Cannabis grow room temperature and humidity levels

21/01/2025

The difference in harvest quality can be staggering when a cannabis seed variety has been grown in optimised temperature/humidity conditions vs uncontrolled conditions. Supplying weed seeds to professional & home growers since the 1980’s has allowed Dutch Passion extensive insight into the improvements that result from well controlled temperature & humidity.

Read on for a full explanation as well as expert tips to allow you to take your crops to the next level via superior control of temperature and humidity.

What temperature / humidity does my cannabis plant need?

Ice Cream Haze cannabis seed to harvest grow report by Johnny Green (6)

Temperatures between 20-27ºC / 68-80F are usually preferred to grow cannabis. Aim to avoid lower than 18ºC/64F or higher than 29ºC/84F.

However, relative humidity (RH) needs to be reduced as the plants reach bloom, typically with a maximum 40-50% RH recommendation in late bloom. These low RH levels maximise your chances of avoiding mold/bud rot (Botrytis) as the dense blooms form.

But during the cannabis seedling stage (RH can be 65-70%), and during veg growth (RH 50-70%) higher humidity levels can be tolerated with little adverse effects.

For cuttings (with no significant cannabis root system) even higher RH levels can be used since most of the initial water absorption is done via foliar feeding.

Note: Indica’s or indica-dominant hybrids can usually take cooler temps than sativa/sativa-dominant strains. Indicas benefit from lower humidity. Sativa strains (or sativa dominant hybrids) can often handle higher temps (max 29-32ºC / 84-90F) than indicas, with higher humidity levels compared to indicas.

Cannabis seeds germination temperature and humidity levels

Ideal temperature range

Around 18-24ºC (64-75F). 20-22ºC (68-72F) is ideal.

Ideal humidity range

Germinate cannabis seeds in moist grow medium or cotton pads

Whether you are germinating autoflower seeds, feminised seeds or regular cannabis seeds you need warm moist conditions. Many germinate their cannabis seeds directly in the grow medium. One other proven method that maximises germination rates is to sprout the weed seeds between two damp (but never soaked) cotton pads.

Cannabis seedlings temperature and humidity levels

Ideal temperature range

Around 22-25ºC (72-77F)

Ideal humidity range

65-70% RH

With small root systems that can absorb only modest levels of moisture, it’s important to keep seedling conditions moist and never too dry/hot. Cannabis cuttings, with no root system whatsoever are often grown in heated seedling boxes with heavy moisture levels (sometimes condensing/dripping on the inner surfaces & roof of the seedling tray) to allow foliar water absorption until the cannabis root system is established.

Vegetative cannabis temperature and humidity levels

Ideal temperature range

Around 20-27ºC (68-80F)

Ideal humidity range

40-65% RH

With an established root system that can handle the moisture/nutrient requirements of a larger specimen, plants in the cannabis vegetative growth stage can tolerate higher temperatures than seedlings.

That said, aim to keep temperatures around 22-25ºC, 72-77F with the indoor lights on. And a few degrees cooler during the lights out period.

As the plant grows, with larger amounts of foliage, transpiration rates (natural water evaporation via the leaf stomata) naturally increase. This can mean that RH levels in your grow room may need more attention. In general, maintain an RH of around 40-70%. And start giving some consideration to the lower 40-50% RH levels that will be needed in a month or two as heavy bloom sets in. Increased air flow rates and dehumidifiers are good options.

Early flowering cannabis temperature and humidity levels

Glueberry OG field of green early flowering buds

Ideal temperature range

Around 20-27ºC (68-80F)

Ideal humidity range

45-55% RH

As bloom establishes and buds start to form, most growers aim to keep RH no higher than 60% and preferably a little lower. Many growers reduce grow room temperature a little during bloom, perhaps by a few degrees. Cooler air carries less moisture, which can make it slightly easier to avoid bud-rotting high humidity levels.

Keep temperatures around 22-25ºC, 72-77F with the indoor lights on. many prefer to aim for the lower end of that scale. And as always, the temperatures can be a few degrees cooler during the lights-out period.

Late flowering cannabis temperature and humidity levels

Auto Banana Blaze stunning autoflower cannabis strain with red coating

Ideal temperature range

Around 20-27ºC (68-80F)

Ideal humidity range

40-50% RH

During the cannabis flowering phase the blooms should be ripening and increasing in volume with a good layer of cannabis trichomes coating the buds and a pungent cannabis terpene profile.

Many growers aim for the lower end of the recommended temperature scale and do their best to avoid quality-limiting high temperatures as harvest approaches.

Reducing RH to 40-50% helps minimise the risks of botrytis/mold/bud rot. Using higher air flow rates with fresh, low RH air, helps to keep humidity low. At this stage many growers that struggle with RH use powerful dehumidifiers to control the RH levels.

Note that some growers deliberately introduce chilled air towards the end of bloom to enhance autumnal colours in the buds. One way to help control grow room temperatures, especially for those growing in summer/warm ambient conditions, is to switch their lights on after sunset – as ambient temperatures are dropping.

Air conditioning, though expensive, is increasingly being considered as an unavoidable cost of growing. Especially as heat-wave summer weather conditions become more common. With air-con your grow-room should benefit from stable temperatures even in hot conditions, allowing you to grow all year round without loss of harvest quality.

Although less commonly seen, some people in desert climates can suffer punishingly low RH levels and may need to use a humidifier.

Best temperature and humidity for drying cannabis

Power Plant huge cannabis harvest drying

Ideal temperature range

Around 15-20ºC (59-68F)

Ideal humidity range

50-60% RH

Now that growing is complete you need to dry your buds, but not too rapidly. A slow dry over around 1-2 weeks (wait until the branches are just dry enough to break) allows you to preserve the maximum amount of cannabis terpenes while the buds are drying. For that reason, lower temperatures are preferred. Ideally the conditions shouldn’t be too dry – you don’t want to rush the last stage. An RH of 50-60% is often favoured.

Best temperature and humidity for curing cannabis

Auto Brooklyn Sunrise cannabis buds curing in a jar

Ideal temperature range

Around 18-21ºC (65-70F)

Ideal humidity range

55-60% RH

Many use self-adjusting humidity control sachets from companies such as Boveda or Integra. These general keep the RH levels in your jars to around 60%. Serious growers often insist on the use of these moisture-controlling sachets for each jar of buds.

Curing is a process where some of the chlorophyll-type flavours gradually break down over a period of a couple of months. This accentuates the cannabis’s flavours and enhances the vibrancy of autumnal hues.

Related:
Best ways to dry and cure cannabis

Best temperature and humidity to store cannabis buds

Auto SFV OG THC-rich dry bud grown by Antonio

Ideal temperature range

Room temperature storage in glass jars, around 18-20ºC (64-68F), works fine.

Ideal humidity range

Once cured, RH is less relevant though many connoisseurs always leave a Boveda/Integra 60% RH control sachet in their jars.

If you get through your jars in a few months, then storing them at room temperature is quite normal. Cold storage is recommended for those in very hot climates (or for long storage periods). The key recommendation for bud storage is to avoid hot, humid conditions which will degrade the bud quality faster than cool-stored buds.

Why temperature and humidity levels matter?

Kerosene Krash with light burn symptoms in the buds

A surprising number of growers don’t monitor either temperature or humidity and probably wonder why they struggle to produce consistently good quality harvests.

Even growers that are aware of the temperature range in their grow room are not always aware of their humidity levels at different stages of growth. Or at different times of the year.

Monitoring both temperature and humidity are the hallmarks of good growers. Once you have more information regarding your grow room conditions you can address them. For some growers that means avoiding growing at certain times of the year. For other growers, it’s an opportunity to better control temperature humidity – using tools such as air-con, dehumidifiers etc.

Note that a thermometer and hygrometer are both inexpensive grow room items yet some of the most informative. You can also buy a ‘combined’ thermometer/hygrometer for added convenience.

More advanced/ambitious growers can buy digital thermometers/hygrometers that electronically integrate with their fan (air extraction) system. This allows automatically increased extraction fan speeds during periods of high temperature and/or high humidity.

Temperature or humidity too high

If temperature is too high, your plants may wilt as the plants lose cellular strength.

If humidity is too high, plants struggle to transpire and remove excess moisture, showing symptoms similar to over watering. This slows down feeding/nutrient uptake. Growth & bloom are stunted, risks of mould or bud rot are increased.

Replacing hot grow room air with fresh cool air (increased air flow) can be a great help. Air conditioning and dehumidifiers are the best permanent fix for those that suffer regular, stubborn high ambient temperatures and high RH issues.

Temperature or humidity too low

If temperature is too low, cannabis plants may droop and wilt. Even if they are receiving enough nutrients/water. Like many other plant species, cannabis struggles with persistently low temperatures.

Understanding the factors (temperature/humidity) which control VPD is actually a key part to understanding how your grow room atmosphere affects your plant at different stages of growth.Understanding the factors (temperature/humidity) which control VPD is actually a key part to understanding how your grow room atmosphere affects your plant at different stages of growth. Growth is quickly stunted. For indoor growers more heat needs to be added to the grow room. Sometimes during winter grows, external air can be too cold to vent directly into your grow room.

If humidity is too low, symptoms similar to under watering can occur. Those growing in desert regions can find that permanently low humidity has a harsh drying effect on their plants causing them to wilt as the transpire rapidly. Outdoor growers can use misting machines to combat low humidity. Indoor growers can use humidifiers.

How to monitor the temperature and humidity in your grow room

Whether you check temperature/humidity manually the old-fashioned way or have an integrated environmental control system, you really do need to monitor grow room temp/RH if you are serious about maximising the full genetic potential from your cannabis seeds. And perhaps give some thought to vapour pressure deficit…

Why measuring grow room VPD (vapour pressure deficit) is important

More advanced cannabis growers may be familiar with the useful term Vapour Pressure Deficit (VPD). This measures the difference (or ‘deficit’) between the amount of moisture currently in the air and how much moisture the air could potentially hold when fully saturated.

In simple terms, VPD allows the grower to better understand the drying effect of their grow room atmosphere on the plants.

Leaf Vapor Pressure Deficit Chart from DimLuxLighting

You can determine the VPD in your grow room by first measuring temperature & RH. Then by cross-referencing temperature & RH on a VPD chart you can assess whether you are in the correct VPD range. The correct VPD range varies according to the stage of plant growth, from seedling to bloom.

Understanding the factors (temperature/humidity) which control VPD is actually a key part to understanding how your grow room atmosphere affects your plant at different stages of growth.

For a deeper dive on VPD, the article below is essential reading.

Cannabis grow room temperature and humidity FAQ

Desfran sativa dominant weed led hps grow indoor halfpipe scrog high yield

Remember to minimise stress on your cannabis plants aim to keep them away from temperature/humidity extremes. Just as you would aim to keep your plants in the nutrient and optical sweet spot, your plants will reward you with superior growth and better results if they can focus their biochemical energy on thriving rather than merely surviving.

Snow Bud near harvest at 1400m above sea level in Slovenia

Snow Bud grown at 1400m above sea level in Slovenia

Pamir Gold did great at withstanding the brutal winds and monsoon rains of Colorado

Pamir Gold 450g harvest at 2100m above sea level!

Those growing indoors over summer without air-con often face temperatures around, or just over 30ºC /86F. Indoors, a maximum temperature of around 35ºC/95F is a realistic upper limit (VPD charts can show more about the temperature/RH correlations). Whilst cannabis can tolerate these temperatures it causes stress to the plant. Cannabis survives rather than flourishes at high temperatures. Note that sativa strains may cope better with higher temperatures than indicas.

For outdoor growers it is difficult to avoid or react to extreme heat – they have to trust that the genetics are tough enough. The realistic outdoor upper temperature limit is, depending on how you grow, around 40-45ºC / 104-113F. Protecting your plants from the very hottest midday sun (e.g. with a tarpaulin shade) could be the difference that helps your plants survive extreme heatwaves.

But indoor growers do have a choice. Lights can be switched on during the coolest part of the day, after sunset. Growing can be avoided in the hottest months. Air conditioning could be the game changing investment that makes the difference.

For indoor growers, temperatures below 15-16ºC / 60F should be avoided. Perhaps the only exception to this could be for the more advanced growers that deliberately chill their grow room to enhance autumnal bud colours during late bloom. This is usually done during the ‘lights off’ periods during late bloom, either by reducing air temperature or piling ice around the main stem.

For outdoor growers, cool year-end temperatures near harvest sometimes can’t be avoided. Here the best advice is to buy quality outdoor cannabis seeds that have been selectively bred for extra toughness and resilience. Or buy one of Dutch Passion’s high altitude cannabis seed varieties (Pamir Gold or Snow Bud) that were developed in the Swiss Alps and can withstand a few days of frost.

CO2 (Carbon Dioxide) tends only to be used by more advanced growers. It can allow plants to cope better with the demands of higher temperatures around 30-35ºC (86F).

Supplemental CO2 (often dosed at around 1200 ppm) is also used by experienced growers who grow their cannabis plants under very high light intensities (above 1000-1500 µmol/m²/s PPFD levels).

Such light intensities can be damaging to cannabis if not well monitored, but with supplemental CO2 cannabis plants are able to use the extra light and thrive at the very highest growth rates. Perhaps at growth rates up to 20-30% higher than normal.

Using CO2, you can run your grow room a few degrees warmer. Some studies suggest optimal growth is achieved at around 29ºC (84F) with supplemental CO2.

Note that some recent studies (by Prof. Bruce Bugbee, Utah University) have also shown that higher CO2 levels are beneficial for photosynthesis even when plants are not being pushed hard at higher temperatures and higher light intensities.

For reference, atmospheric CO2 levels are around 370ppm. Household air indoors typically contains 700-1000 PPM CO2. Bedrooms, overnight, can contain 1000-1500ppm+ CO2 levels. Drawing CO2-rich indoor household air into your grow room is a cheap & simple way to boost photosynthesis.

Not necessarily. RH levels of up to 65-70% can be tolerated in veg.

60% RH is certainly a good level for veg growth. Again, for advanced growers the VPD is important, since it’s the combination of temperature and humidity which really matters. A correct VPD for veg growth could be achieved at 75% RH with a temperature of 30ºC/86F. Or it could also be achieved at 55% RH with a temperature of 20ºC/68F. Understanding VPD is a great help to fully comprehend temperature/humidity at each stage of plant development.

Not necessarily. Again, use of VPD charts indicate that an RH of 60% works so long as temperatures are around 33ºC/91F in bloom. Though such high temperatures might favour the use of CO2 to help plants cope.

However, if grow room temperatures are lower, around 23ºC/73F, then RH levels will need also need to be lower (nearer 35% RH) to create the correct VPD.

As usual, note that sativa strains with a more open bloom structure, will tolerate higher RH than indicas.

Optimised results, and the best way to minimise risks of bud rot, is to keep RH levels between 40-50% during bloom when temperatures are around 27ºC. Many expert growers will run dehumidifiers in an attempt to reduce RH.

You spent a long time deciding which cannabis seeds to buy and several months growing them. Don’t compromise a great potential result by ignoring RH levels in bloom.

Usually, it is placed around the same height as the plant canopy, near the centre. This measures the temperatures experienced by the upper blooms.

Often near the canopy level. Buying a combined thermometer/hygrometer is recommended for those wanting a clean, uncluttered, grow room. Place the thermometer/hygrometer in the shade or make a small cardboard cover to prevent direct light reaching the thermometer – which might cause an artificially inflated temperature reading.

Some professional growers will also use a second thermometer/hygrometer nearer the level of the plant containers. This can provide useful background information about root-zone temperatures.