What’s the Best Humidity for Growing Cannabis?

We are going to figure out the best humidity for growing cannabis in this article.

When first starting out down the cannabis cultivation path, one thing that might not be immediately obvious is how much humidity control matters. Humidity plays a critical role in the growth and development of your plants, and getting it right can be the difference between hitting your yield expectations, or falling way short when harvest time comes around.

In this guide, we’ll go into detail about the best humidity levels for growing weed, look at all the ‘dos and don’ts’ for both indoor and outdoor growers, and help you understand why humidity is so important for the health and success of your Premium Cultivars.

What impacts relative humidity for weed plants?

There is a true plethora of factors that can influence relative humidity (RH) levels, but the big one is where you are located. The climate of your region should be the starting point for any discussion on humidity levels. Other key factors that can influence RH are temperature, time of day, seasonality, and the type of growing environment you’re using (indoor or outdoor).

Region

For all you legends out there growing your weed outdoors, the local climate will obviously be one of the biggest factors that determine how humid it is where you are. But, there are still a few things you can do to make sure your plants are as happy and productive as possible.

You’re going to want to keep an eye on both daytime humidity levels and nighttime humidity levels, as they’ll often vary quite dramatically.

This can also be the case for indoor growers, but the difference is how much easier it is (in almost all instances) for indoor cultivators to control the humidity in their grow space. If you’ve got a few basic pieces of equipment and a plan, hitting consistently high RH levels is going to be no problem at all.

We will go over all the ways to control the RH for both growing cannabis indoors and outdoor grows a little further down.

Seed genetics

The specific cultivar that you decide to cultivate is also going to impact the RH levels that you should be shooting for. Some strains (the more pure Indica side of the genetic split) can tolerate lower humidity levels and even flourish at levels that may be surprising to even the most experienced growers. Sativa dominant strains originate in tropical regions, and so they usually handle a little more humidity than their Indica dominant kin.

Grow stages

As our favorite green friends grow, their tastes and tolerances change. RH levels are no exception to this rule. They need a higher RH level as youngsters, and as they develop into the true queens of the indoor jungle, the RH level they crave becomes slightly lower. Again, we will delve into specifics for each growing stage in the next section later in this write-up.

How do temperature and humidity interact?

Now, you might have noticed a few headings above that we didn’t use just the single word humidity but instead relative humidity (RH). This is a pretty important distinction to make, as it’s relative humidity that we base our growing decisions on, not just the absolute humidity figure.

So, what do we actually mean by this?

Well, simply put, RH is the measure of how much water vapor is actually in the air at a given temperature, and not just the amount of water vapor, but how much it is in relation to what could be held at that temperature.

In other words, we’ll talk about ideal RH levels as a percentage rather than an absolute number. This is because air at higher temperatures can hold more water vapor than air at lower temperatures, which is one of the main reasons why it is essential to remove a lot of the warmer air from an indoor growing space. Very warm air, in the vast majority of cases, just holds a little too much water vapor for ideal weed-growing conditions.

Why is humidity important for cannabis?

Controlling the humidity of your weed grow space is significant for a few key reasons, but one of the main factors is that it affects the transpiration process of the plant.

Transpiration explained

Transpiration is the process by which water is taken into the plant through the roots, and then expelled as vapor through its stomata (small openings on the surface of the leaves). This process is vital for maintaining a healthy balance of moisture in the plant, as well as facilitating gas exchange and nutrient uptake.

When humidity levels are too low, transpiration occurs at a much faster than optimal rate, leading to dehydration and potentially stunted growth. On the other hand, when humidity levels are too high, transpiration slows down and can lead to the development of mold and mildew on the plant’s leaves, internal bud rot, and also root issues. This can quickly result in a lower yield and even loss of entire crops.

Ideal humidity levels for each stage of growth

As we mentioned a little earlier, the humidity levels your plants require will change as they develop through each stage of growth. We also talked about how certain strains can handle differing humidity levels, but for the most part, the following numbers will work great for all strains, no matter which side of the genetic split they fall on.

Let’s take a closer look at what these ideal levels are for each stage:

Seedling and clone humidity 

Seedlings and clones share one major characteristic in common – they both have a higher RH requirement compared to older plants, thanks to their lack of root system. Making sure your relative humidity levels stay above 65% is more important for clones than it is for seedlings, as clones have a larger plant-to-root ratio, meaning they need to uptake more of their moisture needs through their leaves.

Either way, a relative humidity level of between 65-70% is ideal for this stage of growth. You will also want to make sure that the temperature stays around the 72 – 80 °F (22 – 27°C) mark while the lights are on or the sun is out, with a downswing to no lower than around 64 °F (ca. 18 °C) during the dark phase.

Vegetative stage humidity

As your plants start to take off and reach the point where they have a second node, it’s time to start gradually dropping the humidity levels. In this stage, plants don’t require as much moisture and can do with a relative humidity level of around 45-60% during the vegetative stage.

A good rule of thumb is to drop the humidity by around 5% a week as to not shock the crop, as well as giving you the opportunity to make any adjustments if they are required. At this point in their development, your plants will also start to use more water, so keep an eye out for any signs of dehydration, and increase the amount of water they are receiving if needed.

The plants will also begin to be able to handle larger temperature swings better, but if you can keep it in the same region as before (72 – 80 °F or 22 – 27 °C during lights on and no lower than 64 °F or 18 °C when the lights are off), then that would be ideal.

Humidity for flowering weed

By the time that your crop is ready to start flowering, you need to be ready to drop that level, and to do so relatively quickly. In the first 10 to 14 days after you switch the lighting over to 12/12, which induces the flowering cycle, the plant will go through what is referred to as ‘the final stretch’. The plants will add a significant amount of height during this phase, and they also tend to get a bit thirstier than before.

Dropping the humidity down during these two weeks is crucial, as once the flowers start to develop properly, they will start drinking more water than ever before. This is also the time that your plants are most susceptible to mold growth, so controlling the amount of water vapor in the air can be truly make or break in terms of final bud quality. It’s also a good idea to drop the temps just slightly once flower production is in full force, down to around 68 – 79 °F (20 – 26 °C) with a low of no less than 64 °F or 18 °C.

Look to have the RH sitting between 40 to 50% for the first 7 to 8 weeks of flower, and not above 45% for the last two or so weeks.

How to control the humidity of your grow space

Outdoor crops

Outdoors growers are, to a large extent, at the mercy of Mother Nature when it comes to controlling humidity levels. However, by following some basic guidelines and paying attention to weather forecasts, growers can make informed decisions about when to harvest their crop.

If you are growing outside, and want to have a better handle on the weather conditions, you can always look to set up a greenhouse. Doing so will usually raise the humidity, but you can then add vents in the roof and one or more along the sides to help with airflow, as well as setting up a few fans. 

If you are coming into a particularly dry period, make sure you are watering your crop enough and do so in the early or late hours of the day to avoid over-evaporation. You may also need to increase the amount of water you are giving to your plants. If the humidity peaks above what you were expecting, remove any windbreaks you have around the crop, and prune any leaves that are shading others to make sure there is sufficient ventilation and light.

Indoor growing

Indoor growers, on the other hand, have far more control over the humidity levels and can maintain them in a much tighter range. You will have a few key tools at your disposal to help bring that relative humidity within the optimal range.

Humidifiers and de-humidifiers can be of great assistance, but they may also be unneccesary.

If the humidity is too high:

• You can increase the exchange of air in the grow room by opening windows, doors or flaps to help get rid of any excess moisture and look to add an extraction fan if needed.
• Make sure that there is ventilation for your lights, as they produce a lot of warmth and may be contributing to an increase in overall humidity.
• Having your lights on period during the night can help to control the heat, and in turn, the moisture levels.
• If the heat is really out of control, you may have to install an AC unit. These not only drop the temps but also dry the air out.

If the humidity is too low:

• You can use a humidifier to add moisture back into the air.
• Misting your plants and the surrounding area can also temporarily increase humidity levels, but this method is not recommended for larger grow spaces or while the plants are in flower growth.
• Increase watering frequency and/or the amount of water given to the plants, but this is also risky, as you don’t want to over-water your crop.
• You can also hang some damp towels or cloths near the internal fans to increase the humidity.
• Make sure that your fans are not blowing directly onto your plants, as this can dry them out and cause dehydration.
• If all else fails, you can consider moving your plants to a smaller, more enclosed area. This will help trap any moisture in.

The final word on cannabis humidity

Humidity control is just one aspect of growing cannabis, but it is crucial to get it right if you want to produce the quality and amount of sticky-icky we all dream of. If you’re just beginning to grow your own weed, try not to get too caught up in the science of humidity control.

Instead, take note of any different climate conditions that impact your crop and adjust accordingly. In time, you’ll develop an intuitive understanding of how your plants react to changes in humidity levels, and you’ll be able to predict and prevent any potential problems that may pop up.

At the end of the day, growing your own ganga is supposed to be a fun, relaxing, even meditative experience, so don’t sweat the small stuff. Just keep a watchful eye on your plants, make adjustments as needed, and enjoy watching your baby girls grow into the beautiful, resin-covered beasts that the genetics you have acquired from us here at Premium Cultivars are capable of becoming!