Perfect Grow Room Temperature (Ideal Conditions)

rows of plants in a large outdoor greenhouse

So, you got your license to grow your own medicine, and you’re desperate to buy some seeds, shove them in pots, and wait a few weeks to have your sweet smoke.

Let me stop you there.

Getting the climate right in a grow room is probably one of the most overlooked parts when starting to grow your own weed.

Keep reading because this article will discuss the importance of monitoring the climate in your grow room and how you can control these diverse factors.

Cannabis Grow Room Temperature

Grow room temps greatly impact how well your cannabis plant grows. The rate of photosynthesis (the process plants use to convert light into carbohydrates and sugars) is affected by temperature, so if your indoor grow room is too hot or too cold, there will be a negative impact on the potential growth of your plants.

Simultaneously the amount of light available changes the ideal temperature of the grow room. If you are using a high watt/square foot ratio, then optimum photosynthesis happens at the higher end of the temperature range:

  • strong light + low temps → slow growth & short stems
  • medium light + high temps → stretched stems

What’s the Best Temperature for Your Grow Room?

Like every other living being on earth, cannabis plants are different from one another. Some strains thrive with lower temps, while others may tolerate much higher temperatures.

For example, we know that sativas do well in warm climates, while indicas lean to the cooler side of the temperature range.

There’s a consensus on the temperature range cannabis must stay in to survive. However, the ideal temp depends on the development stage of the plant.

Ideal Grow Room Temperature (Lights On)

thermometer in grow room next to marijuana plant

Seeds and Seedlings: 20 – 25°C (max 26°C)/ 70 – 78°F

Jorge Cervantes suggests that the ideal temperature for cannabis seed germination is 25°C (77°F). Too cool a temperature can delay germination, and too high a temperature can upset the seed chemistry, and the emerging sprout (radicle) dry out and die off.

Once the cotyledon (the seed leaves) emerge, drop the temperature by a few degrees as high temps at this stage are associated with stretching and set the plant to develop long node spacing.

Low temperatures in this stage also help maintain a high relative humidity level, which is essential to help your plants thrive. At this point, they haven’t developed a root system, so they basically rely on their leaves’ transpiration to survive.

Vegetative growth: 26 – 28°C (max 30°C)/75 – 86°F

As your cannabis plants grow, increase the grow room temperature.


Because low temperatures reduce the rate at which water evaporates from the leaves.

Picture this: You need to drink a gallon of water packed with other nutrients. As soon as you sweat, you replenish water and nutrients by drinking some more water. However, it’s cold and you’re barely sweating, so you’re not drinking any more water until you feel you need to restock.

Although comparing humans to cannabis plants is a long shot, this example helps you understand why you need to increase temps during the vegetative stage.

That’s why lower temps mean a slower rate of nutrient solution movement, which, in turn, lowers the rate of photosynthesis. Additionally, those nutrients the plant never got to use for growth will ultimately dissolve in the soil and make it more acidic. Furthermore, increased soil acidity reduces root efficiency, leading to diminished water/nutrient uptake and declining growth.

However, take the heat up above 30°C, and you risk a whole new set of problems. Apart from increased node spacing,  transpiration rates will increase disproportionately along with the rate of photosynthesis. This will cause the water to be moved out of the plant, leaving unused nutrients behind and eventually leading to over-fertilization problems.

Flowering: 23 – 25°C (max 28°C) / 71 – 82°F

If you’ve been running a hot grow room during the veg stage, it’s crucial to bring the temperature down to below 28°C during flowering because:

  1. Bud formation  tends to be loose and insubstantial
  2. Bud growth  is slower
  3. Terpenes evaporate, resulting in diminished flavor and smell

During the last 2 weeks of flowering, reduce night temps to 17-19°C (62-66°F). If growing a purple or blue strain, this will help bring out the color by triggering the production of anthocyanin.

Nighttime Temperatures

There can be as much as 10°F (12°C)  difference between day and night temperatures without adverse effects. If your nighttime temps are more than 10°F lower than daytime ones, it can lead to:

  • slow growth
  • stretching
  • delayed flower ripening
  • molds and bud rot:

Why bud rot?

Because the warm/hot environment created by HID grow lights retains water released by plants happily transpiring.

When the lights go out, if the temperature drops too low, this moisture condenses as dew around the plants, possibly leading to mold growth.

Running a dehumidifier during the night can help this problem. However, I find it easier to avoid temperatures dropping below 12°C.

Grow Room Temperatures and Node Spacing

Temperature affects the rate at which cannabis plants grow, but it also affects their overall growth and development.

Understanding Internodal Spacing and Bud Formation

Cannabis flowers start as fine white hairs (pistils) that sprout out at the stem and branch nodesCalyx, the tear-shaped nodules containing high concentrations of trichomes, form at the base of the pistils. 

As these swell and grow, the individual calyx sites join together to form a cola, also known as a “bud.”

However, if the internodal space (the gap where sets of leaves emerge) is too large, the calyx cannot join up and you end up with a lot of small buds and lower yields.

Relationship Between Temperature and Internodal Spacing

Your grow room temp affects the internodal length. More specifically, the difference between day and nighttime temperatures affects how the plant grows.

  • The closer your nighttime temps are to daytime, the shorter the internodal distance will be.
  • If there is a big difference between daytime and nighttime temps, the plants will develop longer gaps between nodes.

  Controlling internodal spacing through temperature management is most effective during pre-flower stretch. If you raise nighttime temps to almost match daytime ones during the accelerated growth spurt that occurs once you put the plants onto 12/12, you will achieve short internodal gaps, which translate into bigger buds and higher yields.

After your plants have been in flower for 2-3 weeks, reduce nighttime temperatures back to normal to avoid stressing the plants. 

What’s the “Cold-Air Dump”?

Scientists have discovered that most of the stem elongation caused by daytime temperatures being greater than nighttime ones (known as +DIF) happens during the first few hours of daylight.

Therefore, you can achieve short compact plants with close internodal spacing by ensuring that temperatures during the first 2 hours of lights are lower than average nighttime temperatures. 

Use a free-standing air-conditioning unit and a timer to create this cold air dump effect.

What Happens if Your Grow Room Temp is Too Low?

Considering what we learned about low temperatures, transpiration, and slow nutrient movement, we know that plant metabolism slows down below 15°C (60°F), and growth stops. Once it falls below 4°C (40°F), plant tissue damage occurs, and it can take several days of warmer temps to repair the damage.

If this happens during flowering, your plants may never mature properly. Generally speaking, if you are using HID grow lamps, these will raise daytime temps to an acceptable level when they are on. However, at nighttime (or daytime if you are using LEDs), your grow room can get too cold. Fortunately, there are simple ways to fix this.

What to Do if Your Grow Room is Too Cold?

  1. Don’t draw your intake air from outside during the winter months
  2. Combat low root temperatures (and believe me, these are more important than room temps) by lifting pots off the floor. An overturned garland tray, or even bread baskets, lift the pots off a cold floor and add insulation. (They work the same way for hot floors growing outside)
  3. Reduce the amount of time your extractor (exhaust fan) is running. Use a timer or speed controller to manage its “on” time so that its main purpose is air exchange rather than heat extraction.
  4. Warm your nutrient solution using a submersible heater.
  5. Add heat using a radiator. Avoid electric fan heaters, which can cause leaf burning, and opt instead for gentle warmth.

What Happens if Your Grow Room Temperatures Are Too High?

Once temperatures rise above 30°C (86°F), performance levels decrease, and by 34°C, cannabis plants really start to struggle. This is because they prioritize transpiration above photosynthesis, and water is moved up through the plant more quickly to cool down the leaves through evaporation.

It’s like you trying to drink not one but two gallons of water and ending up with a soaked, ill stomach.

The plants draw water and nutes disproportionally to the amount of nutrients used for growth, causing nutrient burn and soil acidity. Go above 35°C, and the tiny stomata on the leaves close, plants stop taking in CO2, photosynthesis stops, and the plants deplete stored carbon.

You don’t even have to go above 30°C to damage your plants. The effects of a consistently hot grow room are:

  • spindly long stems
  • loose buds
  • low trichome production
  • increased risk of pest invasion
  • wilting
  • increased likelihood of mildew (especially if also humid)
  • fewer terpenes (lower flavor and smell)

Fortunately, again, you can take a set of preventive and mitigating measures to avoid killing your plants.

What to Do if Your Grow Room is Too Hot?

Dimmable Digital Ballast

HID (High Density) Grow Lamps are the biggest cause of heat in the grow room. Investing in a dimmable digital ballast gives you the ability to control the amount of light produced (250w, 400w, 600w). If your plants are young, using the 250w capacity will reduce the amount of heat (and power consumed).

vivosun 1000Watt grow lamp

Raise the Lights

If it’s a sudden hot spell, raise the lights higher from the plant canopy for a few days to reduce heat levels at plant height. The effects of less light will be better than those resulting from excess heat.

Turn Off Lights

Set your lights to come on at night, ideally after 10 pm, as the ambient temperatures begin to cool down.

Cool-tube Reflector

A cool-tube reflector, uses ducting to draw excess heat out of the grow area.

You can refer to this guide to learn everything you need to know about using a cool-tube reflector.

Oscillating Fans

Make sure you have plenty of oscillating fans running to prevent hot-spot build-ups.

Exhaust Fan

Use a high-capacity extractor (exhaust fan) to remove as much heat from your grow room as possible. Use a thermostatic fan speed controller to switch on the exhaust fan as soon as the temperature gets too high.

Inlet Fan

black inlet fan

Invest in an inlet fan to draw cooler air in instead of relying on passive intake. Remember to pull air in at ground level and remove hot air as high as possible.

Air Conditioning Unit

If you are still battling heat issues, then it’s time to buy an air conditioning unit. Remember that it is easier to cool a smaller area than a large one: use a “control room” – this is a small space with the aircon in (or a heater in winter).

LED Grow Lights

Finally, if you usually have hot summers, think seriously about getting LED grow lights. The initial investment is totally worth it compared to the cost of higher-performance exhaust fans, air-conditioning units and increased power bills. And it will allow you to grow 12 months of the year.

Other Tips to Deal with High Temps in Your Grow Room

  1. Straighten all ducting so that your ventilation system runs as efficiently as possible. If bends are unavoidable use metal connectors.
  2. Bulbs won’t be the only cause of heat.  Move ballasts and all other equipment to outside your grow room.
  3. Water temperature is more important than grow room temperature with hydroponically grown plants. And water temps will rise if your nutrient tank is sited in the grow tent. Check EC and PH more regularly, insulate the tank, and consider investing in a chiller.
  4. As the grow room gets warmer, relative humidity will fall – use a humidifier to add moisture to the air and keep leaf stomata open. Increased humidity will have the effect of reducing temperatures by as much as 2-3°C
  5. Pests thrive in warm grow rooms, so check for them frequently.

Wrapping Up

Replicating the ideal climate for your cannabis plants to thrive inside a new grow room setup can be a delicate task. However, enjoying a nicely-grown smoke produces a superb pleasure.

If you’re about to start your journey as a cannabis growers or are barely taking your first steps, I believe you could use this comprehensive guide on how to grow cannabis indoors

Happy growing!

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