VPD in Hydroponics: A Grower’s Guide

Introduction

Managing humidity and vapor pressure deficit (VPD) isn’t just a detail—it’s one of the most critical factors in successful hydroponic growing. Whether you’re cloning cuttings or pushing your crops to flower, understanding how humidity and VPD affect your plants can be the difference between thriving foliage and failed crops.

In this guide, we’ll explore why humidity matters, how VPD works, and what tools and strategies you need to maintain the ideal grow room environment.

Why Humidity Control Is Essential in Hydroponics

There are three key reasons to manage humidity in your grow room or greenhouse:

1. Prevent Dehydration During Propagation

When propagating cuttings or clones, they have no roots to take in water. You need high humidity—close to 100%—to prevent them from drying out. At this stage, transpiration isn’t necessary; your goal is purely hydration.

2. Limit Pests and Diseases

Once your plants are rooted, you must reduce humidity to around 70% or lower to prevent fungal issues like powdery mildew. In flowering stages, humidity can go as low as 40% RH, depending on your crop.

3. Prevent Leaf Damage and Drying

If humidity drops too low, plants lose water too fast, resulting in crisping leaves and tip burn. This often happens in hot environments without proper humidification.

What Is VPD and Why It Matters

Vapor Pressure Deficit (VPD) is the difference between the amount of moisture in the air and the maximum amount it can hold at a given temperature. It directly affects plant transpiration and nutrient uptake.

• Low VPD = Too humid → Reduced transpiration
• High VPD = Too dry → Excess water loss
• Optimal VPD = Balanced transpiration, healthy growth

Most growers aim for a VPD between 0.8 and 1.2 kPa during veg and flower stages, adjusting humidity and temperature to stay within the “green zone” on a VPD chart.

How to Measure and Manage VPD

You can calculate VPD manually or use:

• Hygrometers to monitor humidity
• Thermometers for room temperature
• VPD charts to find the sweet spot

Some advanced grow room controllers now use AI-powered VPD sensors to adjust:

• Humidifiers
• Dehumidifiers
• Heaters
• Fans

Liquid Gold VPD Chart

💡 Pro Tip: Print and post this VPD chart in your grow room. Use it with your hygrometer and thermometer to keep VPD dialled in.

The Science Behind Humidity and Temperature

• Relative Humidity (RH) is temperature-dependent. Warmer air holds more moisture.
• As temps rise, you may need a humidifier to maintain RH.
• Cold temps = less moisture in the air = different RH behavior.

Example:
At 5°C, air at 100% RH holds much less water than air at 35°C. This means your grow room must adjust dynamically based on temperature swings.

How Humidity Affects Pests and Diseases

• Powdery mildew, rusts, and foliar diseases love high humidity.
• Sap-sucking insects (like aphids and whiteflies) may prefer lower humidity, which increases sap flow.
• Spider mites dislike high RH since it can disrupt their webs.

Balanced VPD keeps disease and pest pressure low while promoting optimal plant development.

VPD vs. Traditional Guidelines

Standard recommendations like “keep your room at 25°C and 60% RH” don’t account for real-time changes. VPD gives you a more accurate, plant-focused view.

For example:

• 40% RH at 25°C ≠ 60% RH at 15°C
• Both may fall within “acceptable” ranges, but only one might be ideal

Advanced Techniques: Leaf Temperature and Thermal Imaging

Take things to the next level by measuring leaf temperature, not just air temp. This is closer to the actual conditions your plant experiences.

Some high-end systems use:

• Thermal imaging cameras
• Leaf surface thermocouples

These tools help you fine-tune environmental controls based on the real stress levels your plants are under—not just what the room sensor says.

VPD and Crop Performance

Ever wonder why some lettuce just sits there and doesn’t grow, while others explode with lush growth? It might be your VPD. Lettuce, like many leafy greens, is incredibly sensitive to microclimate conditions.

Dialing in your VPD can turn “stagnant” crops into vigorous performers.

Final Thoughts: VPD Is the Next Step in Professional Growing

Humidity and VPD are often overlooked, but they’re core to optimizing plant performance in hydroponics. Once you understand and implement VPD control, you’ll see fewer disease issues, better nutrient uptake, and faster, healthier growth.

Takeaways:

• Keep propagation humidity near 100%
• Reduce RH as plants mature to avoid disease
• Use VPD charts to balance humidity and temperature
• Invest in a hygrometer, thermometer, and if possible, a data logger
• Consider smart controllers or leaf temperature monitoring for even better control

And if you are looking for an excellent fertilizer to help out with growing your plants while perfecting VPD, check out Liquid Gold!

Article by Dr Russell Sharp

If you would like to keep up to date with subjects just like this, you can listen to both our podcasts! Links can be found bellow:

Hydroponics Daily Podcast: https://open.spotify.com/show/207T7p7fw9sPjINfSjVXW2

Cereal Killers Podcast: https://t.co/eSEbBkTVHl