Whether you're a home growing enthusiast or aspiring to the rank of Master Grower, cultivating cannabis demands the ability to multitask, pivot on a dime, and pay keen attention to detail. Between deciding which cultivars to grow, what substrate to use, where to grow, which nutrients to use, and much more, humidity might not be something on a novice grower's mind. However, it's something experienced growers track quite closely.
Along with light, temperature, and water, humidity is one of the most important environmental factors impacting your plant's health, development, and yield come harvest day.
Below, find out the hows and whys of humidity and cannabis, guidelines for each phase of the life cycle, and answers to frequently asked questions surrounding humidity.
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Why is humidity important when growing cannabis?
Humidity is a measure of the amount of water vapor in the air. It can be measured in three ways: specific humidity, absolute humidity, and relative humidity. The one that is most easily measured and most commonly used in cultivation is relative humidity (RH).
Relative humidity is the ratio of water vapor present in the air versus the maximum amount of water vapor the air in a given area can hold at a given temperature.
Temperature is an important part of the equation since humidity and temperature are closely related. The amount of water vapor that the air in a given area can hold will increase or decrease depending on temperature. The relationship between relative humidity and temperature is inverse: relative humidity drops when the temperature rises and increases when the temperature falls, assuming the water content in the air remains constant.
Humidity is an environmental variable that impacts all plants, including cannabis. Importantly, it impacts two vital functions of the plant: transpiration and photosynthesis. Humidity directly affects pores on the leaf surface called stomata, causing them to open or close in response to humidity levels. These pores are important for both the release of water vapor — a process called transpiration — and for taking in carbon dioxide, which is needed for photosynthesis.
When humidity is too high or too low, transpiration and photosynthesis, along with myriad downstream processes that are related to these two vital functions, are compromised, which will in turn compromise the growth and vitality of your plants.
Not only is humidity important for plant health, but humidity levels are also important for controlling pests, diseases, and mold since certain insects, fungi, and viruses and viroids thrive in moist conditions.
Humidity can also impact the nutrient levels in your growing medium and how much water and nutrients your plant actually takes up from the medium, too.
Humidity for each stage of the life cycle
The cannabis plant goes through different phases of growth and development in its life cycle. As the plant evolves from a young seedling or clone to its vegetative and final flowering stages, so too evolve its needs for light, nutrients, and water, among other considerations.
Since humidity plays an important part in plant health throughout the life cycle, a grower can adjust humidity at each stage to meet the plant's needs and optimize its growth. Generally speaking, humidity levels are highest when cannabis plants are young and will gradually decrease throughout the life cycle, reaching the lowest point during flowering.
Note that these are general guidelines based on industry best practices and that many growers will evaluate optimal humidity levels for each individual cultivar they grow and the environment they are growing in by observing and tweaking over the course of several crops.
Seedling and young clone phase
Seedlings and new clones do not have robust and established root systems from which to draw the water they need, so higher humidity levels help the plants take in water through their leaves.
Ideal humidity range: High, 75 - 90%
Vegetative phase
During the vegetative phase, the plant's root system is becoming fully established, and its energy is dedicated to growth and developing leaves. Humidity can be gradually decreased incrementally week by week to reach the level you will use while the plant is in flower.
Ideal humidity range: Moderate to low, 70% at the beginning of veg, down to ~40% by flower
Flowering phase
During this phase, the plant stops growing stems and leaves, instead focusing its energy on flower production. Lower humidity levels are ideal during this phase to minimize the chance of bud rot (botrytis). Growers often like to decrease humidity levels even further during the late stages of flowering.
Ideal humidity range: Low, 40 - 50% during early flower, 30 - 40% during late flower
Drying and curing phase
Even after harvest, humidity remains a concern when drying and curing your flower. First, the harvested plants need to be dried. Then, the manicured buds are further dried in a process called curing.
Ideal humidity range: Moderate, drying 50%, curing 55 - 62%
How to adjust humidity levels
If you're growing outside, there isn't much you can do to control humidity, but when growing indoors or in a greenhouse, there are several variables you can tweak to adjust the humidity in your growing environment.
First, you will need a way of measuring humidity. In complex commercial growing operations, the facility will often be equipped with very advanced and sensitive climate control systems that will continuously monitor relative humidity and even make environmental adjustments as needed.
For simpler grow rooms, a hygrometer is a must-have tool that measures relative humidity and air temperature. In fact, it's best to have at least three of these tools that can be placed at the high, mid, and low points of the grow room.
Once you can measure the humidity in the grow room, think about the different factors that impact the humidity of the space, including:
- Region/climate: If your grow room is not completely sealed and insulated, the climate you're in will impact your humidity. Likewise, air exchange systems that pull in fresh air from outdoors will have an impact.
- Lighting: Some lighting systems give off more heat than others — HPS setups versus LED systems, for example. Lighting can significantly impact the temperature in the grow room. Because of these impacts on temperature, the humidity will change when lights go on and when they go off.
- Ventilation and airflow: Air circulation and exchange are crucial for environmental control and healthy plants, but circulating air from fans and your air exchange system will impact both temperature and humidity.
- Temperature control system: While air conditioning and/or heating units have an obvious impact on temperature, they will also impact humidity.
- Level of drainage: Standing water is not only inviting the potential for pests and diseases, but it will increase humidity.
- Plant transpiration: Plants are always transpiring, which increases humidity.
- Watering schedule: Watering your crop will increase humidity, and overwatering can create consistently elevated humidity while inviting other problems for your plant, which is why it's important to understand how to water weed plants.
All of these variables will work in concert to impact humidity, and tweaking any one of them — or all of them — might be needed to optimize the humidity levels in your growing environment.
Keep in mind that humidity levels will always fluctuate in the climate of your grow room, and it will be impossible to maintain a target temperature and humidity level exactly for extended periods of time. Aim for the benchmarks provided and ensure you stay within these ranges over time.
How to increase humidity levels
You can increase relative humidity levels by adjusting either the amount of water vapor in the air or by tweaking the temperature:
- Mist the grow room, including keeping topsoil moist
- Containers of water and/or wet sponges, towels, etc
- Decrease the level of exhaust
- Lower temperature
- Use a properly sized humidifier
While misting and tweaking exhaust and temperature offer quick and easy ways to temporarily increase humidity, investing in a properly sized humidifier/dehumidifier system is the best way to maintain consistent humidity levels.
How to decrease humidity levels
Likewise, you can adjust the vapor levels and temperature to decrease relative humidity:
- Increase temperature
- Increase the level of exhaust
- Increase cool airflow
- Water plants at the start of the light period so moisture evaporates during the day
- Use a properly sized dehumidifier
Air conditioning systems can act as dehumidifiers when you also need to cool the grow room during lights on, but again, the best way to decrease and maintain proper humidity is with dedicated humidifier/dehumidifier systems.
FAQs about humidity for weed plants
Below, find all the answers to your humidity questions.
Bottom line
Humidity plays an important role in the health and development of cannabis plants. By understanding how your plants respond to humidity, and how optimal humidity levels change as your plant develops, you can set yourself up for the best chances at success in your next grow.
