Vapor Pressure Deficit
By following the Vapor Pressure Deficit (VPD), grounds managers can assess the environmental growing potential for their turf. By keeping it within the optimal range, they can achieve healthy grass growth while avoiding pest and environmental problems. VPD has become more important with the transition from HPS to LED grow lighting technology, as LED grow lights produce less heat than HPS.
VPD is used because it controls plant transpiration rates, stomatal opening, CO₂ uptake, nutrient absorption, and plant stress. By mastering VPD, conditions can be optimised for healthy grass growth while preventing limiting factors such as water and nutrient stress.
What is VPD?
Vapor Pressure Deficit is the difference between the amount of moisture present in the air and the total amount of moisture the air could hold if fully saturated. The VPD of a plant is the difference in vapor pressure between the leaf and the air. Normally, air is undersaturated, allowing water to flow from the plant leaves into the air (evaporation). This creates a flow within the plant, transporting the assimilates produced in the leaves and the nutrients absorbed from the soil to the right locations, leading to growth and development.
How to steer growth using VPD?
To raise VPD values, plant temperature can be increased, for instance, by using infrared heaters on your lighting unit or undersoil heating. Turning on lighting will also slightly increase temperature. In stadiums, reduced air circulation leads to high humidity at grass level. Another approach to increasing VPD values is to use Turf Fans and/or Turf Coolers to improve air circulation. By creating this air movement, the boundary layer around the leaf is disturbed, promoting more uniform optimal growing conditions across the whole pitch.
To lower VPD, humidity can be increased, for example, by using your irrigation system or a Turf Cooler (adiabatic cooling). By managing VPD values, the right amount of water can be applied at the right time, optimising yield with minimal water use and improving water-use efficiency.
How is VPD calculated?
VPD is calculated by subtracting the VPD of the air from the VPD of the leaf. The VPD of the leaf is determined using plant temperature, while the VPD of the air is calculated based on air temperature and relative humidity. The unit of measurement is kilopascal (10³ Pa or kPa).
Example Calculation
With a plant temperature of 22.3°C, the VPD of the leaf is calculated to be 2.7 kPa.
With a relative humidity of 75% and an air temperature of 20.7°C, the VPD of the air is calculated to be 1.8 kPa.
Thus, the VPD = VPD leaf – VPD air = 2.7 – 1.8 = 0.9 kPa.
Understanding VPD Levels
- Low VPD values mean the air is highly saturated with moisture, leading to condensation on the leaves. This thin layer of water can increase the risk of disease. Additionally, if the plant cannot release water into the air, transpiration and growth may slow down. Reduced water uptake can also limit nutrient absorption, potentially causing nutrient deficiencies.
- High VPD values (>1.5 kPa) indicate dry air, causing plants to evaporate more water. This water must be drawn from the soil through the roots. In normal conditions, this increased transpiration helps the plant cool itself. However, if more water is lost than can be replenished, the plant experiences water stress. To prevent wilting, it closes its stomata, which also halts CO₂ intake and photosynthesis, making the grass unable to use the available light.
Managing VPD for Healthy Growth
VPD fluctuates based on location and time. At night, VPD values drop as the grass focuses on respiration. During the day, VPD peaks slightly.
SGL recommends keeping VPD values within these ranges:
- C3 grasses: 0.25 – 2.5 kPa
- C4 grasses: 0.25 – 3.5 kPa
While optimal values fall within these ranges, it is best to maintain stable VPD levels and avoid strong fluctuations. The plant stage also plays a role—recently germinated grass can thrive at a slightly lower VPD than more established pitches.
Discover our case studies
- An enclosed stadium structure with a retractable roof
- Insufficient natural sunlight
- 9x LED440 and 2x LED50
- Permanent shaded places in winter
- Challenging Scottish weather
- Full LED system: 4x LED440, 2x LED120 & 3x LED50
- A completely closed stadium
- Growing grass indoor
- 18x LED440, 2x LED120, (1x UVC180, 6x TC50 & 9x TurfPods)
