The environmental conditions we grow our plants in, has a large impact on their efficiency, morphology and health. In this article, we go over several environment characteristics you should control, to be a more productive and efficient grower.

In this article we cover:

• Temperature: Learn the optimal temperature range for growing cannabis and how temperature influences plant health and performance.
• Humidity: Learn the optimal humidity ranges for growing cannabis and how humidity impacts plants.
• Airflow: Understand why air movement is important and what happens if there is not sufficient airflow.
• Air Quality: Learn why fresh air is required and how important carbon dioxide is for sustained healthy growth.

Temperature

Temperature impacts several aspects of plant processes; however, the primary concern is its effect on photosynthesis. Temperature does not affect the light dependent reactions. However, the enzyme activity in the light independent reactions is highly influenced by temperature. Low temperatures naturally slow down the chemical reactions of enzymes and reduce photosynthesis. High temperatures make enzymes lose their shape and functionality which also slows down photosynthesis. To get the most out of your plants, it is ideal to control the temperature of your environment so that enzymatic processes do not hold back the critical systems responsible for photosynthesis.

Species of plants have different optimal levels of temperature because of evolution, genetics and environmental conditioning. Chandra et al. conducted a series of tests on cannabis in 2008 and 2009, which revealed excellent data on the ideal temperatures for photosynthesis. This research has shown that the ambient temperature is best between 75°- 83°F (24°- 28°C). Above 86°F (30°C), enzymes lose their shape and functionality. If temperatures drop below 75°F (24°C), enzymes do not operate efficiently.

Humidity

Stomata on leaves control the exchange of gasses during transpiration. They essentially are like gates on the underside of leaves which open and close to regulate the exchange of oxygen, carbon dioxide and water between the plant and the surrounding air. Stomata are influenced by several factors including temperature, light and humidity. When days are hot, and humidity is low, the stomata will be open to enable high transpiration. However, when this becomes too extreme, the plant will close the stomata to prevent dehydration. This shuts down photosynthesis and slows down plant growth as a result. Conversely, when days are cold and humidity is high, stomata will stay open. However, in this case the potential for transpiration is low because the humid air has very little room for any more water.

Transpiration in plants is important because plants require water to maintain their turgidity and cell flexibility. Transpiration also is critical to keep plants cool in hot weather. In addition, the water that is transported from the roots and out through the leaves serves as a vehicle to carry key minerals and nutrients from the ground to the rest of the organism.

Relative Humidity and Vapor Pressure Deficit

Relative humidity, measures the amount of water vapor currently in the air relative to the maximum amount of water that can be held at that specific temperature. It is different than absolute humidity, which is a measure of the total amount of water vapor in the air. Relative humidity is represented as a percentage and for most applications in horticulture it is the variable that we try to control.

Vapor Pressure Deficit (VPD) is actually a more accurate metric for determining water loss in plants. However, when you are able to maintain grow room temperatures within the ideal range (75°- 83°F, 24°- 28°C), you can achieve the ideal VPD simply by maintaining the relative humidity within the recommended range for each stage of growth. If you are unable to control temperatures within the ideal range, then you should consider VPD for determining the relative humidity that will produce ideal transpiration and growth potential. Look for our article “The Vapor Pressure Deficit”, coming soon.

Ideal Relative Humidity Ranges for Cannabis

The relative humidity that is best for cannabis plants varies over the course of their life cycle. The best relative humidity for germination and early vegetative growth is between 75-95%. This is because young plants typically have small root systems offering little support for transpiration. Providing high humidity allows for a young plant time to develop without having to cope with water stress from over transpiration.

The best relative humidity for the main vegetative growth is between 60-75%. This level of humidity makes the stomata open wider and stay open longer, allowing for more gas exchanges for photosynthesis. It also prevents over/under transpiration and controls nutrient uptake by maintaining ideal levels of fluid transport within the plant.

During flowering, it is often best to lower relative humidity to around 50-60%. This is done to mitigate the risks of pathogens such as powdery mildew, mold and rot. Lower relative humidity is advised during the flowering period only to prevent pathogens, it does not provide other benefits. Despite what many believe, low humidity during the flowering period will not improve the quantity or quality of your buds. In fact, if you are well practiced, you can keep humidity higher to improve a plants growth and performance. However, because of pathogens, we recommend 50-60%.

Airflow

Plants require airflow because all leaves develop a microclimate that is independent of the ambient climate that a plant is in. This phenomenon is called the boundary layer resistance and is largely influenced by air movement. Static or non-moving air, creates large boundary layers. This produces a microclimate around the plant that is increasingly different from the surrounding air. The boundary layer influences how quickly gasses and energy can be exchanged. The boundary layer resistance can become thick enough to impede CO2/O2 exchange, transpiration and thus photosynthesis and plant growth. As such it is important for any indoor environments to have at least some airflow to reduce the boundary layer resistance.

Air Quality

Plants require fresh Carbon dioxide to continue with photosynthesis, when plants utilize CO2, the concentration of CO2 in the surrounding air reduces, lowering the potential for a plant to continue with the same level of photosynthesis. To make sure that the ambient level of CO2 concentration is maintained, fresh ambient air, or air enriched with CO2 must be introduced. Fresh air, either from outside or indoors is a good source of air replacement. You can also get compressed tanks filled with CO2 which can help boost the natural levels of CO2 even higher and improve photosynthesis and yield. For more information on CO2, look for our tutorial, “When and How to Use Supplemental Co2”, coming soon!