Skip to content
  • About
    • AboutAbout
    • BrandsBrands
    • Our philosophyOur philosophy
    • Giving BackGiving Back
    • Social Justice FundSocial Justice Fund
  • Education
  • News
  • Contact
  • Hawthorne 360
  • Login
  • About
    • AboutAbout
    • BrandsBrands
    • Our philosophyOur philosophy
    • Giving BackGiving Back
    • Social Justice FundSocial Justice Fund
  • Education
  • News
  • Contact
  • Hawthorne 360
  • Login
  • https://www.facebook.com/HawthorneGCFind us on Facebook
  • hawthornegc Follow us on twitter
  • hawthornegardening Find us on Instagram
  • https://www.youtube.com/user/sunlightsupplySubscribe to our YouTube channel
  • https://www.linkedin.com/company/the-hawthorne-gardening-companyFind us on LinkedIn
  • https://www.pinterest.com/hawthornegcFind us on Pinterest
Back to 5 Tips for a Seamless Transition from HPS to LED

Use VPD as Your Management Tool

Balance your space’s moisture

Use VPD as Your Management Tool
Download Whitepaper

In addition to monitoring and adjusting room temperature, it’s important to balance the other conditions, like moisture, in your space with a practice like VPD (vapor pressure deficit). Doing so can help promote maximum growth.

What is Vapor Pressure Deficit Management?

VPD is the difference (deficit) between the amount of actual moisture in the air and the amount of moisture the air can potentially hold when saturated. VPD is often measured in pounds per square inch (psi) or kilopascal (kPa). 

When air is saturated with water, the VPD is 0 kPa. As VPD increases, the amount of water (or moisture) in the air decreases. A high VPD, considered greater than 1.0 kPa, indicates that the air can still hold a large amount of water.  

The goal is to maximize how long the stomata are open during the day in order to keep transpiration and photosynthesis at the max. This is accomplished by maintaining a favorable VPD during peak stages of plant growth and development. 

Because VPD is a function of both temperature and humidity, when we raise our ambient room temperatures, we must raise our humidity levels as well to stay within a favorable VPD range. Raising temperatures without raising humidity will have the opposite effect on the plant by creating stress and reducing growth.

VPD matters for two major reasons:

Disease. Moisture, or the lack thereof, can contribute to plant disease. This is because once air becomes saturated, water will condense to form a film of water on leaf surfaces. This in turn, can drive disease-causing spores to germinate and infect the plant tissue. A film of water on a plant leaf makes it much more susceptible to rot. 

Transpiration. The gradient from the plant leaves (nearly saturated with water) and the air, drives transpiration. VPD more accurately expresses the driving force of water loss from a leaf. Unlike relative humidity, VPD has a simple, nearly straight-line relationship to the rate of evapotranspiration (water loss to the atmosphere by evaporation of water and transpiration from plants).

A VPD of zero means the air is 100% saturated and thus plants cannot transpire effectively. As VPD increases, transpiration increases accordingly. 

This is why it’s recommended to keep VPD low for rooting cuttings or seed germination, and higher for finishing a crop and avoiding incidences of disease.

Gas exchange occurs at the stomata. Small pores on the underside of plant leaves exchange water and oxygen out, and CO2 in.

The goal is to maximize how long the stomata are open during the day in order to keep transpiration and photosynthesis at the max. This is accomplished by maintaining a favorable VPD during peak stages of plant growth and development. 

Because VPD is a function of both temperature and humidity, when we raise our ambient room temperatures, we must raise our humidity levels as well to stay within a favorable VPD range. Raising temperatures without raising humidity will have the opposite effect on the plant by creating stress and reducing growth.

Disease Mitigation Tips and Tricks

Many growers are uneasy about raising their RH to 70%+, citing concerns around crop diseases like powdery mildew (PM) or botrytis grey mold. But by following a few recommendations, it’s possible to run high RH and temperatures while mitigating disease.

Monitor temperature
Most foliage diseases prefer cool, damp conditions to thrive. Keeping your ambient night temperature above 75° F is a good first step to keep diseases at bay.
Minimize fluctuations
Sudden drops in nighttime temperatures can cause humidity spikes (particularly immediately following lights off). Keep the day/night temperature differential ≤10° F, to help prevent condensation on your crops’ leaves, which can lead to disease.
Keep air moving
Air movement is critical for a healthy grow space. Lack of uniform, steady air movement can cause stagnant, humid pockets in your grow space, promoting disease. Proper airflow will help keep microfilms of moisture off leaf surfaces (via evaporation).
Adjust per growth phase
For flowering crops, which are sensitive to bud rot, we recommend lowering RH (raising VPD) levels once flowers start to fill out and become dense.
Keep up with the canopy
Defoliation or other canopy management methods can, and should, be employed to promote air movement deep into your crops’ foliage, further alleviating dead spots of ambient air.
Avoid overwatering
Don’t keep pots saturated overnight. Ceasing irrigation events prior to lights off, and allowing for increased drybacks during the night, can be effective at mitigating large humidity spikes at night.
Recommended VPD levels for various stages of plant growth

Keep in mind: There is a balance between disease prevention and optimum plant growth and development. It’s important to understand your facilities’ limitations and make adjustments accordingly.

Continue learning with our next tip ⟶

Download Whitepaper

Continue Learning

  • Tip 1

    Adjust Temperature When Growing with LED

  • Tip 2

    Use VPD as Your Management Tool

  • Tip 3

    Factor in HVAC and Dehumidification

  • Tip 4

    Consider the Effects of Light Spectra and Light Intensity on Crop Growth

  • Tip 5

    Monitor and Adjust Nutrition and Irrigation Based on Transpiration

Education LIghting

Tip 1

Adjust Temperature When Growing with LED

Read

Tip 3

Factor in HVAC and Dehumidification

Read

We know why you grow. And that influences everything we do. We are a company guided by a set of principles that motivate us every day.

Built to serve growers of all sizes — from commercial needs to home hobbyists — Hawthorne houses best-in-class brands across lighting, nutrients, growing environment, growing media, hardware, and more.

Learn more about us
  • Home
  • About
  • Hawthorne 360
  • Education
  • News
  • Dialux Lighting
  • Services
  • Careers
  • Contact
  • Login
  • Conditions of Use
  • Privacy Notice
  • https://www.facebook.com/HawthorneGCFind us on Facebook
  • hawthornegc Follow us on twitter
  • hawthornegardening Find us on Instagram
  • https://www.youtube.com/user/sunlightsupplySubscribe to our YouTube channel
  • https://www.linkedin.com/company/the-hawthorne-gardening-companyFind us on LinkedIn
  • https://www.pinterest.com/hawthornegcFind us on Pinterest
© Copyright 2022. All rights reserved.