Temperature and relative humidity are crucial when growing, drying, and curing cannabis. Click here to learn all about temperature and humidity for weed. Finding ways to control humidity and temperatures is crucial when growing cannabis indoors. This blog shows practical steps for best results. The humidity of your cannabis grow room plays a large part in how your plants function. The right humidity increases the quality and yield of your grow.
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.
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.
Ideal Humidity Level for Cannabis
The humidity of your cannabis grow room plays a large part in how your plants function. It can effect the growth and final yield as well as having implications in the spread of disease and mildew. Through an understanding of exactly what your plant needs and how to make sure it has it, you will increase the quality of your grow and subsequently the marijuana you end up with. Knowledge is power!
Firstly, what is humidity? Well, humidity refers to the amount of water vapor present in the air (water vapor is simply water in its gas form). It is often referred to as a percentage, for example, a humidity of 75% means that the air currently contains 75% of the total maximum water vapor it can hold at that temperature. Also, as air increases in temperature it becomes capable of holding more water vapor, meaning temperature and humidity is related. As a result the percentage used to measure humidity is referred to as Relative Humidity (RH).
A lot of novice cultivators try to maintain a low humidity; this is a common mistake and is largely down to the idea that high humidity encourages disease amongst plants. Whilst partly true, this “play it safe” attitude can have its own adverse effects on how your cannabis plants grow.
The reason it is so important to your plants is because it will affect the rate at which they transpire. During transpiration plants release water vapor into the air, it functions along the same lines as osmosis – that the water levels inside and outside the plant will try to level out to equilibrium. This means that if you have a low humidity, your plants will rapidly transpire as water is drawn out into the air, reducing the amount of water within the plant and potentially having detrimental effects when water levels reach to low. If humidity is high then plants will transpire at a much slower rate and have a lesser potential for loss. It should be noted that cannabis plants have a “humidity” rating of pretty much 100%, so they will always transpire (which is OK as it is an essential part of its functioning).
So why do plants transpire? Well basically, they do it for a number of very important reasons. It is done for temperature control, it is how plants cool themselves and regulate their own temperature. It is also how the cannabis plants move minerals and nutrients about, as water leaves the plant it draws more up from the roots, allowing for the absorption of nutrients from the soil. Finally it is how your cannabis plants get the carbon dioxide they need out of the air – Plants open their stomata to let water vapor out, and in the process carbon dioxide gets in.
If the humidity is too low, then your cannabis plants are going to do a lot of transpiring and it will play havoc with their transportation systems. They will lose a lot of water and begin to exhibit the damage usually caused by dryness – stunted new leaves, shriveling old leaves and dying flowers.
As mentioned, whilst low humidity is usually the pitfall of most novice growers, it is for good reason – they have heard or read somewhere that high humidity is likely to cause the spread of disease, and they are not far off. High humidity has its own perils. In a grow room with excessive humidity and very little air movement (ventilation) you run the risk of exposing your plants to fungal disease, mildew and root rot. However, it is very easy to avoid, with careful grow room planning and management you should not find yourself in a situation where this happens.
The best way to monitor the RH of your grow room is to use a hygrometer, this should give you an accurate read out of the exact water vapor content of the air.
Humidity effect cannabis plants throughout their entire life cycle, below explains how it affects them at each stage and what the optimal humidity is.
At this stage the ideal RH is 70-80%. This RH will ensure that the seedlings do not put too much of their energy into their transpiration process, as there is not much of an imbalance. This will allow your seedling to focus their energy into root and leaf growth. Your little ones will also need to maintain some level of transpiration in order to draw up nutrients from the growing medium.
A great way to control the humidity of you grow room is with good ventilation, and the use of a humidifier when humidity are beginning to get a bit low. A humidifier is an easy to obtain, cheap bit of equipment that simply introduces more water vapor into the air. When used in conjunction with a hygrometer you should easily be able to keep RH within your cannabis plants’ safe limit.
In this stage the RH can be 50-80%. Now that your plants have entered their main growing phase the RH can be a lot more varied. This is because your plant will now have a much bigger surface area with which to transpire, meaning less stress is put on it through faster transpiration.
However, this does mean your plants can transpire at a much greater rate, be sure to keep an eye on your plants. If the levels of humidity drops below a safe level then your plants will transpire so rapidly that they run the risk of over fertilization from their fast uptake of nutrients drawn in from the soil.
Once your cannabis is flowering you will want to consider dropping the RH greatly. This is in order to reduce the risk of the dreaded rot. You ideally want you grow room to have a relative humidity of 40-50% now.
Keeping track and controlling your grow rooms humidity is important if you want to really get the most out of your plants. It is another important factor that is usually overlooked by less experienced cultivators. Now that you have a better understanding of its impact you should be able to utilize the knowledge to improve the quality of your grow.