How to use VPD charts to grow your best plants.

What is a VPD chart & why does it matter?

Ahh yes, the noble (and never ending!) quest to grow the BEST plants indoors. 🌿 Whether you grow for fun, or growing’s your business, you know there’s always room for improvement. Over the last few years, VPD (vapour pressure deficit) & VPD charts have got a lot of press. Let’s take a look at their potential impacts on your grow.

In this post I’ll talk about:

• What VPD is.
• And what VPD isn’t.
• What a VPD chart is and how to read one.
• How to use VPD charts to grow better, healthier plants indoors.
• How to use VPD charts to grow greener indoors & reduce running costs.

As a grower, you want to focus on making real improvements to your grow. You don’t want to rely on guesswork or bro-science, but there’s a sea of information out there.

It’s hard to know what direction to go in to ensure you see meaningful results. I get it. So let’s get to it!

To understand what a VPD chart is and why it matters, let’s first look at the definition of VPD (vapour pressure deficit.)

What is VPD?

According to Wiki, “Vapour-pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated.“ VPD is measured in kPa (kilopascals,) which is a unit of pressure.

Vapour pressure deficit is the difference between the vapour pressure inside and outside of the plant.

What VPD isn’t.

Vapour pressure deficit is not a magic wand or a miracle cure for all your growing problems. The way a lot of people talk about it could lead you to believe otherwise!

If you have an indoor grow that’s working, VPD can help you improve it. VPD is one piece in a large puzzle. You should focus on meeting your plants basic requirements (enough light, heat, water, food and nutrients) first.

VPD charts are no substitute for watching and listening to your plants for any signs they’re giving you.

Why is VPD important?

VPD is important because it allows you to work out the right range of temperatures and humidity to aim for. This gives you the opportunity to optimise and fine tune your grow room. (It’s worth mentioning in passing that different plants or even different strains prefer a different VPD.)

Maintaining the right temperature and humidity in your grow room has several benefits:

• It makes it possible for you to avoid environmental issues like mould and mildew.
• And allows you to evade pest problems.
• All whilst enabling you to grow the best plants.

VPD also has an affect on:

• How your plants take up nutrients and CO2.
• How fast they transpire (give off water vapour through the stomata.)
• And how stressed they are.

What VPD charts are:

There are some pretty hefty maths calculations you can do to work out your VPD. If you’re up for the challenge, you can read about how to calculate VPD here.

To add extra complexity, VPD also differs if your plants are clones, seedlings, in vegetative growth or flowering. So most people use a VDP chart to save time and get results faster.

A VPD chart is a visual representation of how relative humidity and temperature interact to provide an environment for your plants. Normally relative humidity is shown horizontally and temperature vertically.

There are 2 ways that VPD charts are presented:

• There are separate charts for certain phases of growth. Throughout the lifetime of your plants, you’ll use different charts for propagation, vegetative growth and flowering stages. Normally in this sort of chart, you’re aiming to stay within the green band. The further you move away from that, the less optimal the conditions. In these charts sub-optimal temperatures and humidities are often represented as orange or red to or show a danger zone. These areas highlight very low or high VPD and growing conditions that are most likely to cause problems.

• Or there are charts that combine all phases of growth into one chart using a banded approach. With different zones of the chart representing propagation, vegetative growth and flowering.In this sort of chart, the band to stay within for each stage of growth is coloured differently. Any conditions outside those that are optimal for the different growth stages are often coloured red, again to show the danger zone.

How to read VPD charts:

You read a VPD chart by looking at where the relative humidity of your grow room intersects with the temperature of your grow room. This shows you whether the VPD in your grow room is in the sweet spot (great job by the way!) Or if your VPD falls either side of that ideal range.

VPD charts show you whether you need to increase or decrease your relative humidity in combination with increasing or decreasing your temperatures to give your plants the ideal conditions for their current stage of growth.

Interestingly, VPD charts can vary a lot in how they look and how useful they are. Here at Grow, we’re excited to be working on THE MOTHER of all VPD charts.

The Grow VPD chart:

When completed, it’ll come with environmental calculators and a personalised week by week timeline of recommended temperatures, humidities and VPD. The data it gives you will also take into account the equipment you use and the strains you grow. It’ll also include day and night measurements.

We’re creating it specially to give you all the actionable information you need to grow the BEST plants.

Our VPD calculator and VPD chart will help you to stay in the VPD sweet spot, prevent mould and induce colour… so watch this space!! 👀

EDIT: It’s here! check out our ultimate VPD calculator and environmental timeline here!

If you’re strapped for time, you can also take all the legwork out of measuring your grow room’s VPD by using a grow room sensor or grow room monitor like ours to measure VPD.

So now you know that VPD charts show you what temperature and humidity conditions you need to both aim for and avoid, let’s talk about why…

Fun VPD analogy:

Let’s say you decide to take a hike. It’s a balmy 25 degrees and any way you look at it, you’re going to work up a sweat. Sweating is a mechanism to cool the body down.

If it’s humid when you take that hike (there’s a lower vapour pressure deficit,) and it’ll feel much hotter. Chances are you’ll feel clammy and uncomfortable.

If it happens to be a dryer day when you take that hike (there’s a higher vapour pressure deficit,) it’ll feel less hot and much more manageable. This is because humidity affects how sweat evaporates. If conditions are dryer, or there’s a cooling breeze, your sweat can evaporate freely and you feel better.

But when the air is humid (high relative humidity and high VPD) it’s because it has already absorbed more moisture. If the air already holds a lot of moisture, it’ll mean that the sweat on your skin will evaporate much more slowly.

The result is that it feels hotter than it is, your body keeps producing sweat to try to cool you down, but it doesn’t work. The cycle continues and you end up hot and sticky.

Creating all that sweat also leaves your body dehydrated and lacking vital salt and minerals. In the short term, this isn’t much fun, in the long term, it can be dangerous! 🥵

The other end of the VPD scale:

Addressing the other end of the scale real quick. Really low humidity can also be problematic for people. It dries your skin and hair out and can provide the right environment for germs and viruses to thrive. It can make you more susceptible to colds and respiratory problems.

People don’t do great with extremes of temperature and humidity and the same goes for plants. So now let’s look at how high and low humidity scenarios affect your grow room and your plants.

Transpiration is when water evaporates from the leaves of your plant via stomata (the little pores on the surface of the leaves.) This is comparable to sweating in the example above.

VPD extremes:

As relative humidity in your grow environment rises, the rate that your plants can transpire at falls because the air becomes saturated.

When the air inside your grow room becomes saturated, the water which up until now has been a vapour is forced to condense and become liquid. This means you’ll get dew or a film of water on the leaves of your plants, which makes them more susceptible to rot and mildew. Not good.

When the relative humidity in your grow room decreases, it is easier for your plants to transpire. This is because water evaporates more easily into dryer air. The movement of air provided by any fans you’re running mimics the nice breeze on your hike and also increases the rate at which your plants can transpire.

So, say no to too much humidity! Great, got it!

…Well hold on, in drier environments where the vapour pressure deficit is greater, your plants are forced to pull more water from the roots. If your plants aren’t healthy, are clones or are particularly small, they can dry out and die.

High VPD can also cause overfeeding. This is because your plants will be taking up surplus nutrients whilst they take up all the extra water that they need. Also, not good.

How to use VPD charts to grow better, healthier plants:

A happy medium is the aim of the game. This is where your VPD chart comes in. That band down the middle, the sweet spot is where your plant’s going to be fitter, happier and more productive 😃

Either side of that sweet spot and your plants will be using it’s valuable resources inefficiently. They’ll be trying to heat, cool, transpire, fight pests or moulds and deal with excess nutrients. Not putting that energy into growth or flowering.

Whatever you’re growing, if you want the best quality, you have to provide your plants with conditions they need to thrive.

Check out this great ‘deep dive’ interview with Nadia Sabeh from Dr Greenhouse where she talks about the effects and importance of VPD.

“We can save on energy and we can save on cost”

It may all sound like a lot, but knowing how you can avoid the potential pitfalls is half the battle! Don’t worry, you’ve got this!

Let’s have a quick recap:

How to use a VPD chart:

1. Measure the temperature and humidity in your grow room.
2. Look at your VPD chart to see where your relative humidity and temperature readings intersect on the chart.
3. If your readings show conditions are outside the ideal VPD range, use the chart to determine what action to take.
4. Increase or decrease your relative humidity in combination with increasing or decreasing your temperatures.
5. Keep up a cycle of measuring and repeating to stay in the sweet spot.
6. If your figures are within the ideal range, you’re providing your plants with ideal VPD conditions – keep doing what you’re doing!

So, using a VPD chart shows you what optimal conditions to strive for to grow better, healthier plants, but how do you make changes to the VPD of your grow room?

To increase VPD in your grow room you can:

• Increase your temperature by using heating equipment or reducing any air conditioning or cooling.
• Decrease your humidity using a dehumidifier.
• Increase the intensity of your lights by moving them closer to your plants (this raises the leaf temperature.) Be sure to only move your lights closer within the recommended limits provided by your manufacturer.

To decrease VPD in your grow room you can:

• Decrease the temperature using air conditioning or another cooling method.
• Increase your humidity using a humidifier.
• Decrease the intensity of your lights by moving them further away from your plants and so lowering leaf temperature, (Again make sure to pay attention to the manufacturers guidelines.)

The methods to adjust VPD listed above are the most common, but there are more (potentially less energy intensive) things you can do.

For example, you could use an extraction fan to help control VPD. But air extraction and intake will affect both temperature and humidity inside your grow room, which in turn can change the VPD goal posts. (It’s all part of the indoor grow rubix cube puzzle!)

How to use VPD charts to grow greener & reduce running costs

Now we’ve covered how to grow healthier, better plants, you can probably see how using a VPD chart will make your grow greener and reduce your running costs.

But to avoid any doubt…

Your extra rewards for providing your plants with all they need to grow and providing them the optimal conditions using a VPD chart are:

• You’re not running heating, fans, dehumidifiers, humidifiers etc more of the time than you need to, so you save electricity and reduce your bills
• Because you’re not having to water plants more than they need, you save water and reduce your bills further.
• Your plants are running at optimal efficiency, so they’ll be using all the nutrients available to them and less will be wasted, saving you money.
• Using less electricity, water and nutrients is a much more sustainable and environmentally friendly way to grow.
• You can sleep easy at night knowing the changes you’ve made to reduce waste and grow greener are benefitting our planet. BRAVO! 👏

Takeaways:

When you begin the journey of learning to harness VPD in your grow room, it’s tempting to go at it full steam ahead. Remember that VPD is important, but it’s not a magic wand. Get the basics of your indoor grow dialled in first.

VPD charts are a great guide because they highlight the optimum range where the magic can happen, (more on that in future posts!) But it takes time to get your temperature and humidity aligned (even when using VPD charts.) Don’t expect to be able to nail it in a day!

Don’t worry if your VPD isn’t exactly where you want it to be 100% of the time either. Some fluctuation is not the end of the world; perfect is the enemy of good. Relax a little bit.

Enjoy the journey, keep your eyes on the end goal and think of those awesome, healthy plants and cost savings ahead.

You’re green fingered and you love plants, but you know indoor growing can be crazy energy intensive. Do your bit to grow greener using VPD charts, it feels good! 🌎