How you can influence and change VPD:

1. Temperature

   • Increase temperature (run a heater or reduce AC): increase VPD

   • Decrease temperature (increase AC): decrease VPD

2. Humidity

   • Increase humidity (run a humidifier): decrease VPD

   • Decrease humidity (run a dehumidifier): increase VPD

3. Light Intensity

   • Increase light intensity (move lights closer, etc): increase leaf temps: increase VPD

   • Decrease light intensity (move lights farther, etc): decrease leaf temps: decrease VPD

Ideal VPD for Different Stages of Growth

So now you know what VPD is, how it impacts plants, and how you can change VPD. All that’s left is to figure out how to use this information in your own grow. Ideal VPD, as a general rule for plant growth, is around 0.8 – 1.2 kPa (kilopascals). However, your plants have different needs during different stages of growth. You need to tailor your grow environment to the stage of growth that your plants are in. Below are some general recommendations, but keep in mind that these recommendations can vary from strain to strain, and setup to setup. As always, observe your plants and tweak things accordingly.

Infographic: Ideal VPD for Clone, Veg, and Flower

1. Ideal VPD for Seedlings and Clones

   • Clones and seedlings are baby plants, they can’t handle a lot of stress because they are still just trying to form roots. Target a higher humidity and VPD closer to the lower end of the general range.

   • The ideal VPD for seedlings and clones is closer to 0.8 kPa.

2. Ideal VPD for Vegetative Stage

   • In veg (vegetative) stage the plants are bigger and more robust. You can reduce the humidity in your environment to increase VPD. This will increase water and nutrient uptake, but you don’t want to increase VPD too much. This will cause the stomata of the plants to close, causing them to absorb less CO2. CO2 is particularly important in the vegetative stage, because that’s the main ingredient that plants use to grow large.

   • The ideal VPD for the vegetative stage is close to the middle of the general range, around 1.0 kPa.

3. Ideal VPD for Flower Stage

   • In flower stage the plants are robust, but the flowers are sensitive to various issues. You need to avoid excess humidity.

   • The ideal VPD for the flower stage is closer to the top end of the range, 1.2kPa – 1.5kPa.

VPD at Night

If you didn’t already know, VPD is a measurement related to humidity and temperature. It is an absolute measure of how much more moisture the air can hold at the current temperature. VPD is very important for plants because it controls stomatal opening, transpiration rate, CO2 uptake, nutrient uptake, and plant stress. It also directly impacts yields[1][2]. Ideal VPD can boost yields by 20% or more.

VPD Measured At Night. Click for an Interactive Chart

What Plants Do At Night

At night, most plants close their stomata and stop photosynthesis since there is no light available. This closure prevents water from escaping through the open pores, and significantly reduces transpiration. See below a comparison of day and night transpiration as measured in C. Sativa hemp plants:

Transpiration is about 10x lower at night than it is during the day. [1]

Instead of photosynthesis, at night, plants mostly perform respiration. Like in humans, respiration in plants converts stored sugars (made by photosynthesis) to energy. As you may already know, that means plants actually produce CO2 during the night!

How to Make the Most of the Night Period

You may be wondering what you should do as a cultivator during the night period, if the plants aren’t photosynthesizing. There are a few key things you can use the night period to achieve.

• Disease Prevention: use nighttime to keep RH on the lower end to prevent mold such as bud rot and powdery mildew.

• Keep Nighttime VPD Close to Daytime VPD: plants still need to release CO2 created by respiration through the stomata, so keep VPD relatively close to the daytime ranges.

• Minimize VPD Swings:

  

  Vpd swings from day to night. Click for an Interactive Chart.

  According to research[3] done in 2021 Frequent VPD swings greater than 0.4kPa can drop yields by as much as 20%:

  

  Yield Impact from VPD Swings [3]

Ideal Night Period VPD

VPD Dialed In on the Pulse App

Overall you want to keep nighttime VPD close to daytime VPD, but you can be a bit more lenient since VPD matters less at night. Ideal VPD, as a general rule for plant growth, is around 0.8 – 1.2 kPa (kilopascals).

However, your plants have different needs during different stages of growth. You need to tailor your grow environment to the stage of growth that your plants are in. Below are some general recommendations, but keep in mind that these recommendations can vary strain from to strain, and setup to setup. As always, observe your plants and tweak things accordingly.

Nighttime VPD for Seedlings and Clones

• Clones are baby plants, and they can’t handle a lot of stress because they are still just trying to form roots. Target a higher humidity and VPD closer to the lower end of the general range.
• The ideal nighttime VPD range for seedlings and clones is 0.6kPa to 1.0kPa (ideally 0.8kPa)

Nighttime VPD in Veg

• In veg (vegetative) stage the plants are bigger and more robust. You can reduce the humidity in your environment to increase VPD. This will increase water and nutrient uptake, but you don’t want to increase VPD too much. This will cause the stomata of the plants to close, causing them to absorb less CO2. CO2 is particularly important in the vegetative stage, because that’s the main ingredient that plants use to grow large.
• The ideal VPD for the vegetative stage is close to the middle of the general range 0.8kPa to 1.2kPa (ideally 1.0kPa)

Nighttime VPD in Flower

• In the flower stage the plants are robust, but the flowers are sensitive to various issues. You need to avoid excess humidity.
• The ideal nighttime VPD range for the flower stage is closer to the top end of the range: 1.0kPa to 1.5kPa (ideally 1.2kPa)

Summary and Key Points

Night VPD Infographic

1. VPD is mostly about transpiration
2. Plants transpire significantly less at night
3. Use night period for preventing disease by keeping humidity and temps lower
4. Try to minimize VPD fluctuations (less than 0.4kPa)
5. VPD still matters for respiration at night
6. Daytime VPD is about 10x more important than nighttime VPD
7. Optimal VPD can increase yields as much as CO2 enrichment

To sum it up:

Although VPD at night isn't nearly as important as your day VPD, it's still an important factor to consider for vigorous plant growth and increased yields.

Monitoring VPD At Night

Monitor VPD With Pulse

Since VPD is so important and has such an impact on your yield[1], you may be wondering how to monitor and track VPD when you’re not in your grow. My partner Chris, a long-time grower, and I teamed up to build the Pulse One and Pulse Pro sensors along with the Pulse App to track VPD and other key things about your grow.

Pulse helps you boost your yields, optimize your environment, and the instant alerts finally give you peace of mind when you’re away from the garden.

Further Reading on VPD

1. Tang, K., Fracasso, A., Struik, P. C., Yin, X., & Amaducci, S. (2018). Water- and Nitrogen-Use Efficiencies of Hemp (C. sativa L.) Based on Whole-Canopy Measurements and Modeling. Frontiers in Plant Science, 9. doi:10.3389/fpls.2018.00951
2. Jiao, XC., Song, XM., Zhang, DL. et al. Coordination between vapor pressure deficit and CO2 on the regulation of photosynthesis and productivity in greenhouse tomato production. Sci Rep 9, 8700 (2019). doi:10.1038/s41598-019-45232-w
3. Inoue T, Sunaga M, Ito M, Yuchen Q, Matsushima Y, Sakoda K and Yamori W (2021) Minimizing VPD Fluctuations Maintains Higher Stomatal Conductance and Photosynthesis, Resulting in Improvement of Plant Growth in Lettuce. Front. Plant Sci. 12:646144. doi: 10.3389/fpls.2021.646144