Let’s be real—growing strong, healthy plants isn’t just about watering them and hoping for the best. It takes patience, skill, and a good understanding of the delicate balance between temperature and humidity.
If you’re into growing, you’ve probably heard the old rule of thumb: keep temperatures around 24°C with high humidity in the early growth stages, then lower the humidity once flowers or fruits start forming. But what if you could go beyond these general guidelines and fine-tune your environment for even better results?
That’s where Vapour Pressure Deficit (VPD) comes in.
Thanks to advancements in plant science, we now have a precise way to calculate the perfect environment for plants at every stage of growth.
In this article, we’ll break down what VPD is, why it matters, how it affects plant transpiration, and how to calculate it. Plus, we’ll share practical tips and the ideal VPD ranges for different growth phases. Let’s dive in!
INDEX OF CONTENTS
1. What is Vapour Pressure Deficit (VPD)?
2. How do plants transpire water?
3. Why does this difference matter?
4. How do you calculate the VPD of an indoor grow space?
a. What you need to get started
b. Breaking down the formula
c. How to calculate it step by step
d. Making it easier
5. What is a good VPD for your plants?
a. VPD at Vegetative Growth (Early Development & Canopy Formation)
b. VPD at Reproductive Growth (Flowering, Fruiting & Maturation)
What is Vapour Pressure Deficit (VPD)?
Vapour Pressure Deficit (VPD) is the difference between how much moisture the air can hold at a given temperature and how much moisture it actually contains.
For plants, VPD represents the pressure that drives water out of the leaves as vapor.
Temperature and humidity play a key role in this process—they determine how much water your plants lose through their leaves, directly impacting their growth, health, and overall yield. Maintaining an optimal VPD helps plants transpire efficiently, absorb nutrients effectively, and avoid unnecessary stress.
How do plants transpire water?
Think of transpiration as your plants’ way of “sweating.” Just like humans, plants are mostly made of water, and when it gets too hot or dry, they release moisture to cool themselves down.
This happens through stomata—tiny pores on the surface of the leaves that open and close based on environmental conditions.
Here’s how it works:
1. Water absorption: Roots take up water from the soil and transport it through the xylem.
2. Evaporation: Inside the leaves, water evaporates from the mesophyll cells.
3. Release: The water vapor exits through the stomata, creating a “pull” effect that draws more water (and nutrients) up from the roots.
Transpiration isn’t just about water movement—it’s essential for nutrient transport, temperature regulation, and overall plant health. Without it, plants wouldn’t be able to photosynthesize or grow properly.
Why does this difference matter?
Plants thrive when there’s just the right balance between how much water they release (transpire) and how much water the air around them can hold.
• If the VPD is too low (high humidity), your plants may struggle to transpire properly, leading to slower nutrient uptake and increased risks of mold.
• If the VPD is too high (low humidity), your plants may lose water too quickly, causing stress and hindering their growth.
How do you calculate the VPD of an indoor grow space?
At first glance, calculating VPD might seem like an intimidating task, especially if you’re just starting to level up your cultivation techniques. But don’t worry—it’s simpler than it sounds and mastering it will make a world of difference in your results. Here’s a step-by-step guide to help you understand and calculate VPD with ease.
What you need to get started
1. Digital hygrometer
This device measures the air temperature and relative humidity of your grow space. It’s an essential tool for any indoor cultivator, so make sure to have one on hand.
2. Infrared thermometer
This is used to measure the temperature of your plants’ leaves. You’ll notice that leaf temperature is often slightly lower than the surrounding air temperature. That’s because leaves lose heat through transpiration—the process of releasing water vapor.
Breaking down the formula
The formula for VPD is:
VPD = VP(leaf) – VP(air)
Let’s unpack this:
• VP(leaf) refers to the vapor pressure at the leaf surface, which is influenced by the leaf’s temperature. The warmer the leaf, the more water vapor it will release.
• VP(air) refers to the vapor pressure in the surrounding air, determined by the temperature and humidity of the grow space. High humidity means the air can hold less additional moisture, while low humidity means the air can absorb more moisture from the plant.
How to calculate it step by step
1. Measure the air temperature and relative humidity using your hygrometer. Write these values down.
2. Measure the leaf surface temperature using your infrared thermometer. This value is usually 1-2°C lower than the air temperature in a healthy environment.
3. Use a VPD chart or online calculator to plug in your values. These tools will save you from doing the math manually.
If you’re curious to calculate it yourself, here’s the basic process:
1. Convert your temperature readings into vapor pressure values. Look up the saturation vapor pressure (SVP) at your measured temperatures (you can find SVP charts online).
2. Calculate the actual vapor pressure (VP) of the air. Multiply the SVP of the air temperature by the relative humidity (in decimal form, e.g., 60% = 0.6).
3. Subtract VP(air) from VP(leaf) to get the VPD.
Making it easier
There are plenty of free VPD charts and online calculators available. Just enter your temperature and humidity data, and they’ll calculate the VPD for you in seconds! Many even suggest ideal VPD ranges for different growth stages, so you know exactly what to aim for.
Pro tip
• For beginners: Start with a VPD table. It’s visual, easy to use, and tells you the ideal temperature and humidity ranges for different growth stages. Keep it handy to quickly check if you’re in the “right zone.”
• For experienced cultivators: Consider investing in an environmental controller that monitors and adjusts temperature and humidity automatically. This takes the guesswork out of VPD management and keeps your plants in their optimal growth environment at all times.
An example of a VPD chart for growing indoors
What is a good VPD for your plants?
As plants move through their life cycle, they experience distinct growth phases—vegetative and reproductive. Each stage comes with its own specific needs for temperature, humidity, and, ultimately, the vapor pressure deficit (VPD).
Getting the VPD right at every stage is key to optimizing plant growth, health, and final yield. Below, we’ve outlined recommended VPD ranges for each phase, whether you’re cultivating annuals, perennials, or seasonal vegetables, along with temperature and humidity settings to help you create the ideal environment for your plants.
VPD at Vegetative Growth (Early Development & Canopy Formation)
At this stage, plants focus their energy on root development and leaf expansion. Maintaining a higher humidity prevents excessive water loss while their root systems are still developing.
• Recommended VPD: 0.4–0.8 kPa (ideal for seedlings and young plants)
• Air Relative Humidity: 65–75%
• Air Temperature: 22–26°C (72–79°F)
• Leaf Temperature: 1–2°C lower than air temperature
💡 Tip for beginners: If maintaining high humidity is a challenge, placing trays of water or damp towels near your plants can help naturally increase moisture levels.
As plants mature, their nutrient and water uptake increase. Gradually lowering the humidity strengthens transpiration, encouraging stronger stems and lush foliage.
• Recommended VPD: 1.0–1.2 kPa
• Air Relative Humidity: 55–65%
• Air Temperature: 22–26°C (72–79°F)
• Leaf Temperature: Similar to air temperature
⚙️ For experienced cultivators: This is the perfect stage to experiment with fine-tuning VPD levels. Adjusting temperature and humidity can influence plant structure and resilience, leading to healthier growth.
VPD at Reproductive Growth (Flowering, Fruiting & Maturation)
Once plants enter their reproductive phase, they shift energy from vegetative growth to flower and fruit production. Lower humidity levels reduce the risk of mold and ensure optimal transpiration for maximized nutrient transport.
• Recommended VPD: 1.2–1.6 kPa
• Air Relative Humidity: 45–55%
• Air Temperature: 22–26°C (72–79°F)
• Leaf Temperature: Similar to air temperature
🌸 For flowering plants: A well-controlled VPD supports bigger and more vibrant blooms, enhancing color and aroma.
🍓 For fruiting crops: Optimizing VPD can lead to sweeter, more flavourful fruits by improving sugar and nutrient distribution.
🥬 For leafy greens and vegetables: The right balance ensures crispier, more textured leaves, improving overall quality and shelf life.
📌 Pro tip: During this stage, monitor humidity closely. Even a slight increase can trigger mold issues, especially in dense flowers or fruit clusters. A dehumidifier can be a great investment for ensuring high-quality results.
Why is VPD so important?
VPD might seem technical at first, but mastering it is a game-changer. Instead of relying on guesswork, you’ll be creating an optimized environment that promotes:
✅ Better nutrient absorption for stronger, healthier plants.
✅ Enhanced resilience against pests, diseases, and environmental stress.
✅ Improved flower, fruit, and leaf quality, with more vibrant colours and richer flavours.
✅ Higher yields, whether you’re growing for aesthetics, taste, or market value.
Large-scale cultivators rely on precise VPD management to maintain consistent quality. But the same principles apply to home cultivators or novice indoor producers! With just a hygrometer, an infrared thermometer, and a VPD chart or calculator, you can fine-tune your indoor production.
Atami’s Conclusion
Perfecting your VPD is a big step, but it’s one effective way to improve the quality and yield of your crops. Whether you’re cultivating flowers, fruits, or vegetables, understanding and controlling VPD ensures you leave no stone unturned in achieving top-tier quality.
Don’t worry – this isn’t as difficult as it seems, just use VPD charts as a guide, and focus on consistent monitoring rather than perfection! Every cultivator starts somewhere, and small adjustments can lead to big improvements. But remember, the more precise you get, the closer you’ll come to creating that perfect growing environment.
So, grab your tools, check your readings, and dive into the science of VPD. Your plants—and your future harvests—will thank you!
