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 cannabis plants transpire water?
3. How do you calculate the VPD of an indoor grow space?
a. What you need to get started
b. Breaking down the formula
c. Why does this difference matter?
d. How to calculate it step by step
e. An example of a VPD chart for growing indoors
4. What is a good VPD for cannabis plants?
a. Early Vegetative (Seedling) Stage (18/6)
b. Late Vegetative Stage (18/6)
c. Flowering Stage (12/12)
What is Vapour Pressure Deficit (VPD)?
Vapour Pressure Deficit, usually abbreviated as VPD, is the difference between how much moisture the air can hold at a given temperature and how much moisture it actually contains. For cannabis plants, VPD represents the pressure that drives water from the leaves into the air as vapor.
Temperature and humidity play a key role here—they determine how much water your plants lose through their leaves and, in turn, affect their growth, health, and overall yield. Maintaining an optimal VPD helps your plants transpire efficiently, absorb nutrients effectively, and avoid unnecessary stress.
How do cannabis plants transpire water?
Think of transpiration as your plants’ way of “sweating.” Much like humans, plants are made mostly of water, and when it gets too hot or dry, they lose water to cool themselves down. In plants, this process happens through stomata—tiny pores on the underside of leaves that open and close based on environmental conditions.
Here’s the basic rundown of what happens:
1. Water is absorbed by the roots and travels up the plant through the xylem.
2. As water evaporates from the mesophyll cells in the leaves, it exits through the stomata as vapor.
3. This loss of water creates a “pull” effect, drawing more water (and the nutrients dissolved in it) up from the roots.
Transpiration isn’t just about moving water—it’s essential for nutrient transport, maintaining cell pressure, and overall plant health. Without it, plants can’t photosynthesize or grow effectively.
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 growing 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 will measure the air temperature and relative humidity of your grow space. It’s an essential tool for any indoor grower, 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 the 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.
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 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. This involves looking up the saturation vapor pressure (SVP) at your measured temperatures (you can find SVP charts online).
2. Multiply the SVP of the air temperature by the relative humidity (in decimal form, e.g., 60% = 0.6) to get the actual vapor pressure (VP) of the air.
3. Subtract the VP(air) from the VP(leaf) to get the VPD.
Making it easier
There are many free VPD charts you can consult, or even online tools where you just have to enter the temperature and humidity data, and they will calculate the VPD for you in seconds! Many even suggest ranges for the vegetative and flowering stages, so you know exactly what to aim for.
Pro tip
For beginners: Start with a VPD table. It’s visual, easy and will tell you the ideal temperature and humidity ranges for the different stages of growth. Keep it handy so you can quickly check if you’re in the “right zone”.
For experienced growers: Consider investing in an environmental controller that monitors and adjusts temperature and humidity automatically. This takes the guesswork out of VPD management and ensures your plants are always in their optimal growth environment.
An example of a VPD chart for growing indoors
What is a good VPD for cannabis plants?
As your cannabis plants move through their life cycle, they go through distinct growth stages—seedling, vegetative, early flower, and late flowering. 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 yield. Below, we’ve outlined recommended VPD ranges for each growth stage, along with temperature and humidity settings to help you create the ideal environment for your plants.
Early Vegetative (Seedling) Stage (18/6)
At this stage, your seedlings are delicate, focusing their energy on root development and early leaf formation. Keeping the humidity high ensures they don’t lose too much water through transpiration while their root systems are still developing.
• Recommended VPD: 0.4–0.8 kilopascals (kPa)
• Air Relative Humidity: 70%
• Air Temperature: 24°C (75°F)
• Leaf Temperature: 22°C (72°F)
Tip for beginners: Maintaining a high humidity environment can be tricky without a humidifier, especially in colder or drier climates. If you don’t have one, placing trays of water or wet towels near your plants can help boost humidity levels naturally.
Late Vegetative Stage (18/6)
In this phase, your plants are growing rapidly, developing a strong canopy, and preparing for flowering. While they still require relatively high humidity, you’ll want to start reducing it slightly to promote robust transpiration and nutrient uptake.
• Recommended VPD: 1.0–1.1 kilopascals (kPa)
• Air Relative Humidity: 65%
• Air Temperature: 24°C (75°F)
• Leaf Temperature: 24°C (75°F)
For advanced growers: Use this stage to experiment with different VPD levels and observe how your plants respond. Fine-tuning the environment here can have a significant impact on overall growth and plant structure.
Flowering Stage (12/12)
This is the most crucial stage of the plant’s life cycle. Buds are forming, terpenes and resin production are ramping up, and your plants are putting everything into creating high-quality flowers. Lower humidity is key here to reduce the risk of mold and other moisture-related problems while maintaining optimal transpiration rates.
• Recommended VPD: 1.2–1.6 kilopascals (kPa)
• Air Relative Humidity: 53%
• Air Temperature: 24°C (75°F)
• Leaf Temperature: 24°C (75°F)
Pro tip for flowering: Make sure to monitor your humidity closely. Even a slight increase can create conditions for bud rot, especially in dense flowers. Investing in a dehumidifier may be worth it for this critical phase.
Why is VPD so important?
If you’re new to growing, VPD charts might seem overly technical at first. However, they’re an invaluable tool that allows you to balance humidity and temperature with precision, ensuring your plants thrive at every stage of growth.
By consistently maintaining the ideal VPD levels, you’re not just growing plants—you’re creating an optimized environment that promotes:
✅ Better nutrient absorption.
✅ Improved plant health and resilience.
✅ Higher terpene and resin production.
✅ Bigger and denser flowers.
Large-scale commercial facilities rely heavily on VPD charts to maintain consistent quality across all plants. The same principles apply to hobby growers. With just a hygrometer, an infrared thermometer, and access to a VPD chart or calculator, you can start fine-tuning your grow room like a pro.
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 few cannabis plants, 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 grower 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!
DISCLAIMER:
This content is intended for educational and entertainment purposes only. We do not endorse or promote illegal activities related to cannabis. Please adhere to the laws and regulations of your jurisdiction regarding cannabis use.
