The Importance of Temperature, Humidity and CO2 when Growing Cannabis

The Importance of Temperature, Humidity and CO2 when Growing Cannabis

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Growing Cannabis indoors requires a balance of temperature, humidity, air flow, light intensity and fresh carbon dioxide air. As a beginner grower, not knowing how Cannabis plants function and the pressure placed on them to transpire water, can make a huge difference in the overall plant health, nutrient uptake and overall crop quality. In this article, we explain how important temperature and humidity is when growing Cannabis, how CO2 is relative and problems that can arise for a beginner level grower, and how to solve them.

The optimal environment for growing Cannabis indoors
Those who are new to growing Cannabis indoors can often find tutorials and guides online regarding the optimal setting for your indoor grow room. Almost all references to indoor cultivation point to a light on temperature of 24 degrees Celsius, with a 5-6 degree decrease to between 18-19 degrees Celsius when the lights are off.

During the vegetative stage when the lights are set to 18 hours per day, Cannabis plants indoors require an environment which mimics the warm and humid Summer months. During this time when growing Cannabis indoors, the relative humidity levels present in the grow room should measure between 65%-80% relative humidity, especially when supplementing your plants with carbon dioxide.

Once the plants are flowering under 12 hours of light, the ideal humidity levels that should be present will be significantly less. Towards the final 4–5 weeks of bud production, you should aim to have the relative humidity around 30-45% and aim for a dry but cool setting.

How does using Co2 when growing Cannabis affect temperature and humidity?
Outside, there is around 400 parts per million of CO2 available, which means when pulling in fresh air from outside to your indoor grow room, the plants will be able to metabolize and photosynthesize at a set rate. Cannabis plants, however, are able to utilize far greater levels of carbon dioxide, ranging between 1000–1500 parts per million. This is often why huge indoor plants, grown with CO2, produce such heavy harvests and top shelf buds.

One thing that you must know if you are considering using CO2 tanks for your indoor grow room, and that is that the stomata on the underside of the leaves prefer high humidity levels, combined with hot temperatures of up to 30 degrees Celsius. This means that in order to provide optimal levels of CO2, you will have to exceed the comfort zone of 24 degrees and produce an environment which is asking for mold or powdery mildew, or worse case, a nasty spider mite infestation. Plants can become stressed in this environment and, without adequate airflow, can cause dry leaves and for plant health to slowly decline.


● Stomata open up 60 minutes after the lights are on, so wait for one hour before turning on CO2.

● CO2 is a heavy and thick gas that needs to descend on top of the plants from an air hose.

● It is best to use CO2 during early veg and early flowering for the best results and to avoid heat stress.

● Cannabis plants indoors will use more nutrients and uptake more light due to an increase in CO2.

● Carbon dioxide has been proven to increase yields significantly, as well as size and appearance.

● Be vigilant and check daily for signs of pathogens when growing Cannabis at high temperatures.


What equipment will I need to check the environment?
As a beginner grower who wants to become as professional as possible, investing in a digital hygrometer will be the best way of having accurate readings 24 hours a day. Hygrometers are not too expensive and rely on a sensor that is hung inside the grow room close to the canopy. What is great about hygrometers when growing Cannabis indoors, is you will be able to see a low / high setting at the press of a button, allowing you to see the difference in temperature over a 24-hour duration.

The problems that can arise from inconsistent temperature and humidity levels.
There are many issues that may arise which are temperature and humidity related. Ensuring that your indoor garden remains at a consistent temperature of 24 degrees with a fresh air source and optimum range of humidity is not always easy, so below are some of the common mistakes a beginner grower can make when growing Cannabis indoors.

1. Low temperatures and high humidity

Cannabis plants do not perform well in a cold environment, regardless of indoor or outside. A grow room where the air source outside is so cold, the room becomes cold and full of moisture. Healthy Cannabis plants will begin to droop, reduce nutrient uptake, and may even become stunted when the temperature drops below 16 degrees Celsius. In this environment, you will begin to see a spread of powdery mildew, which will cover the leaves with a white powdery residue.

2. Low temperature and low humidity

A cold grow room that is dry due to low levels of humidity will cause Cannabis plants during the vegetative stage to grow slowly, with crispy and dry leaves. The roots and growing medium will become cold, and any plants flowering in these circumstances will produce a small amount of buds that feel dry and crispy to touch. Over time, the entire plant will become sick and, depending on how much air flow is present, may determine the level of stress endured.

3. High temperature and low humidity

This can be the sweet spot if you are well experienced, are using CO2 and around week 4 of flowering. Otherwise, attempting to vegetative healthy plants with low humidity levels, will severely restrict the amount of CO2 that plants are able to uptake, and will only cost you time and money if supplementing the room with CO2 tanks. The roots zone will not be as well-developed as when grown with higher humidity levels, which may ultimately affect yields.

How to fix temperature and humidity issues indoors
Humidifiers - These are devices which are placed inside the grow room, and produce cool vapor and mist that increases the amount of humidity in the room. Humidifiers come in different sizes depending on your requirements and are an excellent solution when growing Cannabis indoors.

Dehumidifier - For those who suffer with high humidity levels due to where they live, will find relief with a dehumidifier. These quite expensive machines will pull the moisture out of the air and overtime fill the tank inside with distilled water. Dehumidifiers are a must-have during rainy reasons, greenhouses late in the year and for drying Cannabis.

Oscillating fans - Keeping a fresh current of air blowing around the bottom, middle and top parts of the canopy is essential for optimal plant health. In the event you find molding buds, use a clear and sterile pair of scissors and cleanly cut away the entire parts which are infected.

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