Environmental control is foundational in modern plant cultivation, allowing growers to precisely tune the atmosphere surrounding their crops. Simple metrics like air temperature and relative humidity (RH) fail to capture the full picture of how a plant interacts with its environment. Vapor Pressure Deficit (VPD) is a superior, single-number metric that combines temperature and humidity to measure the atmospheric “thirst” driving plant function. Optimizing VPD is important for maximizing plant health, nutrient uptake, and final yield during the reproductive phase.
Understanding Vapor Pressure Deficit
Vapor Pressure Deficit is the difference between the amount of moisture the air can hold when saturated and the amount it currently holds at a specific temperature. Measured in kilopascals (kPa), VPD directly relates to the driving force behind plant transpiration. Transpiration is the plant’s process of releasing water vapor through tiny pores on its leaves, called stomata.
When VPD is low, the air is nearly saturated, significantly slowing transpiration because water vapor has nowhere to go. This restricted movement can lead to lower uptake of water and nutrients from the roots, potentially causing deficiencies. Conversely, a high VPD means the air is very dry and pulls water out of the leaves quickly. This rapid loss can cause the plant to close its stomata to conserve water, limiting carbon dioxide intake and slowing photosynthesis.
Maintaining an ideal VPD keeps the stomata open enough to allow for maximum carbon dioxide absorption while driving the movement of water and nutrients from the roots to the leaves. This continuous, balanced flow ensures the plant efficiently absorbs the elements needed to build dense, healthy flowers.
Ideal VPD Targets During the Flowering Stage
The flowering phase requires a gradual increase in VPD compared to the vegetative stage to encourage physiological changes and mitigate disease risk. A VPD range of approximately 1.0 kPa to 1.8 kPa is the general target for this reproductive period, but this value must change as the plant matures.
Early Flowering (Stretch Phase)
In the early flowering stage, a slightly lower VPD is beneficial to support rapid cell expansion and nutrient delivery. Targeting 1.0 kPa to 1.4 kPa supports the vigorous transpiration needed to move water and mineral salts into the developing new growth. This range promotes high metabolic activity, establishing a strong foundation for flower development.
Mid-Flower
As the plants enter mid-flower, the VPD should be elevated to a range of 1.2 kPa to 1.6 kPa. This moderate increase slightly stresses the plant, which can promote the production of secondary metabolites like resins and essential oils. This drier environment also reduces the risk of pathogens like powdery mildew, which thrive in high humidity.
Late Flowering (Ripening Stage)
During the late flowering and ripening stage, the VPD can be pushed higher, often into the 1.4 kPa to 1.8 kPa range. A higher VPD at this stage helps reduce the moisture content within the developing flowers, preventing bud rot in dense structures. This drier air also prepares the plant for harvest and can enhance the expression of certain colors and aromas.
Environmental Factors for Maintaining Optimal VPD
Maintaining optimal VPD requires continuous manipulation of three primary environmental variables: air temperature, relative humidity, and leaf surface temperature. These factors are linked, and adjusting one immediately affects the calculated VPD. For example, increasing the air temperature while keeping the absolute moisture content the same increases the VPD because warmer air holds more moisture.
To accurately control VPD, a grower must monitor the temperature of the leaf surface, not just the ambient air temperature. Transpiration naturally cools the leaf, meaning the leaf temperature is often several degrees cooler than the surrounding air. This difference significantly alters the actual VPD the plant experiences, requiring the use of infrared thermometers or specialized cameras for precise readings.
Practical control relies on equipment designed to regulate both temperature and humidity. HVAC systems, heaters, and air conditioners manage temperature, while humidifiers and dehumidifiers control moisture content. Increasing airflow with fans also helps break the boundary layer of saturated air around the leaf surface, which increases the rate of evaporation and raises the VPD. Climate controllers and sensors monitor these variables, allowing growers to automate the process and maintain the desired range.