Cannabis Seed Humidity

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. 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 … Finding ways to control humidity and temperatures is crucial when growing cannabis indoors. This blog shows practical steps for best results.

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.

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Humidity effect cannabis plants throughout their entire life cycle, below explains how it affects them at each stage and what the optimal humidity is.

Cutting/Seedling Stage:

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.

Vegetative Stage:

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.

Flowering Stage:

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.

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.

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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.

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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.

Growing phase

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.

Early flowering

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.

Late flowering

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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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

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  1. How temperatures and humidity levels interact
  2. Humidity levels and temperatures: from seedling to harvest
    1. Seedling stage
    2. Vegetation period
    3. Flowering period
    4. Late flowering (1-2 weeks before harvest)
    1. A. ways to lower humidity
    2. B. ways to lower temperatures
    3. C. raising humidity
    4. 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.


    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.


    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°)


    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.