Posted on

How to make humidity for cannabis seed

The Importance of Controlled Humidity Levels for Growing Cannabis

Ever wondered how cannabis growers get the perfect aroma? From growing cannabis to consuming it, it all comes down to humidity.

The marijuana industry is booming – generating ~$34 billion every year. As more legal growers take on the challenge, Americans are on the hunt for the best indoor grow rooms (under strict governmental rules & regulations) to obtain the best yield and improve plant growth!

Many factors need to be controlled in a grow room, such as CO2, temperature, and humidity. Controlling humidity and temperature levels is a precise science and is essential for growing cannabis, from planting the first seed to harvesting the end product.

As you probably know, all plants carry out photosynthesis, respiring CO2 from the surrounding environment via their leaves. During this process, the cannabis plant loses some water. If the air is not humid enough, the moisture content within the plant will be depleted. If this continues, the plants will lose more water than they can hold within the foliage, closing the pores in the leaves resulting in cell death.

Some people think that watering their cannabis plants will help, however, this will decrease the amount of air in the soil, causing asphyxiation to the plant and likely root rot. But, with the correct humidity levels, they will thrive!

Controlling humidity levels is not difficult, but getting it right will boost your production levels. This article will tell you everything you need to know regarding humidity, from controlling it, to what equipment you can buy.

Temperature & Relative Humidity (RH)

Humidity and temperature are closely related and play a vital role in harvesting cannabis. Humidity measures how humid the air is in a given area, in your case, this will be the grow room. Obtaining the ‘perfect’ amount of humidity (relative humidity, or RH) is crucial.

When we talk about RH, it refers to how much water air holds at a certain temperature over time. When the RH reaches over 100%, surplus water is no longer detained by air, forming water droplets as condensation occurs on the leaves. The water droplets are usually in 3 forms: fog, dew, or rain.

NOTE: During the growing process, RH should fall slowly.

Temperature is just as important as RH when growing cannabis so controlling both is essential for the best yield. It is important to note that hotter air holds more humidity than colder air. So what happens if the growing area conditions become too hot or too cold?

Too Hot

If temperatures are too high, especially in the flowering stage, the plant’s buds will appear fluffy. This is likely to reduce potency while the growth rate will slow down.

Too Cold

If the conditions inside the growing area are too cold, the cannabis plant will grow much slower. If the temperature goes near freezing (32°F) the plant will be in extreme shock, potentially fatal.

Ideal Humidity Levels for Growing Cannabis

The million-dollar question asked by many grow enthusiasts is: “What should my humidity levels be to optimize cannabis growth”.

Well, the RH level depends on the plant. Plants that originate from warmer/tropical regions thrive with high RH levels as they are well-adapted to these conditions. However, plants from colder regions prefer a less intense RH level.

Cannabis is thought to be native all around the world with Indica cannabis originating from dry mountain regions where temperatures are lower. However, with Sativa cannabis, this strain grows best and is thought to originate from much hotter, humid, subtropical regions.

Nowadays, the seeds you will find are a Sativa-Indica hybrid plant. These hybrids have emerged from heavy cross-breeding which is why most growers will assess the plants’ morphology, and run trials using different RH levels to find the best yield or end product.

Humidity Levels & Growth Stage

Depending on what stage your cannabis plants are at, will depend on how much humidity they require to grow.

There are 4 stages to cannabis growth that every grower should familiarise themselves with: seedling/clones, vegetation, early flowering, and late flowering.

  1. Seedling/Clones
  • This stage involves clones or seeds from the parent cannabis plant
  • Relative humidity level = high (65-70%)
  • High humidity is required to establish root growth & for water uptake in the leaves
  • Day temperature = 77°F
  • Night temperature = ~70°F
  1. Vegetation
  • This stage involves the plant growing well-structured leaves
  • Relative humidity level = moderate to high (40-70% depending on the strain of cannabis) – if the leaves look too wet during this stage, the humidity is too high
  • During the vegetation period, the plants develop strong root systems that increase the uptake of water. You may need to reduce the humidity levels during this period
  • Every week during this period, it is recommended to reduce the humidity levels by 5% and increase the temperature because, at this stage, the cannabis plant’s leaves will be absorbing and evaporating more water.
  • Day temperature = 71-82°F
  • Night temperature = 64-75°F
  1. Early Flowering
  • At this stage, the cannabis plant has established a strong root system, meeting its water requirements, however, it still uses its leaves to absorb water
  • Relative humidity level = low (40-50%)
  • Low humidity is required to prevent mold and mildew (see below for more details on this issue)
  • Day & night temperatures = 68-78°F
  1. Late Flowering
  • This stage usually lasts 6 to 12 weeks as the cannabis plant fully matures
  • Relative humidity level = low (30-40%)
  • Low humidity improves the plant’s yield, taste, and overall appearance
  • Day temperature = 64-75°F
  • Night temperature = 60-68°F

Controlling Humidity

Giving plants the correct amount of humidity, allows their pores to open up inside the leaves, respire more CO2, and grow faster. However, exposing them to too much humidity can cause diseases such as bud rot due to water build-up encouraging bacteria and fungi to overrule the foliage.

Benefits of controlling humidity include:

  • Reduces bud rot from happening
  • Ensures the cannabis plant will survive
  • Improves potency
  • Improves taste

How to Increase Humidity

  • Reduce the power of your exhaust fan – this traps warm air inside the growing area
  • Increase the temperature in the room
  • Gently spray/mist your plants and soil to keep them moist – avoid flowering plants as this can cause the buds to rot
  • Surround your plants with water (in open containers)
  • Use a humidifier to trap moisture in the growing area

How to Reduce Humidity

  • Increase the power of your exhaust fan – this pushed warm air outside the growing area
  • Plug in a dehumidifier
  • Water the plants at the start of their light period/when lights are turned on
  • Increase the amount of cool air in the growing area

Keeping the humidity at the right level throughout your cannabis plant’s life cycle can be difficult, however, it is vital to monitor the plant’s growth during its 4-stage lifecycle.

At Atlas Scientific, we have a humidity sensor that not only reads relative humidity for accurate humidity monitoring, it also measures the dew point and air temperature.

Signs of Poor Humidity Levels

You hear some horror stories from cannabis growers where things have gone wrong, and this is generally from poor humidity management.

Below are a few issues that can occur, and what you can do to prevent them from happening to your stock.

White Powdery Mildew (WPM)

This fungal infection occurs in high humidity environments, which can be difficult as high humidity levels are required for the early stages of cannabis growing. When the cannabis plant is infected, it will be covered by white powder or mildew, which can be prevented if caught early on.

If you notice this taking over your plants, ensure there is ample airflow in the growing area – you can use a swinging fan.

Bud Mold/Rot

Another fungal infection, which affects the dense cores (inner part) of the plant, making it turn white or brown with mold/rot. If you notice this on the plant’s buds, you will have to throw them away as they are now unusable.

Ensure there is enough air circulation, plenty of space between plants, and inspect the plants daily to prevent it from occurring.

Nutrient Issues

Plants need nutrients to grow but give your cannabis plants too much, they will get nutrient burn, also known as nute burn. This is when the leaves turn yellow and the edges and tips turn brown looking burnt.

This process occurs when the plants take up too much water due to high temperatures and low humidity in the growing area. As they drink more water, the uptake of nutrients via their roots is excessively increased causing a nutrient burn.

Reduced Growth

When cannabis plants are young, they require higher humidity levels than mature plants (these grow best in lower humidity levels). If these requirements are not met, your cannabis plants will not reach their potential growth and optimum health.

Summing Up Humidity, Testing Equipment, and Advice

Whether it is personal or commercial use, humidity levels play a vital role in growing cannabis. By controlling relative humidity (RH) levels, you will maximize your yield and reduce issues such as white powdery mildew and bud rot.

At Atlas Scientific, we have the perfect solution to control RH levels and make growing cannabis stress-free! We understand finding the perfect humidity sensor is a mission itself, that’s why we have built our own.

Remember to check your plants daily and keep a close eye on both temperature and humidity throughout the growing process.

Hopefully, after reading this, you will be well on your way to growing healthy cannabis plants, but should you have any questions, please do not hesitate to reach out to one of our staff at Atlas Scientific, part of our world-class team.

How to make humidity for cannabis seed

Over the last decade, more and more states have chosen to legalize the sale of both medicinal and recreational marijuana. Even without national legalization, the U.S. marijuana market was worth $9.2 billion in 2017 and is projected to reach $47.3 billion by 2027, a growth rate of 514% over 10 years (source: Arcview Market Research and BDS Analytics).

Investors are flocking to the marijuana industry, seeing it as one of the safest long-term bets in recent history. Many of them are directing their investments toward dispensaries rather than growers, according to Scott Greiper, president of Viridian Capital Partners.

See also  Cannabis covering sprouting seeds with

As the number and size of dispensaries grow, so do consumer expectations of the quality of marijuana products and the availability of diverse and unique brands. As in the wine industry, premier brand names can command a price ten or more times that of an average product.

On behalf of their customers, dispensaries are looking for consistent quality and a range of products. So while the opportunity to run a lucrative and stable growing business has increased dramatically, there is more and more pressure for growers to produce a consistently high-quality product in order to turn increased demand into increased dollars.

Let’s Get Growing

What are the most important points of growing a consistent crop to build your brand? Again, borrowing from the winemaker’s metaphor, it’s all about what you’re growing, where you’re growing it and the conditions it’s grown under. Below, we’ll look at how to create the best growing environment, with a focus on humidity and temperature controls .

To start with, relative humidity is determined by how much water vapor the air can hold at a specific temperature. Warm air can hold more water than cold air, so the warmer the air is, the higher the relative humidity will be .

As you probably already know, after choosing a substrate (the material you’ll be growing your plants in/on), the strain(s) of cannabis you’ll be growing, lights, watering mechanisms and nutrients needed, the most important element of your day-to-day operation will be managing temperature and humidity.

Each stage of a plant’s life-cycle is prone to different needs. With the right equipment, these needs are easily managed when the focus is on keeping everything within the desired ranges and as constant as possible.

To get started, you’ll want to buy a hygrometer and a thermometer. With these tools, you can accurately monitor and then alter humidity and temperature conditions in an attempt to strive for repeatable quality levels time and time again.

Shop Humidifiers and Dehumidifiers

The Four Stages of Plant Cycle

Seedling Stage

  • Seedlings and clones like high humidity levels of 65-70%
  • Temperatures with lights on: 68-77 F° (lights off: 8-9 F° lower)

The increased humidity levels allow seedlings to take water in through the leaves while their root systems are developing.

Vegetation Period

  • Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
  • Temperatures may be increased slightly if desired.
  • Temperatures with lights on: 71-82 F° (lights off: 8-9 F° lower)

As the roots are now more developed, they’re able to perform most of the needed water uptake with evaporation through the leaves now cooling the plants.

Flowering Period

  • Humidity levels need to be lowered to 40-50%
  • IMPORTANT– Never above 60% humidity!
  • Temperatures with lights on: 68-79 F° (avoid high temperatures)

Late flowering (1-2 weeks before harvest)

While not strictly a necessity, the following can have a direct impact on yield, flavor 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: 64-75 F° (lights off: minus 9-18 F°)

Adjusting Humidity Levels and Temperature

Depending on your lighting solution, season a specific crop is grown in (assuming year-around production), the outdoor air vented in (as it will vary in temperature and its own relative humidity), controlling temperature and humidity in your growing space will be one of the tougher challenges you will face on a day-to-day basis.

While it’s tough, just know that every grower working at a scale similar to you is going through this as well.

Ways to Lower Humidity

  • Run your existing tube vent on higher velocities or upgrade your air flow fans. This applies to both air exits in tents and the room it is placed in! Increase fresh/cool air supply.
  • Try to water your plants right after lights are switched on. The room will be cooler at that time and the immediate absorption will result in less overall humidity.
  • The most consistent option will be: Invest in a decent sized dehumidifier–small humidifiers don’t have large enough tanks to keep up with the demand.

Ways to Lower Temperatures

  • Increase the total airflow and air exchange in your growing room
  • A simple trick: “lights off” during the day — “lights on” at night
  • When you grow with High Pressure Sodium lights (HPS), consider using a cool tube
  • Install an air conditioning unit (AC’s will also lower humidity levels)

Raising Humidity

  • You can temporarily increase humidity levels by misting with a spray bottle
  • Keep the surface of the soil moist, but not wet
  • Hang wet towels/cloths in your grow room or put open water containers in it
  • The most consistent option will be: Invest in a humidifier with a large water reservoir (the need for constant refills makes smaller ones inefficient)

Raising Temperatures

  • Upgrade to a bigger grow light (more watts = more heat)
  • Place a heating mat at the bottom of your tent or grow room
  • Use a simple space heater with a reliable thermostat

Protecting Your Investment

Though it may be exciting to see the fruits of your labor as you begin harvesting, the work is far from done. Correctly processing your crop helps prevent any chances of it becoming damaged or rendered non-smokeable. Proper drying and curing are essential to minimizing the risk of mold contamination and it also greatly improves the taste when smoked.


On top of the need for specific temperature and humidity controls throughout the growing process, it’s worth mentioning that humidity level during the drying process is equally important to the quality of the final product.

Too much humidity in your curing space and you might be left with perfectly grown, but moldy cannabis. Nobody wants that; neither the individual user nor the dispensary buyer. Moldy cannabis is easily the fastest way to ruin your reputation as a reliable grower of high-quality buds.

Again borrowing from Royal Queen Seeds expertise, here’s a how-to on drying your valuable crop:

Wet Trimming versus Dry Trimming

The curing process breaks down chlorophyll remaining in the plant, resulting in a less-harsh taste. Remember: harsh buds are not quality buds. These procedures can enhance the shelf-life of a crop, which is highly important with a product like cannabis that isn’t afforded the addition of preservatives.

Wet Trimming

“Wet trimming” involves trimming as soon as plants are ripe. Remove individual branches and proceed to use sharp scissors or shears to precisely trim excess plant matter. As soon as the plants are cut down, the drying process begins. What’s immediately noticeable is how sticky and wet the buds are. While this is an excellent indicator of potential quality, it’s also a breeding ground for contaminants. Buds left in this state will almost certainly be ruined so it is best to act quickly to begin a controlled drying process.

Widely regarded as the preferred method, “wet trimming” offers the most control and quality assurance. It also has the added benefit of allowing you to collect the “sugar leaves” which contain lower cannabinoid levels and can be stored separately to be processed into edibles at a later time. Remember that “High THC; low Cannabinoid level oil” states list from before? This is where that product comes from.

Dry Trimming

This technique is usually reserved for growers with a large amount of plant matter and little time to process it. This involves cutting off branches and hanging them whole from drying lines. Once the plants are dry, they are then trimmed and processed. It is more difficult to achieve the neatness of a wet trim as sugar leaves will curl in toward the buds while drying. The agitation of handling a dried bud during trimming can also result in the loss of potent resin crystals.

Drying Room Conditions

Whichever method you choose for trimming your product, suitable conditions for your drying room are important. You’ll want a cool, dark environment with temperatures in the range of 59-71°F and humidity at or near 50% . Yes, even after the growing process is completed, you still need to be concerned with temperature and humidity.

The Drying Process

If you opt to use the wet trimming method, you will need to spread them out across a large surface area. Aeration is important so you’ll want to avoid setting the flowers on a surface like cardboard or newspaper as the buds will dry unevenly. Wire mesh or netted drying racks are more beneficial. This allows airflow to reach all sides and lessen the chance of moist air pockets. Make sure to allow yourself enough space to spread your crop out evenly with a small amount of space between buds for the best results.

You may be tempted to speed up the drying process but it has been shown that slow and steady wins the race. Rushing the process and exposing your crop to high temperatures will almost certainly reduce the quality, and result in diminished flavor and even potentially a “poor and uncomfortable high.” Good luck explaining that to your investors or to a discerning dispensary that has numerous growers vying for their shelf space.

Here again is the equipment you can use to help you attain these levels:

  • Air conditioner
  • Dehumidifier
  • Humidifier
  • Portable heater
  • Thermo hygrometer to measure temperature & humidity levels

Check on your crop frequently, approximately every three to seven days. They will become noticeably drier with each check. You’ll know your buds are properly cured once they can snap easily in your hands without any signs of stringiness. Any bendiness when twisting slightly will tell you there’s water still present and that they’re not ready to move to jars yet.

Find the Best Air Cleaner for Your Grow Room

Pure n Natural Can Help

Pure n Natural Systems specializes in the sale of commercial and industrial humidifiers, dehumidifiers, drying fans, air movers, and portable heating and cooling solutions. We have the equipment you need to make your growing operation a consistent success. And as we’ve already discussed, consistency is king.

That said, we are not experts in growing specifics or in the design of custom-growing spaces.

However, if you say you need to remove 50-gallons of water a day from the air in a warehouse setting, we can help!

Indoor Humidity Control for Cannabis Plants

If you want to grow sticky, high-potency buds, your cannabis will demand attention to its many needs.

See also  How does a cannabis plant make seeds from pollen

One need of the growing cannabis plant – that is commonly overlooked – is humidity.

Believe it or not, humidity is actually very important to your plant! Humidity will help determine your plants resilience against mold/mildew in addition to how much your plants need to drink. Getting it just right can improve plant growth and increase your yields.

Luckily, there isn’t much to learn, so you can be the master of humidity in your grow with very little effort! Today we will give you the information you need to have complete mastery over humidity and thus, give your plants a boost in production.

Why Humidity Matters To YOU

In all stages of cannabis growth your plants will have a constant need to intake water, and the amount of water they need fluctuates with the humidity in your grow room. When the humidity is high, cannabis plants use their leaves to absorb moisture from the air which causes them to drink less water from their roots. Conversely, when the humidity is low, they will pull more water in through their roots.

Since humidity changes how much water your plants drink, and the water you give your plants have nutrients in them, being in control of humidity gives you increased control over your plant’s nutrient intake.

But controlling humidity isn’t just about prevention…having the right humidity encourages strong, healthy, leafy plants with vigorous growth. In fact, growing with DWC (Deep Water Culture) / Bubbleponics during vegetation with the correct humidity can actually turn into a struggle to tame out-of-control plant growth!

Additionally, after your plants start making buds in the flowering stage and get close to harvest, you can manipulate the humidity of your grow area to get your plants to produce more resin (trichomes/glitter which contain THC and other cannabinoids) while preventing plants from being attacked by mold. High humidity can sometimes cause mold or bud rot – a grower’s worst nightmare for those fat, dense main colas.

Effects of Poor Humidity Management

We’re not out to scare you, but you should know what happens to a grow room with poor humidity management. We know how important humidity is because we’ve experienced every one of these problems firsthand!

White Powdery Mildew

White Powdery Mildew is a fungal disease that will shows up high humidity environments. This can be tricky since young cannabis plants flourish in humid environments. Luckily, you can stave off WPM by making sure there is proper airflow in your grow area; a small oscillating fan – even on the low setting – works wonders.

Bud Mold or Rot

Bud rot or mold is – in my opinion – one of the worst pains a grower can experience as a result of poor humidity management. Imagine harvesting a huge, dense main cola from your plant only to see the insides are totally white or brown with mold. Buds in this condition are unusable and 100% should be thrown away.

Nutrient Problems

Humidity is a factor that partly determines how much your plants will drink. If the air is dry, your plants will tend to drink more at their roots. If they’re already drinking more due to high temperatures, low humidity can cause them to drink a lot of water through their roots and uptake too-high levels of nutrients. If your plant takes in more nutrients than the plant can use, the leaves will begin to show yellow or burnt tips, which is the result of nutrient burn. Sometimes too-low humidity can cause other apparent nutrient problems.

Slowed Growth

Cannabis plants love higher humidity when they’re young, and lower humidity when they’re mature. To be frank, if they’re not getting these conditions, they may not be growing nearly as fast or healthy as they could.

Humidity Basics

Okay, so now you know why you need to manage humidity when growing cannabis, but you may not know what humidity you are aiming for. This chart is the quick-and-easy answer:

Humidity is actually a measure of how much water vapor is being “held” in the air. There are different ways to measure humidity, including “absolute,” “specific,” or “relative” humidity.

Most growers are talking about Relative Humidity or RH when they talk about humidity in the grow room.

Relative humidity measures how much water is in their air compared to the maximum amount of water that can be held in the air at that temperature.

Below is a quick reference for the major stages of the cannabis life cycle in regards to the relative humidity you are aiming for. We’ll go through them all in more detail


Optimal Humidity is 70% RH

Clones need time to develop a root system to intake water. Young clones are solely dependent on getting water through their leaves from the air and this is only possible with high humidity.

Maintaining high levels of humidity will dramatically increase the chances of clones rooting successfully, and will increase young clone growth.

This is why many growers use a humidity dome for new clones!

Young & Vegetative Plants (seedling to end of vegetative stage)

Optimal Humidity is 40-60% RH

Seedlings and young cannabis plants in the vegetative stage grow a lot of leaf mass in a great growing environment, and plants can sometimes grow foliage at a faster pace than the roots.

But the roots aren’t the only way your plant gets water. Leaves can actually pull water vapor directly out of the air, and higher humidity in the air allows the leaves to get more water to the plant if needed. Until your plant has fully developed roots, low levels of humidity in the air can cause growth to be slower.

The ideas is to create a comfortably warm environment that mimics springtime or summer.

  • Don’t give seedlings a humidity dome unless it’s very dry where you live. They need springtime conditions, but it can be easy to go overboard with seedlings (compared to clones which love high humidity).
  • The RH is too high if the plants are forming wet spots on their leaves even when they’re not touching each other
  • As seedlings get older and become young plants, they want the humidity around 50% RH
  • If the RH drops below 25%, the dry air tends to limit growth and cause nutrient problems to appear on leaves, especially for young seedlings. Some seedlings are fine, but other plants are particularly sensitive to humidity.

Humidity for flowering plants

Optimal Humidity is 40-50% RH

Ideally, the relative humidity of your grow room should be lowered to around 40-50% at the beginning of the flowering stage. As plants approach harvest, some growers lower humidity down below 40% or even less to force cannabis buds to produce more resin, though I’ve seen that can majorly stress some plants, so that technique should be used with caution. Definitely lower the humidity just a little at a time to see how plants react before putting your plants in an extreme environment.

By the beginning of the flowering stage, your cannabis plants have created a large root system to fulfill their water needs. They will still be able to take in water through their leaves, but maintaining a relatively lower humidity than the vegetative stage helps prevent molds or mildews from forming.

Mold is especially dangerous in the flowering stage because it can form on or inside your buds or colas without you knowing. If you find that you have mold growing in any of your buds, or buds are rotting from the inside, you should immediately throw all of the contaminated buds away. Bud rot can ruin entire harvests, so if one bud is infected thoroughly search your grow room for more. If you can’t lower the humidity and already have a rampant mold problem, the best choice might be to cut your losses and harvest immediately to avoid any other buds getting bud rot.

Even if the humidity is low, it is still important to have air moving over and around your plants. Small fans blowing over and under the plant canopy will help keep air moving so that wet spots don’t form around any parts of the plant.

Extra resin production

  • During the last 2-3 weeks before harvest, some growers will use a dehumidifier to drop the humidity of their grow area to between 40-45%. This not only prevents bud rot during the last few weeks, the lower humidity may stress the plant in just the right way to increase resin production. As a result, you harvest extra-potent buds with more sparkly trichomes. While this technique has not yet been proven to increase resin, many growers swear by it. Just be aware that lowering the humidity too much can cause stress to some plants (other strains react well).

Humidity for drying buds (early stage cure)

Optimal Humidity in the Drying Room is 55% RH (for beginners)

Keeping the humidity around the 50% range will allow your buds to dry, but keep them from drying too quickly.

This range is also optimal for preventing mold, so it doesn’t get any better for drying if you’re a beginner!

As an advanced drying and curing technique, some growers like to keep the humidity a little higher so buds dry slower (which could possibly increase the quality of your cured buds). Drying with high humidity is considered an advanced technique because drying must be done with extra care to prevent mold forming on your harvested buds!

It’s good to have air circulation in the drying area, but it’s recommended that no air is being blown directly over the buds to prevent them from drying too quickly. More information in the link below!

Learn how to dry and cure buds for professional results every time (no more guesswork!):

Temperature and How It Relates to Humidity

Temperature and relative humidity are closely related to each other.

Relative humidity is measuring how much water is “being held” in the air compared to the maximum amount of water that can be held at that temperature.

Warm air can “hold onto” more water than cool air. Plants will tend to thrive at different relative humidities depending on the temperature of the air. Once the air becomes too saturated with water, it will tend to form dew or films of water over leaves, which leads to mold.

See also  White rhino feminized cannabis seeds

For those interested in learning more about the science behind this, there is a term used by greenhouse growers known at VPD, or Vapour Pressure Deficit, that roughly measures the temperature and relative humidity.

According to Wikipedia: “the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 kPa.”

When growing plants, temperature is just as important as the Relative Humidity (RH) of the air, if not even more so! You need to control both temperature and RH at the same time to get the best results when growing cannabis.

  • If the air is too hot and dry (high VPD), plants will tend to have stretched growth.
  • If the air is too cool and humid (low VPD), plants are more prone to problems with mold or fungus.

Here’s what that means to you, the grower:

Cannabis likes a comfortable room temperature, or a little warmer – not too dry, not too humid. For most growers, that is all you need to worry about.

If your grow room feels warm or cold, humid or dry, that is a sign that you may want to look into changing the temperature and/or humidity of your grow area.

Generally, cannabis plants prefer temperatures in the 70-80 °F (21-27 °C) range during the day when lights are on. When lights are off (or at night), cannabis plants are happy with slightly cooler temps.

What if it gets too cold?

Colder temps will tend to slow down growth. Temps lower than 60°F (15°C) can upset plant growth and freezing temperatures will shock or even kill a cannabis plant.

Can cannabis stand higher temps? What if my grow room is too hot?

Please note that too-high temps in the flowering stage will not only slow down bud growth and cause fluffier buds, but it will also tend to reduce the potency of your buds by causing some terpenes and cannabinoids to burn off. Keeping the grow room temperature under control is important in the flowering stage!

  • With bright grow lights (such as 600W HPS and larger), cannabis plants can thrive at temps up to 85°F (30°C) even with no CO2 enrichment, as long as there is low relative humidity and plenty of air movement.
  • Cannabis plants can thrive at higher temperatures, up to an ultimate max of about 95°F (35°C), in an environment with low humidity, extremely bright grow lights and enriched CO2 (above 1500 PPM) – this is not your average grow!
  • Cannabis prefers slightly cooler temperature at night (warmer night temperatures will tend to slow down growth – so you can use this knowledge and raise night temperatures to slow down stretching plants that are growing too tall too quickly)

How to Control Humidity in the Grow Room

Step 1: Get a Humidity & Temperature Monitor to see if you need to adjust anything

I like the Ambient Weather WS-07 Wireless Thermo-Hygrometer because it lets you remotely check the temperature and humidity from another room (don’t forget to get 6 AAA batteries).

Step 2: Refer to Chart

Ideal temps are 70-80 °F (21-27 °C) during the day (lights on) and 5-10 degrees cooler at night (lights off).

Refer to the following chart to determine if relative humidity is in the ideal relative humidity range:

Young plants (seedling to end of vegetation)

40-60% RH (if you see wet spots forming on the leaves, humidity is too high or you need to increase air circulation). Beware of white powdery mold in humid grow spaces!

Flowering plants (making buds)

40-50% RH (a little less humidity helps protect plants from mold – during the last few weeks of flowering, a lower RH can possibly increase resin production as buds approach harvest)

Step 3: Correct humidity and/or temperature that is too high or too low

Many indoor growers tend to deal with too much heat and high temperatures – this is because grow lights can get very hot, and cannabis grows are often confined to very tight spaces due to stealth or privacy concerns.

The relative humidity of your grow area depends mostly on where you live, and how you’ve chosen to construct your grow area. Many growers struggle with lowering their humidity, because the mass of leaves and vegetation in a small grow area tends to cause the humidity to go up.

Here’s how you can correct common cannabis problems with heat and/or humidity.



Humidity is too high – how to lower humidity

Dehumidifier – will pull moisture from the air in a small area like a grow room or tent – a good one can be hooked directly to a drain for very humid areas like the UK and other humid parts of the world, otherwise dehumidifiers will automatically shut off when full and you will have to manually remove the water that it collects from the air.

Increase ventilation (add a more powerful exhaust fan or otherwise improve your exhaust system) – this increases the total amount of airflow in the grow area. This strategy will only work to lower humidity if the intake air has a lower RH than the air in the grow room.

Avoid over-watering plants – soggy soil (or leaving extra water in plant trays sitting in the grow area) will tend to raise the humidity of the grow room

Consider slight defoliation – very leafy plants will tend to raise the humidity of the grow room, especially when there isn’t good ventilation or air flow.

Air Conditioner (some are portable, some ACs fit in your window) – tends to lower humidity (in addition to cooling the air)

Humidity is too low – how to increase humidity

Humidifier – will add additional moisture to the air, I like the evaporative type of humidifier because it also works to cool down temps slightly. Make sure you get a humidifier with a large holding tank (holds at least 6 gallons of water at a time). Those “one-room” humdifiers with a 1.5 gallon tank often need to be refilled several times a day.

This humidifier is an evaporative swamp cooler that holds 7 gallons of water at a time – great for hot, dry climates.

Swamp cooler – will cool down air while increasing the relative humidity; works best when humidity is too low and temperature is too high. Read how one user used a swamp cooler to control his humidity.

Temperature is too high (too hot) – how to lower temperature & cool things down

Increase ventilation (add a more powerful exhaust fan) – this increases the total amount of airflow in the grow area. This strategy will only work to lower temperature if the intake air has a lower temperature than the air in the grow room (if the air outside is hot, too, adding more ventilation won’t do much to lower the temperature)

Air Conditioner (some are portable, some ACs fit in your window) – in addition to cooling the air, an AC will also tend to lower the relative humidity of the air

Light Schedule – Consider having your lights turn on during the cooler parts of the day (for example some growers have their lights on at night, and keep lights off during the hottest parts of the day)

Temperature is too low (too cold) – how to raise temperature & heat things up

Insulation – Consider insulating your grow box or grow tent with something that holds in the heat

Bigger grow light – Get a bigger grow light to produce more heat within the grow space (and get plants to grow faster!)

Monitor temperature & humidity in your grow room!

I like the Ambient Weather WS-07 Wireless Thermo-Hygrometer because it lets you remotely check the temperature and humidity from another room (don’t forget to get 6 AAA batteries).

Monitor temperature & humidity even while you’re away!

Here’s a really simple solution that lets you check your temperature and humidity of your grow room in real-time, even if you’re not at home!

Buy an indoor/outdoor temperature+humidity sensor and display, such as the model listed above

Put the “outdoor” module in your grow tent or box.

Put the “indoor” display in front of a webcam (you may have one built into your laptop, or buy a cheap and easy-to-use one like the Logitech HD Webcam C310)

You could also put the “indoor” display near your desk for easy viewing, and then point a webcam at the display.

Install TeamViewer (it’s free for personal use) or you can also use Windows Remote Assistance (not as user friendly in the beginning, but a good choice).

You can now log into your computer from any other computer, open the program for the webcam, and see what your grow room stats are in real-time.

Thanks to Tokiehardo for this awesome tip!

Too Hot, Too Dry – Humidity Story from a Grower in Denver

I’m preparing things for my first grow and trying to make sure that I plan everything ‘right’ and I’m not surprised by too many things, and hopefully none too late to fix.

Humidity has me confused. I’ve seen sources saying the greater the humidity the better, some that say humidity makes no difference as long as the plant is hydrated, and some claiming nothing will grow if you’re not nailed at 45%.

The ambient humidity near Denver is pretty low – the room I plan to grow in sits at 20% on average. Is this too low for my plants to be happy?

This humidity is too low for healthy growth of cannabis plants. This grower decied to build something known as a swamp cooler: swampcooler.html

It moistens the air while keeping the temperature down, which can also be a problem in Denver.

Update from the grower: For anyone stumbling onto this in the future, the swamp cooler in the link above is friggin awesome. I have it connected to my intake ducting and it’s keeping the cabinet at 73F with 80F outside temps! I cannot downplay how great it works, but I recommend creating a way bigger reservoir, so you don’t have to refill it every 6-8 hours.