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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.

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Final results of an indoor grow are greatly influenced by the way growers keep in control of parameters that influence their plants growth. There are two basic factors that can easily be forgotten when we’re busy thinking of other ways to increase yields, size, and overall health of our plants – temperature & relative humidity. This blog summarizes ways to keep both of these factors within an optimum range, and provides specific information what conditions should be maintained to achieve best results.

HOW TEMPERATURES AND HUMIDITY LEVELS INTERACT

It’s important to know that humidity levels and temperatures are closely related to one another. When we talk about humidity, we usually mean relative humidity (RH), which is the ratio of partial pressure of water vapor to the maximum vapor pressure of water at the same temperature. You get the whole idea when knowing the basic principle that warm air holds more water vapor than cold air. This is one of the reasons why it’s necessary to extract a lot of warm air from our grow room, and ideally allow cool air to enter – warm air simply holds too much water vapor in it.

HUMIDITY LEVELS AND TEMPERATURES: FROM SEEDLING TO HARVEST

We need to define what humidity and temperature control actually means when growing cannabis. It makes sense to divide the life of cannabis plants into 4 different stages in which humidity levels, and temperatures, should be adjusted to ensure healthy growth. Don’t think that humidity and temperature control is complicated and not worth it! It’s generally very easy, and more about keeping parameters within a certain range, and as constant as possible.

The first thing you need to do is to buy a hygrometer and thermometer, preferably a digital one with memory function, also showing maximum and minimum values of the past. Some hygrometers aren’t the most accurate, so don’t bother having several devices in your grow room to compare values. Now that we’re able to closely monitor our conditions, we can get to the essence of humidity and temperature control – the actual humidity levels and temperatures we aim for.

1. Seedling Stage

  • Seedlings and clones like high humidity levels of 65-70%
  • Reason: The root system is not established
  • High humidity levels allow water intake through leaves
  • Temperatures with lights on: 20-25 C° (lights off: 4-5 C° lower)

2. Vegetation Period

  • Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
  • Temperatures can be increased a little bit (no obligation)
  • Reason: Roots absorb more water; evaporation through leaves cools plant(s)
  • Temperatures with lights on: 22-28 C° (lights off: 4-5 C° lower)

3. Flowering Period

  • Humidity levels need to be lowered to 40-50% (extremely important)
  • You can get away with 55% (anything over 60% is real bad)
  • It’s best to slightly lower temperatures in flowering
  • Temperatures with lights on: 20-26 C° (avoid high temperatures)

4. Late flowering (1-2 weeks before harvest)

  • The following steps are no necessity, but can improve yield, flavour and appearance
  • Bring down humidity levels as much as you can: 30-40%
  • Lower daytime temperatures, and also increase the temperature difference (day/night)
  • Temperatures with lights on: 18-24 °C (lights off: minus 5-10 C°)

ADJUSTING HUMIDTY LEVELS AND TEMPERATURES

We’ve got a pretty good idea on humidity levels and temperatures we aim for. Now it’s time to get to the practical part, and to find ways to bring things back in balance when they’re not. Most growers will struggle to keep both relative humidity and temperatures down, which is of primary importance in the flowering period – we got that. In some colder regions, and depending on the lighting solution, the opposite scenario might be the case, and temperatures or humidity levels must be raised.

Remember the basic principle that warm air holds more water than cold air? Keep this in mind, and be aware of the fact that relative humidity and temperatures interact with one another.

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

What Is The Best Humidity For Growing Cannabis Plants?

Cannabis is a high-value plant that has come to be primarily cultivated indoors due to its high sensitivity. Each stage of cannabis growth requires a specific kind of tune-up to achieve optimum growth and obtain high-quality yields. Humidity not only plays a roll in the growth of this incredible plant, but also in the proper curing of cannabis, and in proper storage containers and conditions while in storage.

While many cannabis growers identify sensitivity to the condition of the soil and the nutrients available for the plant, it’s environmental conditions that really matter. One of the environmental factors that affect the growth of cannabis is humidity.

Humidity can be defined as the amount of water vapor in the air. Generally, there are three types of humidity:

  • Specific humidity – The specific humidity of an area is the ratio of the amount of water vapor to dry air.
  • Absolute humidity – This is the actual amount of water vapor present in a given area.
  • Relative humidity – This is the ratio of the actual amount of water vapor present in an area compared to the speculated maximum amount of water vapor that the area can hold.

In cannabis growth, as with any other plant, the relative humidity is used. It is also worth noting that humidity and temperature are interrelated. The amount of water vapor that air in an area can hold increases with increasing temperature.

What Factors Affect Relative Humidity Control For Cannabis?

There are three basic factors that weigh heavily in determining the correct relative humidity when growing cannabis plants. They are region, genetics, and stages of growth. Although there are lessor and differing situations, these three have the greatest impact.

Region

Cold regions have less moist air due to the temperature relation stated above. Growing cannabis in these areas means that the relative humidity must be increased to an optimum level. On the other hand, hot or tropical regions require less control of humidity as most cannabis plants are highly favored by relatively higher humidity.

Genetics

A good number of cannabis varieties have been developed over the years. Some cannabis species are adapted to warmer and more humid areas while others are adapted to cold and less humid areas.

Growing Stages

From germination to late flowering, the cannabis plants call for changes in the humidity levels. Since each stage comes with distinct metabolic reactions, the relative humidity needs also vary.

Why Is Humidity Important For Cannabis Plants?

To understand how humidity affects the growth of cannabis, let’s first understand the plant’s respiration process.

The sugars produced in the photosynthesis process must be converted to energy used to support plants’ growth. To convert the sugars, mainly glucose to energy and oxygen, later released to the environment, the cannabis plant must acquire carbon dioxide from the environment.

The carbon dioxide enters the plant via the stomata; tiny pores on the leaves. When this process occurs, some water from the plant’s water reservoir is lost to the environment. Relative humidity kicks in here as it dictates how much water the cannabis plant will lose to the environment.

Two scenarios can occur:

When the air is moist (high relative humidity)

The water concentration gradient is significantly reduced due to the higher amount of water vapor present in the air. This means that the cannabis plant will lose less water and hence remain in its best shape.

When the air is dry (low relative humidity)

The high water gradient created between the plant and the surrounding air causes high water loss. The marijuana plant will try to prevent this by closing the stomata, a process that compromises the vital respiration process.

In summary, relative humidity is vital to cannabis plants as it regulates the respiration process and its subsidiary activities such as water loss. For a cannabis plant to benefit fully from relative humidity, it must be maintained at an optimum level. It will also help if the humidity is also kept constant for an extended period.

Should Humidity Be Low Or High For Cannabis Plants?

Cannabis plants are grown in a relative humidity range of 70-40%. This is neither too high nor too low humidity. As discussed above, both extremes of relative humidity are potential causes of cannabis plant stagnation. It is also good to understand that high humidity may support the growth of mold that may affect the growth of your marijuana plants.

What Humidity Should I Have In My Grow Tent?

When you cultivate cannabis in a grow tent, conditions such as humidity and temperatures must be appropriately controlled. While by now we know that different growing stages of the cannabis plant require different levels of humidity, it would help if you kept the humidity in your grow tent relatively high for cannabis seedlings. Several factors affect the amount of humidity in your cannabis grow tent. These include:

Lighting

If the lights of your grow tent are always on, the temperature of the contained air is raised and hence the relative humidity. Poorly lit grow rooms have cold and dry air (low humidity).

Ventilation

If your grow tent is adequately ventilated, the relative humidity is reduced due to increased air circulation. Ventilation is required for mature cannabis plants that require low relative humidity.

Frequency of Watering

Watering cannabis plants in a grow tent increases the plant’s water intake through the roots. High uptake of water by the plant increases the water loss rate through the leaves, making the room more humid. The induced moist environment, in turn, regulates the respiration process keeping the marijuana plant healthy. Again, watering should be optimized to maintain the grow tent in the recommended level of humidity.

What Humidity Is Good For Cannabis Flowering?

The recommended relative humidity for a flowering cannabis plant is 40-50%. The moisture is reduced to this level mainly to prevent the growth of mold that is very vicious at this stage.

Did you know that mold infestation on one cannabis flower can ruin your entire harvest? Lowering the humidity in this stage is, therefore very crucial and should not be overlooked. The humidity can also be reduced below 40% when the cannabis is close to harvest. This, however, depends on how your cannabis plants respond to changes in humidity.

In conclusion, it can be said that humidity is an essential condition in the growth of cannabis plants. Always start with a high relative humidity of about 70% for your cannabis seedlings and reduce the moisture gradually until the plant is ready for harvest, where the humidity should be around 40%.

Luckily, humidity levels in cannabis growing spaces can be detected easily using manual or automatic hygrometers. If your cannabis plants are in a grow tent, there is also an advantage of automatic reduction of humidity levels. Commercial size dehumidifiers and fans are used for this purpose.

Cannabis is a high-value plant that has come to be primarily cultivated indoors due to its high sensitivity. Each stage of cannabis growth requires a specific kind of tune-up to achieve optimum growth and obtain high-quality yields. Humidity not only plays a roll in the growth of this incredible plant, but also in the