Why getting climate right is key to a successful grow room: Temperature

If you want to achieve awesome results growing cannabis indoors, getting climate right is crucial.

Its probably the most overlooked part of running a grow room, but creating the correct climate for your cannabis plants will make the difference between a so-so grow and an impressive yield.  

What do we mean by “climate”?

The dictionary definition of climate refers to the prevailing weather conditions of an area, e.g. “a Mediterranean climate”. When you’re growing cannabis indoors, you’re aiming to create a perfect micro-climate for your plants. In this context, correct climate control means getting the “environmentals” right: these are all the factors which combine to create the especial climate within your grow room:

  • Temperature
  • Humidity
  • Level of CO2 through air flow
  • Air movement

We’ll look at each of these envirnmental factors in turn, listing correct ranges dependent on plant development and the things you can do to give your plants this. And finally we’ll explain why getting climate right is crucial to a successful grow.  


Grow room temps have a huge impact on how well your cannabis plant grows. The rate of photosynthesis (the process plants use to convert light into carbohydrates and sugars) is effected 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 my grow room?

As with all the environmental factors, the “best” temperature fluctuates depending on the development stage of the plant.

Ideal grow room temps: lights on

(20) 21- 23 (→ 26C) / 70 – 78F

Jorge Cervantes suggests that the ideal temperature for cannabis seed germination is 25C (77F). 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 cotyleden (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.

(24) 26 – 28 (→30C) / 75 – 86F

As your cannabis plants grow, increase the grow room temperature. Why? Because low temperatures reduce the rate at which water evaporates from the leaves. Imagine a suction system where as water exits the plant through the leaves, the empty space created by this action is immediately filled with more water (and nutrients) drawn up from the roots through the stem. Lower temps means a slower rate of nutrient solution movement, which in turn lowers the rate of photosynthesis. In addition to this nutrients which could have been used by the plant for growth that didn’t happen, ultimately dissolve in the soil making it more acidic. Increased soil acidity reduces root efficiency leading to diminishing water/nutrient uptake and declining growth.

Take the heat up above 30C however and you risk a whole new set of problems: apart from increased node spacing, transpiration rates increase disproportionately with the rate of photosynthesis and water is moved out of the plant leaving unused nutrients and the likelihood of over-fertilzation problems.

(21) 23 – 25 (→28C) / 71 – 82F

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

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

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

Nighttime temperatures:

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

  • slow growth
  • stretching
  • delayed flower ripening
  • molds and bud rot:
    • 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. A dehumidifer run during the night can help this problem, but its probably easier to avoid it happening.

Grow room temperatures and node spacing:

Temperature doesn’t only effect the rate at which cannabis plants grow, temperature effects how they grow.

Understanding internodal spacing and bud formation:

Cannabis flowers start as fine white hairs (pistils) which 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 – or what is normally described as “bud”. If the internodal space (the gap between 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.

.Temperature has a direct bearing on internodal length:

or more specifically: the difference between day and nightime temperatures effects how the plant grows.

  • The closer your nightime temps are to daytime, the shorter the internodal distance will be.
  • If there is a big difference between daytime and nightime 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 nightime 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 and in the end, 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.  

The “Cold-Air Dump”

Scientists have discovered that most of the stem elongation caused by daytime temperatures being greater than nightime ones (known as +DIF) happens during the first few hours of daylight. You can achieve short compact plants with close internodal spacing by ensuring that temperatures during the first 2 hours of lights on are lower than average nighttime temperatures. Use a free-standing air-conditioning unit and a timer to create this cold air dump effect.

What do you do if your grow room temperatures are too cold?

Below 15C (60F) plant metabolism slows down, and growth stops. Once it falls below 4C (40F), 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 day time temps to an acceptable level when they are on. At nighttime, or if you are using LEDs, your grow room can get too cold. We’ve explained above what happens to a plant if the temperature is too low. Fortunately its a problem which is easily fixed.

  1. Don’t draw your intake air from outside during 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 a gentle warmth.

Read our review of the best heater for your grow room

Grow room temperatures too high?

Once temperatures rise above 30C (86F), performance levels decrease, and by 34C cannabis plants really start to struggle. They prioritize transpiration above photosynthesis, and water is moved up through the plant more quickly in order to cool down the leaves through evaporation. This happends inproportinately to the amount of nutrients used for growth, causing nute burn and soil acidity. Above 35C and the tiny stomata on the leaves close, and plants stop taking in CO2, photosynthesis stops and the plants deplete stored carbon. Even if you don’t hit the high end temps, 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)
  • less terpenes (lower flavour and smell)

What to do if your grow room is too hot:

  1. HID (High Density) Grow Lamps are the biggest cause of heat in the grow room. Investing in a dimmable digital ballast like the one on the left, 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)
  2. A cool-tube reflector, (shown) uses ducting to draw excess heat out of the grow area.
  3. If its 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.
  4. Set your lights to come on at night, ideally after 10pm as the ambient temperatures begin to cool down.
  5. Make sure that you have plenty of oscillating fans running to prevent hot-spot build-ups.
  6. 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 temperature gets too high.
  7. 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.
  8. Straighten all ducting so that your ventilation system runs as efficiently as possible. If bends are unavoidable use metal connectors.
  9. Bulbs won’t be the only cause of heat. Move ballasts and all other equipment to outside your grow room.
  10. 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).
  11. With hydroponically grown plants, water temperature is more important than grow room temperature. And if your nutrient tank is sited in the grow tent, water temps will rise. Check EC and PH more regularly, insulate the tank and consider investing in a chiller. Read about the importance of water temperature here.
  12. As the grow room gets warmer, relative humidity will fall – use a humidifer to add moisture to the air and keep leaf stamata open. Increased humidity will have the effect of reducing temperatures by as much as 2-3C
  13. Pests thrive in warm grow rooms, so check for them frequently.
  14. Think seriously about LED Grow Lights if you usually have hot summers. The initial investment can be offset against the cost of higher performance exhaust fans, air con units, increased power bills – and will allow you to grow 12 months of the year.