Indoor Marijuana Growing: Relative Humidity and Temperature Guide

indoor marijuana growing guide

Indoor marijuana growing can be a process full of joy. Your cannabis plants are eager to reward your efforts and diligent care with multiple, sticky, deliciously scented and flavored buds upon harvesting.

However, the final outcome of your indoor cannabis growing mission depends on handling an array of factors that affect not only the yield but also the overall health of your green ladies.

Undoubtedly, understanding how to maintain the relative humidity and temperature levels in your grow room is of utmost importance.

Unfortunately, many hobby growers tend to overlook the significance of managing humidity and temperature rates. With so many things to keep in mind, it’s easy to get overwhelmed, right?

We feel you, bud buddies, and we’ve been there before.

But what nobody tells you about indoor marijuana growing is that it is actually an easy task once you take the time to understand and, subsequently, cater, to your plants’ needs.

So what you have to do is to equip yourself with patience and confidence. Confidence is crucial for any cannabis grower because you shouldn’t fall victim of your fear of the unknown. Especially since we are blessed to live in the era of open web communications when anything we want to learn is literally one click away.

If you want to make your indoor cannabis growing journey run smooth and out of troubles, jump straight down to the relative humidity and temperature guide we have compiled below. We will provide all the exact bits of information you need (minus the sweet-talk-nonsense and sci-fi like terms, pfew).

How Humidity and Temperature Affect Marijuana Plants?

During all stages of the growth journey, marijuana plants need water. Of course, the fact that marijuana plants require water to develop doesn’t mean you should over water them.

But even in the case of several consecutive days of no watering, cannabis plants will still use a certain amount of water through their leaves, and not from their roots – this is where humidity steps in.

When the humidity in your grow room is high, your green ladies absorb moisture from the air mainly through their leaves.

When the humidity in your grow room is low, your plants absorb moisture mainly through their roots, and only small amounts of moisture are pulled out through the leaves.

As a rule of thumb, warm air holds more moisture than cold air.

That’s why the levels of humidity and temperature are intricately connected. By manipulating the temperature rates in your grow room, you can also manipulate the humidity rates and vice versa.

For instance, if too much warm air circulates through the grow room, you’ll need to let cooler air circulate in order to block high levels of water vapor in the air.

But what’s the big deal with high humidity, and why should you even bother on that note? Can’t you just control the temperature rates and let your cannabis plants do their job? Isn’t this in all natural process?

For those who are growing cannabis outdoors, the nature does much of the hard work in terms of relative temperature and humidity rates. Humidity is naturally balanced throughout the different months of the spring-summer-autumn season (no need to mention the winter season because you won’t be growing cannabis outdoors in the cold).

Earth releases water vapor gradually throughout the day, starting with low humidity during the early hours before sunrise and increasing the water evaporation as the sun reaches its peak. In the spring season, the sun doesn’t emit the same amount of heat as during the summer season. But humidity levels are typically higher throughout the spring as opposed to the summer when high-temperature cause water to evaporate very swiftly.

When growing cannabis indoors, the changes in the seasons don’t have the same nurturing effect on your plants when it comes to balancing out temperature and humidity levels.

Instead, it is all up to you to provide the most suitable conditions for your cannabis beauties to flourish. If you don’t pay attention to monitoring and managing the humidity and temperature rates in your grow room, you stand a higher chance of ending up quite disappointed.

#1 Nutrients intake related issues

Keeping in mind that humidity affects how the way your cannabis plants absorb moisture, higher or lower humidity than needed can lead to nasty nutrient intake related issues.

High humidity will make your plants “drink” more water through their leaves which can lead to poor nutrient absorption rates.

Low humidity will make your plants “drink” more water through their roots but this can easily lead to nutrient excesses, which can lead to nutrient burns.
That’s especially harmful when low humidity is accompanied by high room temperatures.

High temperatures will force your plants into drinking more water than usual (you know how you feel extra thirsty in the middle of a hot, sunny day, right). As they drink more water to satisfy their thirst due to the high temperatures, low humidity will further force the plants into pulling the water they need through their roots. And without even knowing it, your plants can uptake way too many nutrients. Nutrients should be released slowly and not swiftly as in the case of high temperature + low humidity
Video by Coastal Custom Consulting – CCC – Diagnosing Sick Marijuana Plants – Cannabis Leaves, Discoloration & Troubleshooting

#2 Mold related issues

Mold is a nightmare for your cannabis plants. It can be especially harmful during the flowering stage when the dense, juicy buds become a perfect mold-development spot. Mold (aka bud rot) is not what you want to deal with in your cannabis garden.

In fact, mold can spell disaster for your entire cannabis growing mission.

Picture yourself holding a branch of sparkling, heavy buds set to be harvested so soon that you can already feel their taste and pleasing effects upon consumption.

But as you look closer, you notice that the insides of the beautiful buds are covered with whitish or brownish mold. Several months of your life and efforts might just be thrown away in the trash, and mold is to be blamed. Yes, it is heartbreaking. But the truth is, the grower is to be blamed, and more specifically – poor management of temperature and humidity rates is to be blamed.

Unfortunately, there’s nothing you can do to save the rotten buds – your only choice is to get rid of them.

Choosing mold-resistant marijuana strains can be very helpful to limit the possible issues related to mold.

But the fact that you’re dealing with a mold-resistant strain doesn’t mean it’s 100% protected against mold if you don’t pay attention to the humidity and temperature rates. It just means that the strain will be far more resilient and more forgiving towards your possible mistakes.

#3 Powdery mildew related issues

The only good thing about powdery mildew is that it is relatively easy to spot, as opposed to the signs of nutrient deficiencies and excesses that can be quite overwhelming when it comes to distinguishing between them.

As a rule of thumb, keep in mind that powdery mildew is a fungal disease.

Fungi adore high humidity, high temperatures, and stagnant air.

There’s little you can do to stop fungal diseases from plaguing your grow space if you don’t pay attention to maintaining the relative temperatures and humidity rates.

Powdery mildew can be extra tricky during the vegetative stage. That’s because marijuana plants need higher levels of humidity during vegetation which makes a wonderful environment for the nasty fungi.

But that’s just one more reason why proper ventilation truly matters to every marijuana grower. The constant airflow in your grow room can prevent the fungal disease from ever happening (so, yes, oscillating fans are of utmost importance and you can’t go only with intake/exhaust fans and/or a carbon filter).

Similarly to mold-resistant marijuana strains, mildew-resistant strains are not immune to powdery mildew; they simply have a higher tolerance towards the possible fungal diseases.

If you are concerned about powdery mildew (like I was at the very start of my indoor cannabis growing as I live in a region with very high humidity), then choosing mildew-resistant marijuana strains is a must. However, making sure you bring down the probability of powdery mildew infestations to a minimum is even a bigger must.

#4 Pests related issues

Whether it comes to aphids, spider mites, or just about any type of tiny intruders that wish to come and feast on your cannabis plants, proper humidity, and temperature levels are crucial.

Of course, any of these unwanted visitors can pop out even if you do your best to keep the humidity and temperature rates under control. But the chances for the invasion when the humidity and temperature levels are balanced are significantly lower.

Just remember to keep the airflow running at a higher speed during flowering and/or during any time of your growing journey when the humidity and temperature rates are high. That’s because the tiny invaders (much like fungi) adore stagnant air combined with high temperature and humidity rates.
Video by Grateful Grower – How to deal with Spider mites, Russet mites, Fungus gnats, and Thrips for cannabis plants.

#5 Stunted and/or slow growth related issues

As a rule of thumb, marijuana plants are creatures of habits. They need a particular spectrum of conditions to thrive during each stage of their development.

Simply put, cannabis plants need higher levels of humidity during vegetation, as opposed to lower levels of humidity during flowering.

If you fail to cater to their most basic demands regarding humidity and temperature rates during flowering and vegetation, then you shouldn’t expect that your green ladies will reach their fullest potential.

Stunted or slowed down growth doesn’t only affect the harvesting point. It also affects the overall health of your plants. Respectively, the well-being of your plants affects your well-being as a grower.

Now that we’ve highlighted the ways poor control over the humidity and temperature rates in your grow room affect cannabis plants, let’s get down to learning how to avoid all of these issues from happening.

Quick Heads Up

Before we delve into the practical side of the relative humidity and temperature guide, we want to encourage you that maintaining these important factors in balance isn’t as complicated as it seems at the very first sight.

In fact, even kids can cope with managing the humidity and temperature rates – the process is fairly simple. And when we say simple, we mean it.

Even if you are one of the busy bees out there, don’t sacrifice the humidity and temperature grow room control. It will take you only several minutes a day but the rewards you can reap will be worth every minute (and penny) spent.

Also, growing cannabis plants comes with only relative humidity and temperature rates balance you want to achieve. It isn’t that much about getting obsessed with some strict numbers but much more about being consistent.

All you have to do is to keep the humidity and temperature in your grow room as constant as possible within a certain range depending on each stage of the development of your marijuana plants.

Best Humidity and Temperature Rates for Seedlings

From being seedlings and through vegetation, marijuana plants have NOT fully developed their root system. That means they cannot possibly intake the same amount of water solely through their delicate, young roots, as opposed to mature plants that have entered flowering.

When introduced to high levels of humidity, young marijuana plants will grow much more robustly and vigorously as they will be able to pull out water from the moisture in the air.

And since the seedlings’ root system is far less developed than that of vegetative cannabis plants, seedlings will thrive in slightly higher humidity.

Best Humidity Levels for Seedlings: 65% – 70%
Best Temperature Levels for Seedlings:
20 – 25 degrees Celsius (68 – 77 Fahrenheit)
Acceptable Difference in the Lights ON vs. OFF Temperature for Seedlings: 4 – 5 degrees

In the living nature, temperatures drop lower during the night as opposed to the temperature rates during the day. That’s why it’s fully acceptable (and, in fact, recommendable) to expose your marijuana plants to slightly lower temperatures when the lights are OFF which is also naturally occurring in the absence of the heat emitted by the grow lights. It is important, however, that you don’t allow the difference between the daytime and nighttime temperatures to be more than only several degrees or else, you may shock your plants and stunt their growth.
Video by Nvclosetmedgrower – How to Grow Part 2. Seedling stage.

Best Humidity and Temperature Rates for Marijuana Clones

Marijuana clones, similarly to seedlings and marijuana plants in their early vegetative phase, need time to develop a strong root system.

But what’s more, clones never develop the taproot that regular cannabis plants do since they are not grown from seed. That’s one of the reasons why cannabis plants grown from clones can have a hard time outdoors when faced with the ravages of winds – due to the absence of a taproot they never really get to rely on fully developed roots that can keep them stable and well-connected to the soil.

So when clones are young, they rely heavily on absorbing water through their leaves in order to establish roots.

Best Humidity Levels for Clones: 70% – 80%
Best Temperature Levels for Clones:
20 – 25 degrees Celsius (68 – 77 Fahrenheit)
Acceptable Difference in the Lights ON vs. OFF Temperature for Clones:
4 -5 degrees Celsius lower (8 – 9 degrees Fahrenheit lower)

Bonus Tip: Humidity domes for clones can work wonders for increasing the optimal levels of humidity while also protecting the fragile clones. Plus, it’s easy to make humidity domes by yourself if you don’t want to invest in professional-grade humidity domes. But do NOT use humidity domes for seedlings unless you happen to live in an extremely dry region.
Video by INDICA INSTITUTE – Marijuana Cloning Steps & Process – Growing Cannabis 201: Advanced Grow Tips

Best Humidity and Temperature Rates for Vegetative Marijuana Plants

As a rule of thumb, as the vegetative phase progresses, humidity levels have to be gradually lowered.

That’s because your marijuana plants are slowly but surely walking towards entering their flowering phase. Therefore, the root system becomes better developed. As a result, you no longer need to strive for high levels of humidity since your green ladies will be able to intake a greater amount of water through the roots and only a limited amount of water through the leaves.

However, keeping the balance of humidity and temperature in your grow room becomes even more crucial with the approach of the flowering phase. With this in mind, the vegetative phase determines the structure, as well as the future well-being of your green ladies, so do not underestimate it.

Best Humidity Levels for Vegetative Marijuana Plants: 70% – 40%
Best Temperature Levels for Vegetative Marijuana Plants:
22 – 28 degrees Celsius (71 – 83 Fahrenheit)
Acceptable Difference in the Lights ON vs. OFF Temperature for Vegetative Marijuana Plants:
4 – 5 degrees Celsius lower (11 – 12 degrees Fahrenheit lower)

Bonus Tips: During the vegetation period, you only want to start with high humidity levels of approximately 70% (but not any higher!) Your major goal is to keep lowering the humidity with about 5% each week. The final objective is to reach humidity levels of about 40% once the flowering stage begins. But this rule has certain exceptions. For instance, if you are trying to limit the size of your indoor marijuana plants, you may want to keep them in vegetation for half of the typical time needed (let’s say only a single month as opposed to the usual 2 months of vegetation). In such case, just figure out the time you’ll keep your plants in vegetation and make sure you lower the humidity levels gradually each week – for instance, 6%-8% each week.

With auto-flowering strains, though, this process can be even trickier. Since the autoflowers transition to flowering without the need of the typical photoperiod (controlled hours of undisturbed light vs. undisturbed darkness), it can be quite confusing to figure out the best approach in lowering the humidity rates. But then again, things are easier than they seem at the very first sight.
Video by Nvclosetmedgrower – How to Grow Part 3, Early Veg.

Due to the short period of the vegetation of auto-flowering cannabis strains, you want to lower the humidity levels with about 8% each week. So once you get started with no more than 70% humidity at the beginning of vegetation, you’ll end up with about 40% humidity after 4 weeks which is when your autos are supposed to enter flowering.

Don’t forget that there might be exceptions on that note, too. Different auto-flowering cannabis strains have a different growth schedule – some might be ready to harvest within as little as 8 weeks but others may take 10 or even 11 weeks from seed to harvest.

Your best move is to choose seeds from reputable retailers because only high-quality seeds possess high-quality genetics. It is by relying on the authentic genetic inheritance of the strains that growers can understand what to truly expect in terms of both growth patterns and growth timelines.

Best Humidity and Temperature Rates for Flowering Marijuana Plants

As soon as the flowering stage begins, your best move is to lower the humidity levels to about 40% – 50% and keep them within this range up until the final stages of flowering. In fact, it can be perfectly fine to keep the humidity at 40%-50% all the way up to harvesting but if you choose to follow the sweet tips we’re sharing below, you can get much more impressive yields in terms of both quantity and quality.

We feel obliged to reiterate once again how crucial it is to keep high humidity at bay during flowering. At this point, your cannabis plants will have already established strong, robust roots that can intake water and nutrients to nurture the hungry buds.

Meanwhile, letting the plants intake too much moisture through their leaves can easily lead to molds and mildews. Mold, in particular, is such a nasty issue for cannabis growers because it can take place without you ever noticing what’s really going on – it remains hidden and eating the buds from within secretly and sneakily.

Best Humidity Levels for Flowering Marijuana Plants: 50% – 40%
Best Temperature Levels for Flowering Marijuana Plants:
20 – 26 degrees Celsius (68 – 79 degrees Fahrenheit)
Acceptable Difference in the Lights ON vs. OFF Temperature for Flowering Marijuana Plants:
4 – 5 degrees Celsius lower ( 10 – 11 degrees Fahrenheit lower)

By slightly lowering the humidity rates, as well as the temperature rates during flowering, you will make sure the buds are well-protected from disease and infestations that thrive in high temperatures and high humidity. But that won’t be possible without providing a reliable and consistent air circulation in the grow room.

During the last 1-2 weeks of flowering, you can further boost the yield, flavors, and appearance of the buds you’ll get upon harvesting by following the short guidelines below.

Best Humidity Levels for the Last Stage of Flowering: 40% – 30%
Best Temperature Levels for the Last Stage of Flowering:
18 – 24 degrees Celsius (64 – 75 degrees Fahrenheit)
Acceptable Difference in the Lights ON vs. OFF Temperature for the Last Stage of Flowering:
5 – 10 degrees Celsius lower (10 – 11 degrees Fahrenheit lower)

Mind that the humidity and temperature rates guidelines for the late stage of flowering are merely recommendable. However, following these pieces of advice will not only let you prevent bud rot right before harvesting but it can also increase the potency of your green medication. Dropping down the temperatures mimics the natural phenomenon that occurs as the autumn season kicks in and it gracefully stresses the plants so that resin production is increased.
Video by INDICA INSTITUTE – Cannabis Flowering Stage – How to grow marijuana course for dummies – Growing Cannabis Indoors 101

Don’t feel stuck into thinking that maybe you are supposed to know the optimal humidity and temperature rates by heart. As long as you keep our humidity and temperature neat cheat sheet handy, you can get down to growing with confidence and a smile.

Now that you are familiar with the best relative humidity and temperature rates for each stage of the development of your green ladies, it’s high time to learn how to manipulate these rates whenever needed.

Indoor Marijuana Growing 101: How to Control Humidity and Temperature?

So far we’ve highlighted the unique and extremely important ways that the relative humidity and temperature rates in your grow room interact with each other. Do always keep in mind that humidity is relative to temperature (and vice versa). You can make this work in your favor, and not the opposite.

Certainly, there are some crucial tricks you want to know in order to balance out the humidity vs. temperature levels confidently.

How to Lower Humidity in your Grow Room

#1 Increase ventilation

The exhaust system in your grow room truly matters. By increasing the total amount of airflow in your grow space, you can significantly lower humidity levels and keep them intact within the desired rates.

You may need to either adjust the existing vent by letting it run on a higher speed or you may need to upgrade your entire ventilation system so that it suits the size of your grow space and the number of plants you are cultivating.

Opting for an air conditioner can also work great for lowering the humidity levels. Plus, an air conditioner is helpful when it comes to cooling the air which can be extremely beneficial if you’re dealing with grow lights that input a lot of heat (in particular, HIDs). But do keep in mind that you need the right size of AC for your grow room – bigger or smaller ACs than needed will not work well.

#2 Get a dehumidifier

Dehumidifiers can work wonders for pulling out moisture from the air. Some dehumidifiers can be easily hooked straight to the drain while others will require manual maintenance which means you’ll have to pour away the accumulated water once the dehumidifier shuts off.

Most importantly, only a dehumidifier of suitable, decent size can work fine to lower humidity levels efficiently so skip on smaller ones as they won’t really do the trick (except in the case you’re growing only a handful of 1-2 plants).

#3 Master the watering schedule of your marijuana plants

You can significantly lower humidity if you water your green ladies as soon as the lights are switched ON.

If you keep watering your plants after the lights have been switched OFF, this can cause high humidity because plants are in sleep mode and don’t really interact with neither the higher day temperatures that speed up water evaporation nor the light that is crucial for the completion of the process of photosynthesis.

Also, don’t leave extra water in the trays of your plants. Letting the excess water stay in the trays for a while will inevitably increase the humidity rates in your grow room
Video by Future Cannabis Project – More Frequent Watering for Cannabis Plants

#4 Defoliate your cannabis plants

While this is only an optional step and it is not recommendable if you are just getting started with growing your own green medication, defoliation can work great to limit the excess humidity. Cannabis plants that are super leafy will raise the humidity in the grow room while those that are well-groomed will work in the opposite direction.

In fact, proper defoliation will also help you optimize your grow space so that the plants get lighter exactly where needed – straight onto the gorgeous flowers.
Video by Justintime2Grow Network – Beginner Tips #3 – Defoliation

How to Increase Humidity in your Grow Room

#1 Increase humidity levels manually

There are several ways to increase humidity without spending a dime. However, it’s important to keep in mind that these methods are not your most reliable option.

One quick way to increase humidity is to use a spray bottle filled with water. Just spray the walls and the floors, and you’re done but the increase in humidity will only last for a very limited period. You can also gently spray the top of the soil without making it wet – just keep it damp.

You can also place several containers/bottles filled with waterin your grow room. The water will start to evaporate, and thus, to increase humidity. Nevertheless, you can hang or place wherever suitable wet towels or cloths.

#2 Get a humidifier

Just like the name suggests, humidifiers work for increasing the humidity levels in a room. Humidifiers gradually add an extra bit of moisture in the air. Evaporative humidifiers are helpful for lowering the temperatures, as well.

Just make sure you avoid small-sized humidifiers such as these that come with 1.5-gallon tanks. The smaller the size of the tank, the more regular you will have to refill it with water (in the case of 1.5-gallon tanks, that means refilling several times a day).

How to Lower the Temperature in your Grow Room

 #1 Increase ventilation

Just like the case with lowering humidity levels, increasing the ventilation in your grow room will help significantly in lowering the temperatures. You can either switch your vents to a higher speed or you may need to invest in a larger tube vent.

Keep in mind, though, that if the outside air is too hot, your intake fan can’t really work wonders on its own to lower the temperatures inside your grow room as if by a miracle.

Air conditioners can be a good alternative when you want to cool down the temperature rates. Plus, it also lowers the humidity levels.

#2 Consider the light schedule of your plants

Most growers will keep their lights ON during the day and OFF during the night.

But if high temperatures are bothersome for your indoor marijuana growing mission (which highly depends on the climate in the region you live in), then you can switch to introducing your cannabis plants to lights ON at nighttime vs. lights OFF at daytime.

This will most certainly decrease the overall temperature in your grow space because the heat emitted by the grow lights will be lower due to the natural way temperatures of the air fall down at night.

Quick Tip: If you’re having a hard time trying to handle the heat emitted by HID grow lights, it can be a wise move to use a cooltube.

How to Increase Temperature in your Grow Room

#1 Switch to bigger grow lights

The bigger the grow lights, the more heat they emit, and as a result, the higher the temperature in your grow space.

#2 Heating mats and/or simple space heaters

As obvious as it seems, opting for a simple space heater that comes with a thermostat is a quick and reliable way of increasing the temperature in your grow room.

Meanwhile, heating mats can be easily placed under the plants and work great, too. In the case you’re struggling with quite low temperatures, a combination of a heating mat and a simple space heater can be very beneficial.

#3 Extra insulation

Insulating your grow space prevents outside factors of affecting the environment of your indoor cannabis garden.

Excellent insulation has turned growing marijuana in shipping containers into a whole new trend, and this only points out to the huge significance of growing weed indoors in a well-sealed space. You can use different inexpensive materials from just about any hardware store to improve insulation.

Undoubtedly, the more you invest in sealing your indoor grow space, the better the ease of control over each of the factors that affect the cannabis growing environment
Video by Monster Gardens – Sealed Grow Room 101: Indoor Garden Setup, Designs, Configurations Layout Grow Room Diagrams Example

Indoor Marijuana Growing: Relative Temperature and Humidity Guide – The Takeaway

By using a reliable hygrometer and room thermometer (or a hygro-thermometer device), you can keep track of the temperature and humidity rates in your grow room effortlessly.

Adjusting the levels of humidity and temperature by decreasing or increasing them when needed is not a difficult task at all.

Then again, these rates are called relative for a good reason. There’s enough room for experimenting as long as you strive to limit any abrupt spikes that can stress out your cannabis plants.

Last but not least, it’s important to get familiar with the growing specifications of the strains you’re taking care of. Some cannabis varieties can tolerate higher/lower humidity and/or temperatures better than others.

Monitor your cannabis beauties and you will gradually learn to speak their language – meaning, you will be able to understand their needs at a glance.

We truly hope that the relative humidity and temperature guide will be of your best assistance because we’re on a mission of spreading the love for growing the green medication with joy and confidence.


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