Vapor Pressure Deficit (VPD) quantifies the difference between the amount of moisture the air can potentially hold when saturated and the amount it currently holds. This difference is expressed as a pressure and is measured in kilopascals (kPa). During the light cycle, VPD is monitored because it directly governs the rate at which a plant transpires and absorbs nutrients. While plant activity slows when lights are off, nighttime VPD remains a factor in plant health. Managing this period is important for preparing the plant for the next day’s growth and preventing disease.
Understanding Nighttime Plant Physiology
When the lights turn off, the plant immediately ceases photosynthesis. However, the plant continues to respire constantly, a process sometimes called “dark respiration.” Respiration involves the intake of oxygen and the release of carbon dioxide as the plant consumes stored carbohydrates to fuel basic metabolism, maintenance, and growth processes.
To conserve water when transpiration is absent, plants close their stomata, the tiny pores on the leaves. This closure significantly reduces the rate of water loss compared to the daytime, but some low-level water movement and gas exchange still occur. The dark period is an active time of recovery and preparation, where the plant redirects energy away from sugar production and toward structural maintenance.
Turgor Recovery and Water Movement
The dark period serves as the plant’s rest cycle, during which it aims to fully restore cell turgor, the internal pressure that gives leaves and stems their rigidity. Throughout the day, high transpiration rates can lead to a slight loss of turgor, which the plant repairs overnight. This restoration is accomplished through the generation of root pressure, where water uptake continues and pushes water up the xylem vessels.
If the nighttime VPD is too high (dry air), the plant may continue to lose excessive water, preventing a full recovery of turgor and causing stress that can lead to morning wilting. Conversely, if the VPD is extremely low (high humidity), the evaporative pull from the leaves is almost eliminated. This lack of pull can slow the movement of water and dissolved nutrients, particularly immobile ones like calcium, throughout the plant’s tissues, potentially limiting growth the following day.
Preventing Pathogens and Condensation
The primary reason for managing nighttime VPD is to reduce the risk of fungal diseases, which are triggered by free water on the leaf surface. This free water forms when the temperature of the leaf surface drops to or below the air’s dew point. The dew point is the temperature at which the air becomes completely saturated, causing water vapor to condense into liquid droplets.
When the lights turn off, the leaf temperature often drops quickly. If the air is too humid, the leaf reaches the dew point, resulting in condensation. This film of liquid water provides the necessary environment for fungal spores, such as those causing Botrytis (bud rot) and Powdery Mildew, to germinate.
Effective disease prevention requires maintaining a slight positive VPD, above 0 kPa, to ensure the leaf surface remains dry. Adequate air movement is also necessary to break up the humid boundary layer of air that surrounds the leaf surface. This continuous circulation prevents pockets of still, saturated air from forming around the canopy, which could lead to localized condensation.
Determining Optimal Nighttime VPD Setpoints
Nighttime VPD setpoints must balance the need for turgor recovery with preventing condensation and disease. The range for most plants is between 0.4 kPa and 1.5 kPa, depending on the growth stage. Plants in the early stages, like seedlings or clones, benefit from a lower VPD (0.6 kPa to 1.0 kPa) to encourage root development without excessive water stress.
For plants in the later flowering stages, the target setpoint is raised to a drier 1.0 kPa to 1.5 kPa to reduce the risk of mold in dense flowers. Achieving these VPD targets involves coordinating the temperature drop with humidity control. Since cooler air holds less moisture, a temperature reduction at night will cause the relative humidity to increase, which must be counteracted, often with dehumidifiers or ventilation, to maintain the desired VPD.