If you’ve bought a new inline fan with app support lately or been looking at new gear, you’ve most likely seen VPD pop up, promising new insights into plant growing and another level of environmental understanding and control.
I came across it a couple of years ago when I bought my first Vivosun E42A controller. I was intrigued by this new number everyone seemed to care about, so I read up on it. I tried to understand it. And honestly? I lost interest pretty quickly and moved on.
But today, I want to revisit the topic—because it keeps coming up. I want to explore whether VPD is actually a useful metric that growers should base gear-buying decisions on. And more importantly, I want to explain how it can be misleading, especially for new growers, and how chasing it without context might lead to poor decisions.
What the AI Says About VPD
After asking a couple of AIs to explain VPD to me, Gemini was very clear and thorough. Here’s what Gemini said:
What is VPD?
At its core, VPD is the difference between the amount of moisture the air can hold and the amount it is actually holding at a given temperature.
Think of it like air’s “thirstiness.”
- High VPD: The air is very “thirsty” and will pull a lot of water from plants.
- Low VPD: The air is “full” of moisture and won’t pull much water from plants.
VPD is measured in kilopascals (kPa) and is a direct result of your temperature and humidity readings. This is crucial: you can’t control VPD directly; you control temperature and humidity, and those create your VPD.
How VPD Connects Temperature, Humidity, and Transpiration
The plant’s ability to “breathe” and move water, a process called transpiration, is directly influenced by VPD.
- High VPD: Increases transpiration. Plants lose water faster.
- Low VPD: Decreases transpiration. Plants lose water slower.
Maintaining the right VPD is key to optimal water and nutrient uptake, cooling the plant, and overall vigor.
When I asked about the perfect VPD number, Gemini provided the following:
The “Perfect” VPD Numbers (According to Advocates)
VPD charts suggest optimal ranges change with the plant’s life stage:
Clones/Seedlings:
- Target VPD: 0.4 – 0.8 kPa
- Why: Young plants have undeveloped roots and don’t want to transpire heavily. Lower VPD (higher humidity) reduces stress.
Vegetative Growth:
- Target VPD: 0.8 – 1.2 kPa
- Why: Encourages vigorous transpiration for strong growth and efficient nutrient uptake.
Flowering/Fruiting:
- Target VPD: 1.0 – 1.5 kPa (can go slightly higher in late flower)
- Why: Supports active nutrient flow during early bloom. In late flower, a slightly higher VPD (achieved by lower humidity) is often deliberately targeted to reduce water content in flowers, potentially increase potency, and critically, prevent fungal issues like bud rot.
The Problem: Notice Something Missing?
Gemini hasn’t mentioned temperature and humidity specifics!
This means for seedlings, assuming the range is correct and we aim for 0.6 kPa, this number can represent an endless number of combinations. Here are just a few:
| Temperature (°C) | Humidity (%) | VPD (kPa) |
| 10.0 | 51.0 | 0.602 |
| 15.0 | 65.0 | 0.597 |
| 25.5 | 81.5 | 0.604 |
| 35.0 | 89.5 | 0.59 |
| 40.0 | 92.0 | 0.59 |
As you can see, a VPD of 0.6 kPa is only good for seedlings in one case, and the rest will kill the plant.
Testing AI Consistency
I went back to AI and asked: “What is the perfect VPD for a two-week-old cannabis seedling under 24-hour light?” because I don’t know what number I should be chasing.
I used a platform called Askodin.org, where you can ask one question and get several AI answers at the same time. Here are the answers I received:
ChatGPT: “The perfect VPD for a two-week-old cannabis seedling under 24 hours of light is between 0.8-1.0 kPa. It is important to maintain the proper VPD to ensure optimal growth and avoid any stress on the plant.”
DeepSeek: “For two-week-old cannabis seedlings under 24-hour light, the ideal VPD (Vapor Pressure Deficit) is 0.4–0.8 kPa. This range ensures proper transpiration and growth while avoiding stress. Maintain 70–75% RH at 22–25°C (72–77°F) for optimal conditions.”
Gemini: “The ideal VPD for two-week-old cannabis seedlings under 24 hours of light is 0.8-1.0 kPa (or 4-6 mmHg). Maintaining this range promotes healthy growth and development.”
As you can see, these answers are not consistent and all over the shop. Even DeepSeek’s answer, which is mostly correct, is confusing to someone trying to chase VPD, while ChatGPT and Gemini didn’t even mention temperature and humidity.
What Google Says
After these inconclusive results, I thought I’d Google VPD, and to be honest, Google’s answer was a bit better but also not spot on.
Here’s the top Google result for a VPD chart for cannabis plants in all stages:
Understanding the Cannabis VPD Chart
| Growth Stage | Temperature (Day/Night) | Ideal VPD (kPa) |
| Seedling Stage | 68-77°F (20-25°C) | 0.4-0.8 |
| Vegetative Stage | 72-82°F (22-28°C) | 0.8-1.2 |
| Flowering Stage | 68-79°F (20-26°C) | 1.2-1.6 |
| Late Flowering | 65-75°F (18-24°C) | 1.3-1.8 |
This is a bigger problem because look at the following table for a few possible combinations that achieve 0.6 kPa in the range from 20 to 25°C:
| Temperature (°C) | Humidity (%) | VPD (kPa) |
| 20.0 | 74.0 | 0.608 |
| 21.0 | 76.0 | 0.597 |
| 22.0 | 77.5 | 0.595 |
| 23.5 | 79.0 | 0.608 |
| 24.0 | 80.0 | 0.597 |
None of these temperatures is ideal for a seedling. 25.5°C is the minimum ideal daytime temperature for most cannabis plants, especially at that young age and up until mid-flowering. So where can I find the correct chart?
My Conclusion: VPD is Overcomplicating Things
The more I dug into VPD online, the more confused about its use and value I became. All this information got me to believe that this is unnecessary fluff in the grower’s life and a confusing number that growers have been doing great without for decades.
I know for a fact that I can grow a plant using a temperature gauge only. All I need to know is the temp, and I can feel and see the rest. Is it ideal? Not at all, but it has been done by many growers. But I can’t use a VPD reading alone for anything. The fact is that I have to look at the temperature and then look at a confusing chart and use calculators to figure out what the humidity should be—it’s just overcomplicated. There are charts for ideal temperature and humidity for each stage, and I don’t need to do any extra calculations.
Why VPD Doesn’t Add Value
VPD isn’t something you directly control. You can’t just “set” a VPD. What you do control are your temperature and your humidity. These are the fundamental dials you turn using your heaters, humidifiers, AC units, and dehumidifiers. If your automated systems are already keeping your temperature and humidity in check for each growth stage, then in my view, you’re already giving your plants what they need. Knowing the specific VPD number just tells you what that optimal environment is in another unit, but it doesn’t change your actions if your core controls are effective.
A single VPD number can represent wildly different, even harmful, environments. This is a major point of confusion, especially for new growers. For instance, a VPD of 1.0 kPa – which is often cited as a good target – could exist in several very different scenarios:
- 20.2°C and 56% RH = ~1.0 kPa VPD. In my experience, 20.2°C is simply too cold for optimal plant metabolism and growth, even if the “drying power” of the air is technically “perfect.”
- 26°C and 60% RH = ~1.0 kPa VPD. This combination is often a good range for plants but on the hot side.
- 30°C and 70% RH = ~1.0 kPa VPD. Here, the temperature is too high, causing heat stress for the plant, regardless of the VPD.
It can misguide new growers and distract from crucial basics. If a new grower is told to “chase a perfect VPD” number, they might get confused. They could end up thinking that just hitting that VPD number is enough, even if the underlying temperature is way off. This can lead to serious problems like plants freezing or cooking. In my view, focusing on temperature and humidity directly, and learning to observe your plants, is far more important for a new grower.
It’s adding complexity where it’s not needed. In my opinion, VPD feels like an extra layer that doesn’t add practical value to my results. It’s like having an F1 wheel nut gun in your toolbox – yes, it’s cool and high-tech, but useless.
Should You Worry About VPD When Buying New Gear?
I don’t believe so.
But if you have a different opinion and you believe that I’m talking nonsense and VPD is a vital tool for growers today, please let me know by commenting on the post.
For now, keep growing and peace out.
Written by Dirar Flazi