Have a read. There is lots out there you just need to look past the mountain of old info. The reason ppl think 40% o think is due to not understanding how significant that humidity spike can be at lights out. This is why VPD got a bad rap from those ppl with the same view. That and it was largely used improperly.
By Mark June-Wells, Ph.D.
What Can Happen When Humidity Is Unbalanced?
Plants acquire carbon dioxide from the atmosphere in which they are growing; this requires that the plants open their stomata and capture carbon dioxide. Opening the stomata also causes water to be drawn up through the plant from roots to shoots to the leaves and, ultimately, the atmosphere. This process causes the plant to acquire water from the soil medium, which is laden with nutrients. Therefore, the following can occur if humidity is not balanced correctly:
1. With very low humidity, the plant is drawing water from the soil at a very high rate, and if the humidity is too low, the plant is unable to draw water at a rate equal to loss through the stomatal openings. The result is that the plants close their stomata, which slows the photosynthetic process (due to carbon limitations) and leads to stress, slow growth and compromised yield. Under severely low humidity, the plant will wilt and die because even its protective mechanisms cannot offset the water stress imposed by the lack of atmospheric water.
2. When humidity is too high, the rate at which plants draw water from the soil is reduced because transpiration is slowed by the smaller-than-normal water gradient between the plant and the atmosphere. This can result in diminished nutrient uptake efficiency, which can lead to nutrient deficiencies. This phenomenon is particularly evident in the uptake of calcium.
Additionally, saturated media pH has a tendency to climb over time, which can also result in many nutrients (e.g., metals, phosphorus and calcium) becoming unavailable to the plant because of altered chemical interactions under elevated pH conditions.
What Are Optimal Humidity Levels?
While cannabis cultivation research is a growing field, scientific evidence characterizing optimal growing conditions has not been fully explored. Fortunately, a vast body of research exists in the areas of indoor and greenhouse cultivation of many food species. Some of these food crops are suitable surrogates for determining potential cannabis cultivation conditions because of similarities in natural, physiological and environmental requirements.
The general humidity range for indoor cultivation appears to exist between 60% and 75% for all growth stages. When humidity surpasses 80%, particularly during the lights-off phase, a higher risk of fungal, bacterial and viral infections exists. Conversely, if humidity drops much below 50%, crop photosynthetic efficiency may be affected and yield will suffer.
Based on peer-reviewed research in the field of indoor/greenhouse produce cultivation, the following relative humidity (RH) conditions would be suitable for indoor/greenhouse cannabis cultivation.
Cloning: 70%-75% RH
Vegetation: 65%-70% RH
Flower: 60%-65% RH
Night phase of flower: 55%-60% RH
Tips: Always be sure to avoid stagnant air during all light and dark phases and swap air to reduce humidity/carbon dioxide levels during light-dark-phase transitions. Finally, be sure your temperatures are suitable for optimal plant growth during light-phases (75°-80°F) and slightly cooler temperatures during dark-phases (~70°F).
Mark June-Wells is laboratory director for Connecticut Pharmaceutical Solutions (CPS), one of four licensed medical cannabis producers in the state. Dr. June-Wells holds a Ph.D. in botany/plant ecology from Rutgers University, and has engineered CPS’ cannabinoid extraction efficiency and tracking programs, developed one of the largest production databases in the United States, and created efficient and repeatable production methods informed by rigorous data collection and statistical model building.
