Vapour Pressure Deficit (VPD) Video Transcript

For those who were struggling to take in all of the information, here's a copy of the script from our recent YouTube video on vapour pressure deficit!

Hello to all of you growers out there! Welcome to the second part in our series on VPD or ‘Vapour Pressure Deficit’. One thing that experienced growers know only too well is that the environment in your room or tent plays the most important role in deciding your success. Replicating your chosen plant species’ optimal natural conditions as closely as possible is the key to extremely productive growing!

In part one, we touched on plant transpiration – essentially, the way that temperature and humidity conditions in your grow room affect your plants’ growth rates, the quality of any fruits produced and, ultimately, your ability to maximise yields. But to make sure we’re all on the same page, and for anybody who hasn’t already seen the first video, let’s have a quick recap:

- On the undersides of leaves are tiny pores, called ‘stomata’ which are what the plant uses to breath, absorbing CO2 from the surrounding air and releasing oxygen.
- This breathing happens as stomata open up during daylight hours, forming a vital component of the process of photosynthesis.
- For the stomata to breath properly, their linings need to stay wet, which requires a huge volume of water from the plant. In fact, this transfer of water accounts for up to 90% of a plants’ water loss!!!
- BUT, water is a precious resource… When plants are having to use too much of it to maintain moisturise levels, the stomata will start to close and growth slows. The plant effectively makes the decision that it’s more advantageous to maintain water levels than to keep the stomata open and photosynthesize.

But you want to know about vapour pressure deficit! Many indoor gardeners grow successfully with only a simple hygrometer, which measures temperature and relative humidity. Hygrometers are standard kit - they’ve served us well for many years and every grow room should have one. However, measuring VPD gives us much better information on the drying ability of the air around your plants, which provides a clearer insight into how much water is required to by the plant to maintain stomata moisture levels.

VPD as a measurement talks about the difference between the moisture content in the air and the moisture content the air can hold when saturated. In terms of plant growth, it relates to the difference between the pressure inside the leaves (leaf pressure) themselves and the surrounding atmosphere. It’s kind of like rolling temperature and humidity into one measurement to get a clearer picture of what’s going on directly around stomata, as opposed to just knowing what conditions are like in the room itself.

Putting it more simply, the higher the VPD, the more drying ability the air has and the more plants transpire. The key is in making sure that everything is sitting in the right ranges. In most cases, a certain amount of pressure deficit between the leaves and the outside air has a positive effect because some level of transpiration from stomata is needed to promote the flow of water from the roots to the rest of the plant. And it is this flow that brings along with it the nutrients that are required in countless roles around the plant.

If the VPD is too high, plants will lose too much water through transpiration and the stomata will close.

A low VPD indicates that the air is holding a lot of water, which slows down plants’ transpiration rates. Transpiration rates that are too low will slow down the movement of water and nutrients from the roots, inevitably causing problems and leading to nutrient deficiencies issues.

As VPD is a pressure reading, it’s usually written in mb or millibars, but you may also see VPD expressed in kilopascals or kPa. Converting between the two is easy enough: just divide the figure in millbars by ten to get the appropriate number in kPa – that’s it.

Here are some typical ranges:
For low transpiration 4 – 8mb or 0.4 - 0.8kPa (cutting / veg)
For healthy transpiration 8 – 12mb or 0.8 – 1.2kPa (flower)
For high transpiration 12 – 16mb or 1.2 – 1.6kPa (useful during peak flower)

So, now we know what VPD readings relate to, we need a way of calculating them!

By far the easiest way of getting accurate readings is to use a piece of equipment that does the job for you! The Maxi Controller is a complete grow room control system from Dimlux – a true leading edge company that specialises in high-end horticultural equipment. There isn’t much in a typical grow room that can’t be connected to the Maxi Controller – lights, co2 generators, CO2 sensors, heaters, humidifiers can all be controlled and regulated digitally from a central point.

To calculate VPD, you’ll need a temperature probe, a humidity probe and the revolutionary plant camera, which takes readings of the leaf temperatures at the top of the canopy. Once you got the reading you can then make the relevant changes to your set up to keep your VPD in the sweet spot at all times. By far the best way to do this is to alter humidity levels with the use of humidifiers or dehumidifiers. You may also need to increase or decrease your atmospheric temps as required using duct fans or your Opticlimate. Adjusting your plant canopy temperature can be done as simply as moving your light closer to or further away from your plants.

The Maxi Controller has it covered it either way, though, because you can set it up to automatically dim your lighting if temps go above your required settings. You should only ever use this as a last resort though – you want your lights running at full power as much as possible if you want to really maximise your yields.

The Maxi Controller may be the easiest way to calculate VPD, but there’s also a relatively simple way to work it out for non-Dimlux users. The I-R Pocket measures leaf temperatures, which you can then use to calculate VPD for yourself. You’ll need a hygrometer to take temperature and relative humidity readings from in the room, too. But if you’re growing indoors, you should have one of those around anyway. You just need to calculate leaf pressure, then the surrounding air pressure, and then you just subtract the two!

Here’s how:
- Point the I-R Pocket at the canopy and press ‘measure’ to get the leaf temperature. Keep the distance relative to the size of the square that you want to measure. If the leaf area to measure is 30cm x 30cm, keep the I-R Pocket 30cm away.
- Have a look at the table we’ve included and find out what the pressure would be for that temperature at 100% RH. You’ve now got your leaf vapour pressure!
- Then, just take the air temperature and relative humidity readings from your room with your hygrometer. Again, just check out where they intersect on the table and make a note of the number, which gives you your air vapour pressure
- Subtract leaf VP from air VP and you’ve got your vapour pressure deficit!

But what does all of this tell us and what changes should we make?
Because we have more accurate information we can see that at different temperatures the acceptable humidity levels change – the higher the temperature is in your room, the more you need to increase the humidity in order to maintain an optimal VPD. Looking solely at relative humidity is not enough to give us the information we need to maintain optimal plant health, though it can certainly help.

Hope this has been helpful for you.
Happy growing guys, take care

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