Temperature and relative humidity are crucial when growing, drying, and curing cannabis. Click here to learn all about temperature and humidity for weed. Climate is key to cannabis growing. In this post we analyse the importance of relative air humidity at every growing stage. We’ll define the ideal hygrometric level …
Best temperature and relative humidity for growing cannabis
From germinating your seeds right on through to curing, temperature and humidity have a huge impact on the cannabis plant, and properly managing these conditions can make or break a harvest. Find out what you need to do to keep these conditions optimal during each stage of a cannabis grow, so you can secure a huge bounty of pristine buds.
- UNDERSTANDING RELATIVE HUMIDITY AND TEMPERATURE
- HOW TO MEASURE RH AND TEMPERATURE WHEN GROWING CANNABIS
- OPTIMAL HUMIDITY AND TEMPERATURE FOR GROWING CANNABIS
- HOW TO ADJUST HUMIDITY LEVEL AND TEMPERATURE
- BEST HUMIDITY LEVELS AND TEMPERATURES FOR DRYING AND CURING
Temperature and relative humidity are crucial measures when growing weed. And while cannabis is renowned for being a hardy plant, prolonged exposure to subpar temperature and humidity levels can cause stress, attract pests and pathogens, and even kill plants altogether. In this article, we’ll teach you all there is to know about the optimal temperature and humidity conditions for cannabis at all stages of your grow, from seedling through to harvest—and beyond!
UNDERSTANDING RELATIVE HUMIDITY AND TEMPERATURE
Whenever we talk about humidity in relation to cannabis cultivation, we’re referring to “relative humidity”. Relative humidity is a measure of how much water vapor is present in the environment relevant to the temperature. This might seem trivial, but it’s actually very important; since warm air can naturally hold more vapor than cold air, a humidity reading of 60% at 20°C isn’t the same as 60% humidity at 30°C, for example.
As an indoor cannabis grower, you need to be armed with both a thermometer and hygrometer, or an all-in-one thermo-hygrometer, to accurately measure temperature and humidity levels. You’ll find a wide variety of digital and analogue devices online or at your local nursery or grow store.
Measuring relative humidity and temperature is crucial for growing happy, healthy cannabis plants, as constant exposure to extreme temperatures and humidity stresses even the toughest weed strains. And excess stress can lead to stunted growth, reduced yields, hermaphroditism, or death of a crop—so it pays to be mindful.
THE PROBLEM WITH EXTREME TEMPERATURE OR HUMIDITY
Extremely hot or cold temperatures can affect many biological processes within plants. At temperatures above 30°C, many of the enzymes involved in photosynthesis start to work less efficiently. This leads to stunted growth, which you’ll either have to compensate for with a longer veg time, or, in the case of autoflowers, settle for smaller plants with reduced yield potential.
Another major concern with high temperatures is pests. Many common cannabis pests (such as spider mites, thrips, whiteflies, and white powdery mildew) love hot temperatures. Warm soil temperatures also affect cannabis’ ability to uptake nutrients, which can lead to nutrient deficiencies or nutrient lockout.
Cold temperatures also affect the enzymes responsible for photosynthesis, and thereby cause stunted growth, and can also create an ideal environment for the growth of certain forms of mould. Botrytis, for example (the fungi that causes bud rot), thrives in cold temperatures.
Just like subpar temperatures can have detrimental effects on cannabis, so can subpar humidity levels. The main concern with high humidity is mould (some moulds thrive in warm, humid climates, while others prefer cold and damp conditions). Both super-humid and super-dry conditions can also attract pests; for example, fungus gnats and spider mites like humid and dry conditions, respectively.
HOW TO MEASURE RH AND TEMPERATURE WHEN GROWING CANNABIS
The simplest, most accurate, and most reliable way to measure the temperature and humidity in your grow room or tent is with the aforementioned thermo-hygrometer. There are many different makes and models on the market, but we recommend the digital models for their ease of use and cheap prices.
Make sure to keep your device out of direct light while in use. Also, temperature and humidity levels can vary greatly above and below the canopy, especially if you’re growing dense indica strains that are particularly effective at trapping warm air and water vapor. Using multiple thermo-hygrometers will help ensure balanced temperature and humidity throughout a large room or tent.
OPTIMAL HUMIDITY AND TEMPERATURE FOR GROWING CANNABIS
The optimal temperature and relative humidity conditions for a cannabis plant depend primarily on the stage of development. Seedlings, for example, like warm, humid conditions, while flowering plants usually prefer cooler nighttime conditions and less humidity. Below, we’ll highlight the optimal temperature and humidity levels for cannabis in the seedling, vegetative, and flowering phases.
SEEDLINGS / CLONES
- Seedlings and clones like temperatures of 20–25°C and RH of 65–70%.
- These warm, humid conditions promote rooting and allow seedlings/clones to uptake water via the leaves until they develop roots.
Cannabis seedlings and clones are very fragile, and stark changes in their growing conditions can quickly lead to stunted growth or even death. As they develop roots, seedlings typically like warm and humid conditions. Most growers keep their seedlings/clones in a dome or seed germinator under cool lights, and maintain high humidity by regularly misting the specimens with a spray bottle.
- Vegetative plants like warmer, less humid conditions.
- We recommend raising the temps in your grow room/tent to 22–28°C and reducing humidity levels gradually until you reach a sweet spot of roughly 60% RH.
Cannabis’ hardy nature shines through best during its vegetative growth phase. While the seedling phase tends to be a bit touch-and-go for most beginner growers, the vegetative phase tends to be a lot easier to manage. Once your plants have established roots and developed a few nodes and true leaves, you should see them shoot up in height, producing plenty of healthy, lush foliage.
- Flowering cannabis plants like cooler temperatures and low humidity.
- Keep temperatures between 20–26°C and RH at 50% during the lights-on period. If possible, drop temperatures by roughly 5–8°C during lights off.
The flowering stage is super exciting, but can prove challenging for inexperienced growers. Some strains can stretch considerably as they prepare to bloom, and changes in nutrients, lighting, temperature, and humidity conditions should be implemented gradually to avoid shocking plants.
- During the final 2 weeks of flowering, we recommend dropping the temperature to 20–23°C during lights on and 18–20°C during lights off, and reducing RH to 30%.
If you got excited when you first saw your ladies start developing flowers, wait until you see them as they approach harvest. Modern cannabis strains have been specially bred to produce big, dense, and super resinous buds. To accommodate the dense structure of these flowers, it’s important to keep humidity levels in your room/tent as low as possible. Colder temperatures, especially during lights off, can promote more resin production and also bring out vibrant colours in some strains.
HOW TO ADJUST HUMIDITY LEVEL AND TEMPERATURE
You now know what temperature and humidity levels are best for cannabis during the different stages of its life cycle. But what are the best and most efficient ways to adjust these metrics and create the ideal environment for your plants?
In what follows, we outline some of the best ways to raise and lower the temperature and RH in your tent or grow room.
HOW TO INCREASE HUMIDITY LEVELS FOR CANNABIS
- Invest in a humidifier.
- Keep open containers of water placed around the grow room.
- Regularly mist the air in your grow room with a spray bottle.
HOW TO INCREASE TEMPERATURES FOR CANNABIS
- Use a high-watt HPS grow light (the more watts, the more heat it will produce).
- Place a heat mat on the floor of your grow room/tent.
- Invest in a small space heater with a thermostat that will automatically turn off once the ideal temperature in the room is reached.
HOW TO LOWER HUMIDITY LEVELS FOR CANNABIS
- Invest in a dehumidifier.
- Increase airflow around the room using oscillating fans.
- Invest in more efficient intake fans to increase the amount of fresh air drawn into the room/tent.
- Water your plants before lights on so the soil dries out faster under the head of the lights.
HOW TO LOWER TEMPERATURES FOR CANNABIS
- Increase airflow into and around the room/tent using oscillating fans.
- Run your lights during the night and keep them off during the day.
- Use a cool tube when growing under HPS lights, or switch to LEDs if possible.
- Invest in an air conditioner (which will also reduce the RH in your room).
BEST HUMIDITY LEVELS AND TEMPERATURES FOR DRYING AND CURING
Monitoring humidity and temperature is just as important during drying and curing as it is when growing cannabis. While every grower has their own approach to drying and curing, below are the standard temperature/RH levels used during these steps.
- First 3 days: 20°C and 50% RH.
- From day 3 onward: 17–18°C and 60% RH.
- Keep a fan on somewhere in your drying room to keep air circulating.
- Keep temperatures at roughly 20°C.
- The ideal RH for curing cannabis is 59–63%. Remember to “burp” your jars regularly (3–5 times a week) during the first weeks of curing to keep humidity levels optimal.
- Invest in humidity packs for your curing jars to maintain optimal levels in the long term.
From the time you germinate your seeds to the day you harvest, trim, and dry your buds, temperature and humidity have a direct impact on the health of your plants and the quality of your harvest. In this article, we’ve done our best to summarise optimal temperature and humidity levels for cannabis at all stages of the plant’s life, so make sure to keep it handy as you hone your skills in the grow room.
Steven is a long-time veteran of cannabis journalism, having delved into every aspect of the subject. His particular interests lie in cannabis culture, the emerging science of cannabis, and how it is shaping the legal landscape across the globe.
What is the optimum humidity level for each stage of cannabis growing?
In order to start growing in the best possible conditions, it is adamant to understand the meaning of this technical term. Relative air humidity levels give us information on the concentration of water vapour in the air.
It is important to realise that air humidity is closely connected to air temperature, as hot air is more humid than cold air.
That’s why an air conditioning unit dehumidifies and cools the air at the same time.
When air humidity levels come close to 100%, the air is not capable of retaining the excess moisture, which leads to condensation in the shape of small droplets of fog, morning dew, or rain.
For instance, if the temperature of your grow space is 30ºC, with a relative humidity of around 33-35%, and the temperature suddenly plummets to 10-12ºC, the humidity level can quickly reach 100%.
This can become a critical situation at the end of the flowering period if the ‘dew point‘ is reached. This is a parameter that reflects the cannabis flowers’ humidity level. Bad management of the relative humidity levels of your grow space, caused by abrupt temperature variations, could lead to the appearance of mould in your buds.
Air circulation for cannabis growing
So what is the most favourable humidity level for each growing stage?
Here we show you the ideal humidity levels for every growing phase of your cannabis grow.
Germination / beginning of the cuttings’ rooting period
It is really important to maintain a relative humidity of 80-90% in your mini greenhouse during this phase. This applies to seeds that have just sprouted and also to young seedlings (0-10 days old). These still don’t have a strong radicular system and are therefore pretty fragile.
They require high moisture levels so they can concentrate all their resources on the development of a strong and vigorous root mass. The correct day temperature should be 25-26ºC, and 21-22ºC at night.
Young seedlings and 10-day-old clones
At this point, the required hygrometric conditions should range between 70 and 80%, with similar temperatures to those in the initial stage.
The moisture level for this phase needs to be quite high as the root mass is not very vigorous and actually non-existant in the case of the young seedlings. That’s why it’s necessary to point all technical parameters towards this objective.
It is essential to maintain the correct relative humidity as cuttings will start to develop their first roots between 12 and 20 days after having been planted in the growing medium. This is a crucial point for the appropriate evolution of these young plants.
At this stage the young seedlings, derived from seeds or cuttings, will have developed a radicular mass that is significant enough for them to feed through and therefore reach their maximum potential before being transplanted to a new pot for further development.
Establishing new climate conditions at this stage is vital as the aim is to achieve a reasonably high humidity level, but slightly lower than at the beginning. A good balance should range between 60 and 70%.
Temperatures must be kept between 22-28ºC during the day and 18-22ºC at night. These conditions guarantee the plants’ well-being and facilitate good metabolic function, as well as overall optimum development.
At this point, the radicular system is strong enough for the plant to absorb a higher amount of nutrients and develop a much more resistant immune system.
At the start of the flowering period, when the ‘stretching’ or the final growth boost occurs, plants need specific climate conditions for this transitional phase.
As this is still a transition period in which plants progressively end the veg phase, the humidity level needs to be set at 50-60%. It is also essential that the temperature remains between 20-26ºC both day and night.
This prevents thermal shock and guarantees that plants will be ready to produce those delicious flowers. It is vital that their well-being is taken care of so they can concentrate all their resources on their final objective: quality and quantity. These measures are valid for the first 3 to 4 weeks of flowering, depending on the strain.
In the second part of the flowering phase the humidity level must be reduced to 40-50%. In the two to three final weeks it’s preferable for this figure to be closer to 40%.
This is absolutely essential, particularly if you’re growing strains that are sensitive to mould or that produce dense flowers.
If you follow our guidelines, chances of botrytis hitting your flowers will be slim. As far as temperature goes, we advise you to stick to 18-24ºC during the day and 17-20ºC at night.
This way you’ll be able to control your flowers’ dew point, which is crucial in order to prevent fungi and avoid ruining all your hard work.
Brian Worms After completing my studies in International Trade and Marketing in France, I’m more interested in the world of communication today. Blogger, grower and real cannabis toker for more than 20 years, always looking into the rarest and finest cannabis genetics. My passion for cannabis is so deep that it’s become something of a lifestyle now… My work is like a dream come true.
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