Temperature and relative humidity are crucial when growing, drying, and curing cannabis. Click here to learn all about temperature and humidity for weed. How to measure and adjust humidity levels in your marijuana grow room? A guide with a schedule for the best humidity levels for your grow. Finding ways to control humidity and temperatures is crucial when growing cannabis indoors. This blog shows practical steps for best results.
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.
We are going to take some time to delve into some much-needed information to further your understanding of the importance of humidity for seedlings when growing marijuana plants.
What is humidity
Humidity is expressed as the quantity of water vapor in the air. Humidity plays a necessary role in influencing how much evaporation occurs in cannabis plants.
Your plant will take in more nutrition and water when the humidity is low.
If for any reason the evaporation strain gets to be too much, marijuana plants will guard themselves against scarcity by closing their stomata. So naturally, this will stunt the growth of your plant due to the lack of water absorption.
This is why you must have the proper humidity levels in your marijuana factory. When the plant is in the growth stage, it will need high humidity, unlike the blooming phase.
This is mainly because the roots of baby plants are much smaller. Measure the humidity using a hygrometer. In the first stages of growing your plant, the humidity can be about 70%, and you can reduce it by 5 every week until it gets to 40%. Included in this article is a helpful schedule to assist you with the right information on humidity levels for both indoor and outdoor growing.
Relation to temperature
Before we go any further, we have to discuss the connection between temperature and humidity. First you must understand that the percentage of absorbable water is determined by the temperature. At the temperature of 68 degrees Fahrenheit, air can take in up to 7.2 ml of water. In this case, air has a corresponding humidity of 100%.
With temperatures and humidity such as that you are guaranteed to see absolutely nothing because the water vapor is very high. At 32 degrees Fahrenheit, the air will take in around 5 ml of water. For this reason, air is much drier during the winter time than the summer months.
‘The hotter it is, the more moisture the air could hold at the same temperature.’
Since you will be airing out your grow room quite frequently, you will end up releasing the humidity as well. So you have to be sure that your marijuana factory is always moist so the humidity level goes up. Later in the article you will be informed of options for adjusting the humidity levels.
At first, your marijuana cuttings will have very small roots so they are not able to take in much water yet. At this stage, you want them to evaporate a very small amount of water. Cutting will only evaporate a small amount of water when the humidity is high, and require fewer roots.
Another way to reduce the amount of evaporation is chopping the bigger leaves halfway from the cutting. A humidity of 70% is exactly what you want when putting the cuttings in your marijuana factory.
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Your cuttings will begin growing roots in a clone box when the temperature is around 71.5 degrees
Fahrenheit in a very humid environment and fluorescent lighting.
I suggest the cuttings go beneath a 600-watt HPS lamp in an environment with 30% humidity for good results.
For as long as the 2 nd or 3 rd week of blooming the roots are going to grow the size of the actual plant. The roots must be durable for proper water and nutrient absorption.
If you’ve produced a bountiful harvest with your desired attributes, cloning these marijuana plants is the next best thing to do so you can recreate the same results.
Seedlings require a separate set of rules since they have a taproot at germination that takes in moisture and grows fairly quickly. Never clip the leaves of a seedling. The leaves are necessary for absorbing light and evaporating water.
To help the seedlings take in water and nutrition through its leaves, have the humidity at high levels. Begin at 60% and slowly work your way down to 40%. Click this link to buy marijuana seeds
As your plant start flowering, you can gradually decrease the humidity level.
At this stage the roots are very mature, so the plant is able to take in the most nutrients and water. You will want to reduce the humidity level in the flowering stage since mold has a tendency to flourish in high humidity environments.
With age, there is a higher chance of your marijuana plant getting mold.
As a result of the high humidity, water gathers around the tops and that when mold has the opportunity to develop. Bud rot is the most frequent type of mold.
Levels of humidity in cannabis plants
Once the plant is fully grown you will need to start thinking about harvest time. Our free little Harvest Guide will help you determine the best moment to cut your plants.
Follow this schedule to ensure you give your plants the right amount of humidity for optimum growth. Notice there is a difference in clones and seedlings.
Clones humidity level
|Growth Week 1||70%|
|Growth Week 2||70%|
|Flowering Week 1||65%|
|Flowering Week 2||60%|
|Flowering Week 3||55%|
|Flowering Week 4||50%|
|Flowering Week 5||50%|
|Flowering Week 6||45%|
|Flowering Week 7||45%|
|Flowering Week 8||40%|
|Flowering Week 9||40%|
Check this complete Marijuana Grow Schedule with temperatures, humidity, watering and nutrients tips.
Seedlings humidity level
|Growth Week 1||60%|
|Growth Week 2||60%|
|Flowering Week 1||55%|
|Flowering Week 2||50%|
|Flowering Week 3||50%|
|Flowering Week 4||50%|
|Flowering Week 5||50%|
|Flowering Week 6||45%|
|Flowering Week 7||45%|
|Flowering Week 8||40%|
|Flowering Week 9||40%|
How to raise the humidity in your grow room
So, you have a few options for raising the humidity in your grow room. For starters, you can try spraying water on the walls and flower.
Also, try putting the lights further up so the temperature will go down a little near the plants. This way you won’t have to turn on the extractor fan as often. If you put bottles or buckets of water around the grow room, or even wet towels, this will raise the humidity level.
How to decrease the humidity
Dehumidifiers are most certainly your best option. It has the capability of taking moisture from the air and draining it or holding it in a reservoir. Make sure to get a larger humidifier because the smaller ones fill up quickly.
Once your marijuana plant starts flowering the humidity will need to be decreased, so it is necessary to dehumidify. Try using the extractor fan on a higher setting than usual or even you can even shoot cold air into the room.
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Keep in mind that the humidity outside may influence the humidity in your marijuana factory. For instance, if on a rainy day the humidity increases quickly, you can shut off the extractor fan or turn it to a much lower setting. Make sure the temperature doesn’t rise too quickly because you’re getting less cold air from outside.
How to measure humidity levels
Hygrometers are used to measure humidity levels (wiki on hygrometer at this link). All you have to do is put it over the plants, so long as it is in an area that easily aired out. The analog hygrometers cost $5 and you can buy a digital one for $10.
The higher priced hygrometers are usually of better quality. For ease of use, get the one that has a wire attached to it. This makes it easier to see the humidity without having to turn the lights on in the room. These devices have a built-in storage bank that tracks the highest and lowest values, to help you determine how well you stayed within the preferred range. Buy the best hygrometers at this link.
Watering your plants causes the humidity to go up quite a bit, which is fine when in the growth stage because you just spray the floor and walls to raise the humidity. During the flowering stage, humidity levels often go up way too much after you water your plants.
Once the lights are no longer on in the grow room, the temperature decreases and extracting a lot of hot air is unnecessary, which usually is the cause of humidity increases.
Nourish your plant with water once the lights are on, that way they will evaporate most of the water throughout the day. Understand that spraying water on top of the buds is pointless because you will raise the possibilities of the humidity rising and leading to mold.
Humidity is much less of an issue when growing your cannabis plants outdoors. The springtime and the start of summer offer higher humidity which works just fine since there are no buds present on the plant for moisture to gather. Any moisture on the plant from morning dew will easily evaporate throughout the day.
Once Summertime ends, the flowering stage starts and there is a shift in the climate, resulting in cooler days and more rainfall. This usually makes the humidity higher. In the cooler months, the morning dew could be an issue since the sun is not guaranteed to come out and the temperatures are sometimes too low to evaporate it.
Luckily a tad bit of rain will not cause the buds to rot, but just in case you may want to be on the lookout for this. In the last of the blooming stage, it might be a good idea to brush the dew off your marijuana plants in the mornings. If you notice that rain is expected, go ahead on move your plants to a location where they will be kept dry, away from the rain. It is always best to take the necessary precautions.
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Indoor Cannabis Growing: Relative Humidity and Temperatures
The most refined techniques to grow cannabis become irrelevant when relative humidity and temperatures are not being controlled – learn more about these two major factors.
Cannabis cultivation, cannabis history, cannabis culture
- How temperatures and humidity levels interact
- Humidity levels and temperatures: from seedling to harvest
- Seedling stage
- Vegetation period
- Flowering period
- Late flowering (1-2 weeks before harvest)
- A. ways to lower humidity
- B. ways to lower temperatures
- C. raising humidity
- D. raising temperatures
Final results of an indoor grow are greatly influenced by the way growers keep in control of parameters that influence their plants growth. There are two basic factors that can easily be forgotten when we’re busy thinking of other ways to increase yields, size, and overall health of our plants – temperature & relative humidity. This blog summarizes ways to keep both of these factors within an optimum range, and provides specific information what conditions should be maintained to achieve best results.
HOW TEMPERATURES AND HUMIDITY LEVELS INTERACT
It’s important to know that humidity levels and temperatures are closely related to one another. When we talk about humidity, we usually mean relative humidity (RH), which is the ratio of partial pressure of water vapor to the maximum vapor pressure of water at the same temperature. You get the whole idea when knowing the basic principle that warm air holds more water vapor than cold air. This is one of the reasons why it’s necessary to extract a lot of warm air from our grow room, and ideally allow cool air to enter – warm air simply holds too much water vapor in it.
HUMIDITY LEVELS AND TEMPERATURES: FROM SEEDLING TO HARVEST
We need to define what humidity and temperature control actually means when growing cannabis. It makes sense to divide the life of cannabis plants into 4 different stages in which humidity levels, and temperatures, should be adjusted to ensure healthy growth. Don’t think that humidity and temperature control is complicated and not worth it! It’s generally very easy, and more about keeping parameters within a certain range, and as constant as possible.
The first thing you need to do is to buy a hygrometer and thermometer, preferably a digital one with memory function, also showing maximum and minimum values of the past. Some hygrometers aren’t the most accurate, so don’t bother having several devices in your grow room to compare values. Now that we’re able to closely monitor our conditions, we can get to the essence of humidity and temperature control – the actual humidity levels and temperatures we aim for.
1. Seedling Stage
- Seedlings and clones like high humidity levels of 65-70%
- Reason: The root system is not established
- High humidity levels allow water intake through leaves
- Temperatures with lights on: 20-25 C° (lights off: 4-5 C° lower)
2. Vegetation Period
- Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
- Temperatures can be increased a little bit (no obligation)
- Reason: Roots absorb more water; evaporation through leaves cools plant(s)
- Temperatures with lights on: 22-28 C° (lights off: 4-5 C° lower)
3. Flowering Period
- Humidity levels need to be lowered to 40-50% (extremely important)
- You can get away with 55% (anything over 60% is real bad)
- It’s best to slightly lower temperatures in flowering
- Temperatures with lights on: 20-26 C° (avoid high temperatures)
4. Late flowering (1-2 weeks before harvest)
- The following steps are no necessity, but can improve yield, flavour and appearance
- Bring down humidity levels as much as you can: 30-40%
- Lower daytime temperatures, and also increase the temperature difference (day/night)
- Temperatures with lights on: 18-24 °C (lights off: minus 5-10 C°)
ADJUSTING HUMIDITY LEVELS AND TEMPERATURES
We’ve got a pretty good idea on humidity levels and temperatures we aim for. Now it’s time to get to the practical part, and to find ways to bring things back in balance when they’re not. Most growers will struggle to keep both relative humidity and temperatures down, which is of primary importance in the flowering period – we got that. In some colder regions, and depending on the lighting solution, the opposite scenario might be the case, and temperatures or humidity levels must be raised.
Remember the basic principle that warm air holds more water than cold air? Keep this in mind, and be aware of the fact that relative humidity and temperatures interact with one another.