellow and curl inward. This happens as the plant tries to protect itself from the heat. To see it in action, put a lamp too close to your plant. It’s a simple mistake that can quickly cost your plant its life.

Prevent this from happening to your plants by keeping your lamps a bit higher than your plants and/or by using a fan to blow away some of that excessive heat.

Check out my marijuana seedshop  for a wide selection of top-quality seeds. We ship to the United States, Canada, and a number of other countries. If the order doesn’t get to you or the seeds don’t germinate, then you can get new seeds free of charge.

Happy growing,

Robert

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Comment Section

27 thoughts on “Ideal Temperature For Marijuana Plants


By GeoBot on 31 May 2014

Good Info, Thanks


By chrissy on 9 October 2014

OK just a quick question… My girl isn’t growing very much and is only 13 inches IF THAT at 42 days of vegging. She was badly burnt by I guess getting hold of a TR nute ball in the soil and was horrible looking she bounced back quickly nicely and super fast at least a node a day and a half. Now it’s like a node for every 2 days or even three. I don’t have much for her heat wise and sorta wondering if I should get a heating pad for her. My house runs cold natrually but I’ve been able to manage 75 to 82 degrees Fahrenheit since germination. Hope to hear back soon and the article was great to read I even wrote some things down too lol thanks again


By sam and maria roberts on 2 November 2015

We are getting ready to start growing… we’ve been doing a lot of research on lighting. We’re wondering why or if LED lights are even a possibility for growing? They have an abundant amount of lighting without all the heat plus they use very low wattage compared to other lights… is this something that would work or do we need to use another type of light for the heat element as well? What do you think/
Completely confused,
Sam and Maria


By Ed boham on 20 May 2016

Led is by far the way to go! No matter what the energy savings alone are worth it but also the quality of light! If you need proof go to YouTube and put hps v led the results are iutstanding


By perfectgrowth on 25 August 2016

the LUSH brand of lights are the original and best lights on the market. there are a lot of led knockoffs that do not work at all. so be careful


By Chapito on 9 July 2016

I used the LED flourencents from Walmart with a T8 bulb in the 6100-6300k color range for veg. takes 4-6 weeks to veg rather than 3 bit the flower cycle is roughly the same length so you can veg under LED and flower under good lights and have a constant rotation and save on power. T5 are best but the 2 bulb T8 LED at Walmart works too. I use it and I get great results. my last veg was 2 weeks longer than expected and I ended up with more used space and a yield increase from 24 cured OZ per plant I got nearly 34 cured oz per plant. I have stayed with this system because it works. keep in mine I am using a hybrid hydro system of bubbleponics and RDWC so it can be tricky but the yields are insane especially with the CO2 injection enrichment.


By Nick on 4 November 2016

Yes I have a 15 way led and it works great


By Alba on 13 February 2017

I’m using a 600w led in a 12 sq ft room on at night and cycling off in day keeping the temp between 70 -77 deg the light is full spectrum led working like a charm 7 weeks into the process


By ColoradoRosin on 12 April 2017

It’s a myth LED’s dont generate heat. They do. Enough to keep a room at a good temp. I had to cool my LED down but removing the glass, and turning teh fans around.


By latewood.ILGM on 3 November 2015

This is all a matter of choice. Without knowing what size grow you are considering I can only offer my opinion.

LED is expensive compared to HID. The newest digital hps/mh switchable, adjustable power output all in one systems are not that hot. In fact; When set up with proper ventilation, you will enjoy the warmth.

If you are worried about electrical consumption; Your yield is always going to suffer. I use a portable AC unit with dehumidifier built in. You cannot merely use LED lamp as a cure all for heat, humidity or electric consumption.


By BotanicMike on 6 November 2015

For Sam & Maria Roberts

1st we must know what your climate is like in order to determine which lighting setup is right for you. I grow with both. So there are a few factors to look at before deciding which is best for you. As for yields – you can expect the same GPW using either light source… No miracle 2-3 GPW claims here. There are a lot of factors that play into this rather than merely just the light source.

If you live in the Southern Hemisphere LEDs would be an ideal choice if you do not wish to spend an arm and a leg on AC. Temps from 75-83* are ideal for growing with LEDs. If ambient temps are on average above 90 in summer time. This might be a more ideal cooling or just venting situation vs if you used HID in this climate. Lower temps and your plants won’t be able to perspire, and growth will be slow and may have root rot from roots staying wet too long.

On the opposite side of things, if you live up North where it can get quite cold, HID would be the choice to go with. Using the HID would supply the heat needed to keep the area warm. ( be sure to block air vents into room! Otherwise this could cause the room to be to warm. Adjust airflow as needed. ) Keeping your canopy at around 72-75 (80 w/Co2 enrichment) is ideal.

It gets a little cold where I’m at in the winter while it can be quite hot in the summer.. This just takes extra steps to ensure a quality grow. Heaters, AC, running when temps are not extreme.

But anyways in one area I have 2 8″xxxl w/ 1000 watt Hort. Super HPS. With fans on temps never get above 80* 12″ from light source. So I drop my hood as low as the plants can handle. It’s cold here so I run while the temps are at there lowest, however during the day it’s still pretty cold and even though this has been insulated very well I have to add heat during the day. I also grabbed a few of those mats you stand on while working in the shop to place under the containers as a barrier for the cold concrete floor and the roots.. 4 pack is like $8 at HF. Approx 20″x20″ (heating pad for 2 humidity domes fits nicely as well)

Last but not least is your overall budget:

Let’s face it lights can be expensive, not to mention everything else that is needed. You can spend over $2000 easy on one LED light fixture… Crazy! While you can find cheaper ones… Get one that has proven results, cheap imitations can leave you feeling frustrated with poor results.
HIDs are much cheaper, and they have worked great for over 20 years. While they have had advancements and become cheaper in recent years. As for brands there are so many… If budget isn’t a problem then get what ever you fancy.. Some have high end looks too if that’s your thing…
But if your just looking for something to get the job done and well too, you can pick up a 1000 watt switchable digital ballast, Horti. Super HPS bulb and 8″xxxl hood for right @ $400 with free shipping.

For more direct response you can find me on FaceBook under the same name.
Hope this helps!

BotanicMike


By T Bear on 20 May 2016

Hi Mike I’m growing ultimate and power plant at the moment one tent has 4 power plant in 25litre air pots 5ft by 5ft and the other same size tent in an eb and flow system at the start we had some heat problems and still an issue at times both have hps 600 watt duel light bulbs have 1000 to 1500 ppm of co2 going in both tents at the start as I said temps went up to 103f which we had to solve by putting lights on at night and leaving tents open we have intake fans on 15 minute intervals and outage on all the time with good air movement in both was thinking after this harvest switching to LEDs as it’s the start of the summer in Scotland and can get hot also the cost is more than expected at £100 gbp per month I managed to keep on top of temp when growing at home but because its elsewhere my friends first grow and he ain’t the freezer space as what I did was freeze 2litre bottles and place behind fan and I managed a great crop would you advise LEDs or a air cooled lamps any advice the hydro grow are drinking 10litres water a day and never drank half that when I’ve been doing in the past here’s hoping my friend can keep on top of temps any advice most helpful like is an ax unit expensive and would this help kind regards T Bear ps was really thinking on going auto what are your thoughts on this and do you have seeds for a hotter climate regards


By Joao Ferreira on 26 May 2016

You mention temperature what about humidity ?



By Gongalee on 27 May 2016

Great info, well appreciated.


By pokediok on 6 December 2016

I dont get the fuss about LEDs, frankly, HID produce 130 – 150 lumens per watt, the same as LED and cost TIMES cheaper. So there are no savings on your electricity bill.
And for the quality of light? Plant needs AMOUNT of photo energy, not the quality, it is not a human eye, its biology and chemistry


By Michael Coutts on 11 February 2017

I have used both HP and LED in my grows. I am sticking with LED. My electricity bill is 43% of what it was , I water less frequently and at least in my veg room the results are no less tha phenomenal. As far as product output I get about 10% less but my buds are much tighter due to the lower running temperature of the lights. The PAR and spectrum seem to be better ths the HP lights.


By Rob on 12 February 2017

You mention a max temp difference of 50F but that can’t be right. You’re converting 10C to Fahrenheit, but since you’re talking about differences in temp, you should use only the slope in the calculation and drop the intercept. So 9/5 * 10C = 18F max difference. I hope this is helpful.


By Roy ILGM on 13 February 2017

Thanks Rob, I will have it looked at!


By Chris on 7 March 2017

You say “So when it’s 82ºF (28ºC) during the day, it cannot go below 64ºF (18ºF) at night. A temperature difference of 40ºF (5ºC) is ideal.” If its 82ºF and temp drops 40ºF then you are at 42ºF.. Too Cold!


By latewood_ILGM on 7 March 2017

IN the same expanation it is made clear that it is not advisable to allow temps to drop more than 18f. Common sense dictates that this is probably a typo. It is a typo. I went through the article, found your issue, and reported it to the office for editing. Thanks for sharing. Peace, lw


By Dendo on 19 March 2017

What is your take on high DO (dissolved oxygen)? Have you used high DO in the cloning stage? Thanks.

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