Are you ruining the quality of your bud by ignoring transpiration rates?
Transpiration: How Cannabis Sweats
Water movement within a plant is driven by evaporation of water at the leaf surface. This water loss is called transpiration. Relative humidity (RH), vapour pressure deficit (VPD), and the temperature of air at and above the plant canopy will influence how much water a plant loses through transpiration.
In order to monitor these factors, growers will often use sensors and thermometers that can be linked to a control system that alerts growers when conditions are outside of the ideal range. Some systems, like GrowerIQ, can even automatically activate equipment to adjust climate control system to maintain ideal environmental conditions.
Since fluctuation of environmental conditions can influence potency of cannabis, it is important for environmental conditions to be well maintained. If the plant experiences too much water loss from transpiration, it could ruin the quality of the bud.
What is the Best Temperature and Relative Humidity for Cannabis?
In cannabis, ideal environmental conditions and water needs will vary depending on the growth stage. For indoor growing, 25°C to 30°C is ideal for most varieties of cannabis through most of its lifecycle. For juvenile cannabis plants, relative humidity around 75% is ideal. As the plant matures into the vegetative and floral growth stages, 55% to 60% relative humidity is recommended.
For indoor growing, 25°C to 30°C is ideal for most varieties of cannabis through most of its lifecycle.
Basic Water Needs of Cannabis
Other factors influencing water needs need to be considered in addition to temperature, humidity, and transpiration rates. The water needs of cannabis will be influenced by temperature and relative humidity, but also depend on the growth medium, container size, and growth stage. Cannabis will have the greatest need for water during floral growth since that is the time when the plant is working hardest to produce buds.
How does transpiration work?
In transpiration, water is transported up the xylem from the roots by passive forces. From the xylem, the water moves up into the cells inside the leaves. The plant's leaves have pores called stomata. Water diffuses through the stomata and evaporates into the air. Water loss of transpiration is a necessary part of plant metabolism because the stomata must open to let out carbon dioxide that is produced as a waste product of photosynthesis.
Of the water given to a plant, 97 to 99.5% is lost to transpiration and guttation, a process where plants exude water droplets. Plants regulate transpiration by opening and closing stomata, but transpiration is also influenced by environmental conditions around the leaf. If conditions are too warm, the plant will lose water to help cool itself, much like when humans sweat.
Other environmental conditions also play a large role in how much water will be lost through transpiration. Here are some of the factors and effect that influence transpiration:
Number and Size of Leaves
More leaves and larger leaves means more surface area for gas exchange, resulting in greater water loss. This factor should encourage growers to practice good defoliation when growing cannabis because large fan leaves not only block light, they also contribute to more water loss through transpiration.
Number of Stomata
Some plants have more stomata than others, plants with more stomata will have higher demand for water because there are more pores allowing it to leave the plant.
Trichomes are tiny hair-like structures that are abundant on the cannabis plant. Trichomes on cannabis are the source of cannabinoids, but they also help prevent water loss by holding humidity at the leaf’s surface.
Stomata generally open at night to let out carbon dioxide when the environment is cooler. If stomata open during the day, more water is lost because of the heat that is associated with light. Transpiration will increase when the PPFD (or the amount of PAR that actually arrives at the plant) of the grow light increases, so it is important to consider that stronger lighting could increase the need for irrigation to make up for the increased water loss from transpiration.
Temperature and Relative Humidity
When temperature rises, evaporation rates will rise and plants will lose water faster. Drier conditions with lower relative humidity also increase the rate of transpiration.
Unless you are growing cannabis outdoors, you may not think of the effect of wind on transpiration, but indoor grows and greenhouses may have ventilation equipment like fans that have similar effects. Wind will move water vapour away from the area on and around the plant causing plants to transpire more water. Circulating fans will help moisture, temperature, and humidity be more even and consistent, but directional fans can have detrimental effects to transpiration when they are moving air at high velocities.
Drought stress occurs when a plant is not provided with sufficient irrigation, or when it is losing too much water to transpiration. When plants are drought stressed they use adaptive measures to try to preserve themselves. This may mean that they close stomata and have carbon dioxide build up inside them.
Drought stress can also stimulate plants to produce more secondary metabolites. These metabolites are accessories to nutrient production and often have defensive purposes. In the case of cannabis, it is thought that a slight degree of drought stress may stimulate cannabinoid production since cannabinoids are secondary metabolites.
How to Measure Environmental Factors
If you are wondering what "normal" transpiration rates look like, the answer would be that they would remain steady relative to environmental conditions, lighting, and irrigation frequency. So long as all of these factors are balanced, the transpiration rate of the plant will be balanced.
Most growers do not take the time to compute vapour pressure deficit and transpiration rates for their cannabis crops. That sort of information may be useful when designing a new facility to ensure ventilation is adequate, or if crops are not doing well. What is more likely is that growers will use devices like thermometers, psychrometers, and automated sensors to ensure that grow room conditions remain stable.
Special devices called potometers, lysimeters, and porometers can be used to measure transpiration directly, but are not as practical as the trained eye of a master grower who will have the hands on knowledge of how cannabis varieties respond better or worse to different environmental set points.
Most growers do not take the time to compute vapour pressure deficit and transpiration rates for their cannabis crops.
Understanding cannabis transpiration is an important part of understanding what makes for a successful grow. The process can be complicated however, which is why it's so important to consult with an experienced Master Grower. For more information, and actionable tips from our Master Grower on how to improve your yields and processes, please visit our Seed for Success consulting program.
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