If you’re here, it’s because you’re driven to grow the best quality plants indoors. Knowing vapour pressure deficit (VPD) could help you is all well and good, but how exactly can VPD improve your indoor grow? Is it really worth it?
Making the decision to embrace measuring and using vapour pressure deficit (also sometimes known as vapour pressure differential) might seem a big undertaking, but it isn’t.
I’m pretty sure you’re taking the measurements that you’ll need to work out the ideal VPD values for your grow room already! 😃
If you’re not completely new to the concept of vapour pressure deficit, but looking for the how’s and the why’s, you’re in the right place!
The importance of vapour pressure deficit:
Vapour pressure deficit is important because when you grow indoors, it’s something that you have the ability to control. And when you do, it can help your plants to thrive.
Some growers use relative humidity (RH) measurements to optimise their grow space. But using vapour pressure deficit values is preferable because it takes into account the important relationship between humidity and temperature.
Vapour pressure deficit describes the difference between the humidity inside a leaf (which is always going to be 100%, it’s wet in there!) and the humidity outside the leaf (which you have the ability to control when growing indoors.) VPD is measured in pressure.
If conditions outside the leaf are dry, the leaf will transpire and release water vapour through the stomata to try and equalise the pressure inside and outside the leaf. A vapour pressure deficit value tells you about how comfortable your plants are, or how much stress they’re under.
Plants are like people – there’s an optimum range of conditions that ensure they function their best. I mean sure, we can survive right through from -20’C to 40’C+ but there’s definitely a range of temperatures that’s comfortable and that we can be most productive at.
The same goes for plants. The less stress plants are under, the more comfortable and happy they are, the more efficient and productive they can be.
Different plants thrive at slightly different vapour pressure deficits (and there’s a post in the pipeline about that too!) But for the purposes of this post, I’m going to be talking about VPD and how it can improve your grow if you’re growing medicinal marijuana.
Just a ‘lil disclaimer:
Any information given on this site is for educational purposes only. Please ensure if you’re growing medical marijuana you’re doing so in accordance with the law and subject to appropriate permissions and licenses of the applicable country.
Now that’s out of the way…
To work out vapour pressure deficit, you need to measure both the temperature and humidity in your grow room using a thermometer and hygrometer. With those readings taken, you can simply use a vapour pressure deficit chart to work out the VPD in your grow room.
Conveniently, we’ve just finished putting together the mother of VPD charts.
There’s plenty of VPD charts and VPD calculators scattered across the internet. It maybe didn’t need another… But we’ve worked long and hard to create something that gives growers like you a higher level of insight and value.
What we’ve created is a VPD calculator and environmental timeline combined.
Here’s a bit of info about it:
- It enables you to grow healthier plants.
- It’s highly customisable, taking into consideration the information you give about what lights you’re using, what species (indica or sativa) and strains you’re growing etc.
- Then you’ll get a FREE personalised, week by week, VPD grow calendar.
- You get recommended conditions for every stage of your grow cycle.
- You’ll learn how to stay in the VPD sweet spot.
- You gain understanding your ideal temperature range and relative humidity.
- You’ll get day and night time measurements.
- You will be able to use it to prevent mould.
- You’ll discover how to induce colour.
- It offers the ability to collect data for optimising your future grows.
Sounds good right? Download the ultimate VPD calculator & environmental timeline here.
We also recently covered how to use a VPD chart to grow your best plants so check that out too to learn more about how to read and make the best use of VPD charts.
Right, let’s get to what you came here for and jump into how vapour pressure deficit can improve your indoor grow…
5 ways vapour pressure deficit can improve your indoor grow.
The optimum vapour pressure deficit for plant growth generally falls within the range of 0.8 – 1.2 kPa (kilopascals.) BUT… your plant’s needs (and therefore the ideal vapour pressure deficit) varies at different stages of plant growth.
In this post, we’re going to focus on how paying close attention to VPD can improve your indoor grow at every stage of your plant’s life cycle.
Disclaimer here: bear in mind here that the both strain you’re growing and your set up will affect VPD. It’s really important that you keep a close eye on your plants and act or react according to the feedback they’re giving you.
1. Use vapour pressure deficit to ensure healthy clones and seedlings.
Both clones and seedlings are baby plants. They’re not able to withstand a lot of stress and can quickly be negatively affected by extreme conditions.
Clones and seedling both lack a sturdy and well developed root system to efficiently draw water and nutrients up through the plant. For that reason it’s recommended to provide a low (and consistent) vapour pressure deficit.
A lower VPD (higher humidity) will slow the rate of transpiration. This prevents your young plants from losing excessive moisture from their leaves when they’re not easily able to replace it.
So what’s a good VPD for clones or seedlings?
An appropriate vapour pressure deficit for seedlings and clones (in early vegetative stage) is 0.8kPa. 🌱
2. Promote strong growth during vegetative stage with vapour pressure deficit.
Whilst the flowering stage of your plants life cycle is arguably the most exciting and important, it’s the foundations you lay during vegetative growth that are crucial to a successful grow cycle. Optimising during vegetative growth lines your plants up for success during flowering.
Vapour pressure deficit during vegetative growth is particularly important because it affects many crucial processes in a domino effect:
- It regulates stomatal opening. (Plants react to a higher VPD by reducing the size of the stomatal openings.)
- It determines the rate of CO2 uptake. (When stomata are smaller at high VPD values, CO2 uptake happens more slowly.)
- It controls the speed at which plants transpire. (At higher VPD, plants transpire and give off water vapour quicker because there is a bigger difference between the amount of water vapour inside and outside the leaf.)
- It controls the amount of nutrients roots are able to take up. (When transpiration happens faster because of raised VPD, plants pull more nutrients from the roots.)
- It influences plant stress levels. (When VPD’s higher, plants can take up less CO2, they’re respiring faster and pulling more nutrients and water from the soil. Just like people, when you’re spinning more plates, you’re under more stress.)
Optimise your VPD in Vegetative growth
By the time your plants are in their vegetative stage, they’re more robust with a better established root system. This means you can start to reduce humidity levels in your grow room to increase the vapour pressure deficit. This in turn kickstarts faster transpiration, ensuring your plants take up more water and nutrients.
Manipulating vapour pressure deficit during vegetative growth gives you the opportunity to optimise plant growth and development and ensure peak performance. Adjusting your VPD enables you to increase chances of healthy robust plants and maximum yields come flowering.
A word to the wise though, it’s really important to make small adjustments as you go about dialling your grow. Pay close attention to any changes and the results you get before making further amends to your environment.
It’s not uncommon for growers starting out with VPD to grab a chart and follow it to the letter without looking for visual queues from within the grow room about how it’s working out for them.
Each grow has a different combination of species, strains, grow spaces, equipment and therefore environmental conditions, so tailor your VPD to your specific grow’s needs.
This is why having a product like the Grow sensor is so valuable, because it gives you a live view of your environment at all times.
VPD pro tip: Disperse the boundary layer.
The boundary layer is a thin, still and humid layer of air which surrounds the surface of a leaf – a sort of microclimate. It occurs naturally and is created by the plants themselves when they transpire.
If the boundary layer is thicker (due to densely packed plants or thick foliage,) transpiration rates will be slower. When air surrounding the leaves becomes too humid, water vapour cannot exit the stomata via transpiration due to the low vapour pressure deficit.
So whilst you may be taking measurements and striving for the optimum VPD, your plants could be experiencing quite different conditions to those you’re measuring due to the boundary layer. This puts your plants at risk of mould.
What can you do about it? Measure vapour pressure deficit close to the leaves of the canopy. Measuring VPD close to the stem or roots of your plant won’t help you optimise your grow in the same way as if you take that reading close to the canopy.
Then take the necessary measures to disperse the boundary layers around the canopy with moving air to maintain appropriate levels of relative humidity.
So what’s a good VPD for vegetative growth?
An appropriate vapour pressure deficit for plants in vegetative growth is 1.1 kPa.🌿
3. Improve yields by managing vapour pressure deficit during flowering.
So, having kept your vapour pressure deficit low up until now to promote vigorous, strong growth, your plants will have a lot of leaves, large roots and they’ll (hopefully) be robust and healthy.
They’re now going to be able to withstand a higher vapour pressure deficit. Which is good because when plants enter the flowering stage, you’ll want to raise the VPD.
Raising the vapour pressure deficit once your plants are well established allows them to take up as many nutrients as possible.
Evidence suggests that increasing your vapour pressure deficit in the latter stages of flowering increases quality and trichrome production.
The flowering phase is when your plants are of most value, so there’s also the added benefit that an atmosphere that’s slightly drier will prevent powdery mildew and mould.
So what’s a good VPD for flowering?
Vapour pressure deficit during flowering should be towards the top end of optimal range.
During early flower 1.2 kPa is recommended.
For mid flower: 1.3 kPa is advised.
For late flower: 1.4 kPa is better.
4. Effectively optimise all stages of plant growth with vapour pressure deficit.
Growing indoors allows you to completely control your grow environment and vapour pressure deficit is a key piece of the puzzle. VPD is a dependable and tested agricultural theory that you can apply to optimise your grow room, at every stage of your plant’s life cycle.
As much as you’ll try, conditions in your grow room will never be static. You’ll find yourself in the VPD sweet spot one moment and slightly out soon after as plants transpire and steadily release moisture into the environment.
It’s important to have processes and equipment in place to deal with increases in relative humidity. This is especially important at night when temperatures drop and humidity creeps up.
Make sure you’re prepared to manage the humidity changes as you increase your VPD through flowering too.
When you work towards to optimising your grow space for VPD, there are also cost savings to me made.
Your plants won’t necessarily need as much heat or cooling as you might have provided them if you weren’t using VPD to help you determine the ideal conditions.
Your plants will also waste less water and nutrients if they’re not having to transpire at a faster rate than is comfortable for them.
Economising on electricity, water and nutrient usage is doing your bit for the environment and going greener.
There are some compromises and balances to strike between vapour pressure deficit and a range of other factors, but using the appropriate VPD value for where you’re at in your growth cycle is the best place to start to get results.
Optimise your grow room for each stage of plant growth using the suggested VPD values in combination with keeping a beady eye on your environment and plants. Observe, tweak, repeat!
5. Avoid extremes of vapour pressure deficit for optimal plant health.
So by now, vapour pressure deficit could sound like a magic formula to supercharge your plants… but you can have too much of a good thing!
Because the rate at which plants move water and nutrients from the root to the rest of the plant is directly linked to VPD, you need to tread cautiously when making tweaks to your environment.
Extreme VPD values, whether they are high or low provide conditions that mean that your plants won’t be able to transpire like they normally should and that obviously negatively impacts growth.
What happens when vapour pressure deficit is too low?
- High levels of humidity prevent plants from transpiring.
- The slow in metabolism and reduction of nutrients being taken up will hinder development.
- When air inside your grow room is too humid, transpiration slows and this in turn means your plants won’t be able to take up carbon dioxide like they should.
- This subjects plants to stress and if the situation persists, eventually they’ll die.
- When VPD is too low, it reflects the air around the plants being too humid and that creates an environment susceptible to mould and mildew.
What happens when vapour pressure deficit is too high?
- When VPD is too high, low levels of humidity mean that plants will transpire too much.
- Plants that respire too much are at risk of nutrient burn from the excess salts they’ve taken up (with all the water they needed) building up in the leaves.
- Air that’s too dry creates an environment that makes your plants susceptible to spider mites and other insects that favour low humidity conditions.
- Extremely high vapour pressure deficit and water stress forces plants to close stomata to try to retain water. This has the knock on effect of preventing any further uptake of CO2 which restricts growth.
- Plants also experience high levels of stress and eventually they’ll die.
…If all that doesn’t make a strong case for maintaining a comfortable environment for your plants, i don’t know what will! 😳
Vapour pressure deficit is important at every stage of your plants’ lives. You can improve the success of your indoor grow cycles by having a good understanding of VPD and putting that knowledge into practise.
Vapour pressure deficit is a great tool to have in your arsenal but it’s really important to remember that when we talk about VPD, there’s an assumption that the roots of your plants are constantly supplied with enough water. If there’s not enough water VPD ain’t a thing.
All you need to start implementing VPD best practises are a thermometer, a hygrometer and a VPD chart… But you can make tracking VPD even easier using a grow room sensor like ours to automatically report on VPD in real time.
Remember – haste makes waste. Any adjustments you make when using vapour pressure deficit to optimise your plant grow cycle should be made incrementally.
Vapour pressure deficit isn’t a magic wand and you need to be meticulous about observing your plants health and their environment and reviewing your results before responding accordingly.
The best advice we can give is to keep your vapour pressure deficit in a range that’ll keep your plants (and you!) comfortable.
Keep experimenting and determine what works well for your style of growing. 🌱
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