Ecothrive’s VPD Guide

What is VPD? 

VPD is an abbreviation for Vapour Pressure Deficit. VPD is a measurement of how much more “room” there is for humidity (water vapour) in the air at the current temperature. This is important because if there is no “room” left then the plant cannot transpire properly, potentially causing mineral deficiencies, such as reduced calcium uptake and lower growth rates in general. 

What do you mean by “room” in the air?

At different temperatures, the air around us is able to hold different volumes of water. The measurement of the volume of water in the air is called relative humidity. As the air temperature falls, the amount of water that can be held will reduce. This will help us in understanding why VPD is important. 

Why does it matter if there is too much or too little water in the air?

If you have ever visited a tropical greenhouse in summer you will have felt the effects of high temperatures with high humidity. It can become uncomfortable as it’s harder for your body to perspire (sweat). This is because our skin releases water, the water evaporates and this pulls the heat away from the body. A similar process happens with plants, called transpiration. Similar to our skin, leaves have tiny pores (stomata) that release water. Aside from evaporative cooling, transpiration also helps in the transportation of water and nutrients to different parts of the plants. 

Low VPD will slow your plant’s transpiration, potentially slowing the uptake of nutrients. Low VPD can also be associated with high humidity and condensation in the surrounding environment leading to mildew and bud rot in plants. Conversely, if your VPD is too high it allows your dry ambient air to create excessive transpiration from plants. This creates a situation in which the plants are forced to uptake a lot more water and nutrients, resulting in tip burn and leaf curl. The worst-case scenario with high VPD is the plant can’t keep up with the excessive transpiration and has to close its stomata to conserve water and plant growth stops altogether.  

How will learning about VPD help me as a grower? 

This is a question we have been asked many times over the years. Keeping your VPD in the correct range is one of the easiest ways to master your environment. VPD is basically using a balance of temperature and humidity to find the sweet spot for your plants to thrive in. VPD will control plant transpiration rates, CO2 uptake, nutrient uptake and plant stress. 

VPD for indoor growing; HPS vs LED lighting.

Lighting can be a big influence on your grow room environment. A 600W HPS fixture will usually give off a lot more radiant heat than a 600W LED fixture. This is evidently more noticeable when we take a leaf surface temperature reading, a grower with a stable air temperature and good air circulation would usually see around a 1-2°C cooler leaf temperature compared to the air temperature. 

Typically growers who change from HPS to LED grow lights find the plant temperature can be much cooler than the air, leading to low VPD, slow growth and nutrient deficiencies. To correct this, LED growers often use heaters to maintain a higher air temperature, which increases the plant temperature and keeps the VPD in the sweet spot. Ideally, the plant temperature should be around 25- 26°C at lights on, for optimal growth. 

Every environment is different so that’s why having the ability to read your own leaf temperature is so helpful. 

How do we measure leaf temperature?

The best way to measure leaf temperature is with an infrared thermometer. For growers looking to take things a step further, the AC Infinity VPD Gun (AC-TRV3) measures leaf temperature, ambient temperature, relative humidity, air VPD and leaf VPD in one device.

Unlike a basic infrared thermometer, the VPD Gun provides a more complete picture of your growing environment, helping you understand the difference between the temperature of your plants and the surrounding air. This information can be used to fine-tune your environment and calibrate compatible smart controllers for more accurate climate management.

The VPD Gun uses infrared technology with 12-point laser positioning to help target the area being measured. It also saves previous readings, making it easier to track changes in your environment over time.

When measuring leaf temperature, take several readings from larger leaves exposed to direct light. These are usually found towards the top of the canopy during vegetative growth, or just below the tops of the plants during flowering or fruit production. Taking multiple readings and calculating an average will give you a more representative picture of conditions across your growing area.

Shop the AC Infinity VPD Gun

How do I calculate my VPD?

As we have learnt, environmental control is key to growing happy plants! Calculating your VPD is really easy! All you need is three simple measurements:

• Air temperature
• Relative humidity
• Plant temperature using an infrared thermometer

A good quality digital thermo-hygrometer is fine to read the air temperature with relative humidity, just make sure it’s not in direct light and has good air movement around it. 

VPD combines the air temperature, humidity & leaf temperature into one reading. You use this to determine how well plants are going to transpire. 

Step 1

Look at the VPD chart, on the left-hand side you will see a temperature scale. This is the air temperature.

Step 2

Look along the top row of the VPD chart for the “Relative Humidity” line. Match your relative humidity reading to the closest option on the chart.

Step 3

Now we need to look at the stage of growth. The VPD chart has been colour coded to make it easier; light blue for propagation and early vegetative growth, green for late vegetative and early flower growth, and orange for mid-late flower. The red areas are the “danger zone”; drifting into these areas can result in poor growth, nutrient uptake issues and ultimately lower yield. 

Step 4

Now it’s time to get the leaf surface temperature, with this information we can navigate the charts more efficiently. 

Step 5

As an example, say we have an air temperature of 27°C and a leaf temperature of 25°C. We can now easily see there is a 2°C difference between the air and plant temperatures. For this scenario, we would now use the -2°C VPD chart as our reference point for this current environment. 

VPD Targets  

We have covered why VPD is important and how to take the measurements required to calculate it. Now we can look at the target VPDs and we can introduce the VPD tables. The table below is a good guide to the VPD band you should be aiming for. 

Transpiration Rate/Growth Stage	VPD (KpA)
Low Transpiration / Propagation /Early Veg	0.4-0.8
Healthy Transpiration / Late Veg / Early Flower	0.8-1.2
High Transpiration / Late Flower	1.2-1.6
Danger Zone	<0.4 / >1.6

If you would like to calculate your exact VPD, we have created this open-source Dynamic VPD Calculator here for anyone to use for FREE! All for the love of the plant.  Download this Excel spreadsheet below. It can be opened via Google sheets for easier mobile access. For unrestricted access to the formulas for your own projects, email us for the password – info@ecothrive.co.uk

My VPD wasn’t in the optimum zone. How do I fix it?

Getting your VPD into the right range usually comes down to making adjustments to your air temperature and relative humidity. Maintaining a consistent daytime temperature of around 26–28°C and a lights-off temperature of 24–26°C is a good starting point, but the ideal conditions will depend on your plants, growth stage and environment.

When making significant changes to your growing environment, remember that plants can experience environmental stress if conditions change too quickly. If you're transitioning from one VPD target to another as your plants progress through their growth stages, make adjustments gradually over the course of a week.

Pro tips: When growing indoors, we generally aim to keep leaf temperature around 25-26°C, with your lights on, to support healthy growth. Plants can perform well at higher temperatures when VPD is kept within an appropriate range, but running too cool can slow growth and reduce plant activity.

It's also worth considering the limitations of your equipment. Carbon filters and other extraction components may not perform as intended in very high humidity, so many growers aim to keep relative humidity below 75%. If you're running a low VPD environment with high humidity, pay particular attention to temperature fluctuations. A sudden drop in air temperature can cause condensation on leaf surfaces, increasing the risk of foliage diseases.

Take control of your VPD with AC Infinity

If you're serious about maintaining consistent growing conditions, investing in environmental control equipment can make a huge difference. At Ecothrive, we're big fans of AC Infinity. Their range of equipment makes it much easier to manage temperature, humidity and airflow, taking much of the guesswork out of maintaining your target VPD. We have plenty of happy customers using the system, and many wouldn't choose anything else.

The AC Infinity AI+ Controller is at the heart of a fully integrated setup, allowing compatible equipment to work together to maintain your desired environmental conditions. Rather than constantly adjusting individual settings, you can use environmental targets and automation to help keep your grow room on track.

You can build a setup around your needs, with Cloudline extraction systems to manage airflow and temperature, Cloudforge humidifiers to increase humidity, Hydrone dehumidifiers to remove excess moisture, Thermoforge heaters to bring temperatures up, and the Terraform air conditioning unit to help manage cooling and wider environmental conditions.

One of the best things about the range is its flexibility. Some AC Infinity products offer built-in intelligent controls and VPD-targeting functions, so you can automate individual pieces of equipment without necessarily needing a separate controller. If you want to take things further, compatible equipment can be integrated with an AI+ Controller to coordinate multiple devices and give you much greater control over your entire growing environment.

Whether you're looking for a simple way to manage humidity or want a fully automated grow room, there's an AC Infinity setup to suit your needs. Keeping your VPD within the right range becomes much more manageable when your equipment can respond to changing conditions automatically.

If your VPD is frequently outside your target range, start by identifying whether temperature, humidity or airflow is causing the problem, then choose the equipment that addresses it. And remember, environmental control is only one part of healthy plant growth. Products such as Biosys can also help support a healthy root zone and the beneficial biology in your growing medium.

Thanks for reading,

Team Ecothrive!