In our previous blog post, we talked about the importance of plant transpiration and how to ensure plants transpire at an ideal rate for optimal growth. One way that growers can determine if the right amount of transpiration is occurring is by measuring the vapor pressure deficit (VPD).
VPD indicates the air’s moisture saturation level. Lower moisture in the air allows more plant transpiration, while higher saturation reduces it. In this post, we’ll explore how VPD works, its impact on plant functions, factors affecting VPD, and how growers can manage it.
What is VPD?
Vapor pressure is the point where a liquid converts into a vapor—think of boiling water turning to steam. As the water heats up, the pressure increases until it matches the vapor pressure of the surrounding air and turns the water into steam.
VPD measures the difference between the moisture in the air and the maximum moisture it can hold when fully saturated. Measured pounds per square inch (psi) or kilopascals (kPa), a high VPD means the air can absorb more moisture, while a low VPD indicates a highly saturated atmosphere.
In plant cultivation, VPD specifically refers to the difference in vapor pressure between the inside of a leaf and the surrounding air. As explained in our blog post on understanding plant transpiration, liquid water movement through a plant—from the roots to the leaves—is driven by water potential. However, its evaporation as water vapor, a gas, depends on the VPD difference between leaf interiors and the air (Figure 1).[1]
Figure 1. Water uptake and transport. The solid and dotted lines represent the waterflow pathways in the liquid and vapor phases, respectively. ∆Ψ represents the water potential drawdown between the two compartments of the soil-plant-atmospheric continuum. MPa is a megapascal, equivalent to 1,000 kPa.
Image source: Zhang, Dalong, Qingjie Du, Zhi Zhang, Xiaocong Jiao, Xiaoming Song, and Jiaming Li. 2017. “Vapour pressure deficit control in relation to water transport and water productivity in greenhouse tomato production during summer.” Scientific Reports 7: 43461. https://doi.org/10.1038/srep43461.
A high VPD allows plants to release more moisture through transpiration, while a low VPD limits this release. Since transpiration impacts photosynthesis, respiration and nutrient uptake, VPD is a key factor for optimizing plant growth.
VPD versus relative humidity
Although humidity also measures air moisture and affects transpiration, it differs from VPD. Unlike VPD, humidity only indicates the amount of moisture in the air, not the amount of moisture it can hold.[2]
That’s because humidity is temperature-dependent, and warmer air can hold more moisture: For every 20°F (11°C) that the temperature increases, the air’s water-holding capacity doubles. Thus, if the temperature reaches 80°F (26°C), the air can hold twice as much water as at 60°F (15°C).[3]
VPD, on the other hand, measures the gap between current and potential moisture, regardless of temperature. For example, when the relative humidity is 70% at 60°F, VPD is 0.55 kPa. But if the temperature rises to 90°F with the same humidity, the VPD jumps to 1.45 kPa. This makes VPD a more accurate indicator of transpiration than humidity alone.[4]
How VPD affects plant growth
VPD levels, whether high or low, directly influence plant growth.
High VPD increases the atmosphere’s demand for water through evapotranspiration (ET)—the water loss from evaporation and transpiration. However, plants have evolved to partially close their stomata—the pores on leaves that regulate the exchange of water, carbon dioxide (CO2) and gases with the atmosphere and through which transpiration occurs—in response to dry atmospheric conditions. As a result, an increase in VPD might reduce ET due to stomatal closure.
If atmospheric demand dominates and VPD is too high, plants may struggle to meet their water needs, leading to blockages in their water transport system (xylem).[5] But if plant regulation dominates, the plant’s stomata close to minimize water loss, reducing transpiration.
Because high VPD levels can cause stomata to close, it also reduces CO2 assimilation, as stomatal closure limits gas exchange between the leaves and the atmosphere. Lowering the VPD expands the leaf’s surface area for gas exchange, which enhances CO2 intake and thereby improves photosynthesis, water-use efficiency and yield.
If transpiration is reduced too much from a high VPD, water uptake is slowed and can cause heat stress, as transpiration is the plant’s cooling mechanism. Plants can also become water stressed, resulting in wilting and leaf curling. One study found that a high VPD of 2.2 kPa resulted in fruit cracking in tomato crops.[6]
Since plants uptake nutrients with water, reduced transpiration can also cause nutrient deficiencies, while a higher VPD increases transpiration, helping to ensure nutrients are effectively transported throughout the plant.
While the growing medium does not directly affect VPD, it influences how plants respond to it by determining how efficiently roots can access water and nutrients. For example, a growing medium that allows roots to absorb water quickly is beneficial when VPD—and thereby transpiration—increases, enabling more water and nutrient uptake to meet crop demands.
Low transpiration rates also occur when VPD is too low and there is too much moisture in the air. Prolonged low VPD levels can cause excessive humidity, increasing the risk of mold, fungal diseases and root problems. Studies show that VPDs of less than 0.43 kPa and 0.20 kPa are optimal for the survival of fungal pathogens and disease infections, respectively.[7]
Factors affecting VPD
Several factors affect VPD:
- Elevation: Higher altitudes tend to have naturally higher VPD due to lower atmospheric pressure.
- Temperature: As temperatures rise, VPD increases because warmer air can hold more moisture.
- Humidity: High humidity lowers VPD, while low humidity raises it, influencing transpiration rates.
- Light Intensity: More intense light increases plant transpiration and raises VPD by adding moisture to the air. More intense light also increases the grow room temperature.
- Airflow: Increased airflow from wind or ventilation reduces the boundary layer—a thin zone of still air around leaves—accelerating transpiration and elevating VPD.[8]
In summary, growers can raise temperature, light and airflow to increase VPD or lower these factors to decrease it.
In our next post, we’ll explain how to calculate VPD based on temperature and humidity, ideal VPD levels for cannabis, and strategies to manage it effectively.
Emerald Harvest Team
[1] Fricke, Weland. 2016. “Water transport and energy.” Plant, Cell and Environment 40: 977-994. https://doi.org/10.1111/pce.12848.
[2] Wollaeger, Heidi. 2015. “Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?” Published July 30. https://canr.msu.edu/news/why_should_greenhouse_growers_pay_attention_to_vapor_pressure_deficit_and_n.
[3] Wollaeger, Heidi. 2015. “Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?” Published July 30. https://canr.msu.edu/news/why_should_greenhouse_growers_pay_attention_to_vapor_pressure_deficit_and_n.
[4] Wollaeger, Heidi. 2015. “Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?” Published July 30. https://canr.msu.edu/news/why_should_greenhouse_growers_pay_attention_to_vapor_pressure_deficit_and_n.
[5] Song, Xiaoming, Ping Bai, Juping Ding, and Jianming Li. 2021. “Effect of vapor pressure deficit on growth and water status in muskmelon and cucumber.” Plant Science 303: 110755. https://doi.org/10.1016/j.plantsci.2020.110755.
[6] Leonardi, Cherubino, Soraya Guichard, and Nadia Bertin. 2000. “High vapour pressure deficit influences growth, transpiration and quality of tomato fruits.” Scientia Horticulturae 84: 285-296 https://doi.org/10.1016/S0304-4238(99)00127-2.
[7] Cayli, A. and A.N. Baytorun. 2021. “Analysis Of Climate And Vapor Pressure Deficit (VPD) In A Heated Multi-Spa Plastic Greenhouse.” Journal of Animal & Plant Sciences 31 (6): 1632-1644. https://doi.org/10.36899.JAPS.2021.6.0367.
[8] Boundary layer thickness depends on how quickly gasses and energy are exchanged between the leaf and surrounding air. The thicker the boundary layer, the less heat, CO2 and water vapor are transferred outside the plant. In reducing the boundary layer, transpiration is accelerated, further elevating VPD. Runkle, Erik. 2016. “The Boundary Layer and Its Importance.” GPN, March. https://www.canr.msu.edu/uploads/resources/pdfs/boundary-layer.pdf.
