If your mission is to grow the highest quality cannabis plants then you are in the right place.
Heard about vapour pressure deficit (VPD) but not sure how it can benefit your grow room? Let’s fix that right now!
It may seem that you already have enough on your plate without adding any more to think about. The good news is that there is a high chance that you are already taking the measurements and checks needed to optimize your grow room with VPD.
So without much extra work at all, you could supercharge your grow!
The Importance of VPD for Your Grow Room
Indoor growing allows for a high level of control over the growing environment. The ability to micromanage your grow room helps to produce high quality, healthy cannabis plants.
Many professional growers already measure temperature and relative humidity within their grow spaces. Measuring VPD instead is actually more beneficial because it recognises the important relationship between temperature and humidity.
Vapour pressure deficit relates to the difference (deficit) between the humidity levels found inside a leaf of one of your plants and the humidity outside that leaf (in your grow room). VPD is displayed in a unit named kilopascals (kPa) and is measured as pressure.
Let’s say the conditions outside of your leaf are dry. This will cause the leaf to transpire and release water vapour through the stomata, in an attempt to match the pressure inside itself with that of the outside. That’s a stressed plant!
A VPD reading will tell you how much stress your plants are experiencing and how comfortable they are within your grow room.
If you stop and think for a moment, plants are just like people. Yes, we humans can physically survive in a temperature range of say -20°C and +40°C, but within that range is an optimum level of temperatures and humidities that we are most comfortable and productive in.
Like humans, different plants and strains will tolerate and thrive in different VPD ranges.
Rather than delving into that here, we are simply going to focus on VPD and how it can improve your grow if you are growing cannabis indoors.
How to Work Out the Ideal VPD Range for Your Grow Room
The first thing you will need to do is to measure the temperature and humidity in your grow space. You can do this simply with a thermometer and hygrometer (but a grow room sensor will also be able to give you this data plus much more too!)
Once you have your readings, it’s time to use a VPD chart or VPD calculator to work out the VPD of your grow room.
Using a VPD Calculator
There are many different VPD calculators and VPD charts available to access online, but with professional growers in mind, the team at Grow created a particularly comprehensive one. Get your own free, customisable VPD calculator and environmental timeline here.
This particular useful digital calculator will benefit you and your grow room by…
- Enabling you to grow healthier and happier plants
- Helping you to prevent mould
- Assisting you to induce colour
- Providing you with recommended day and nighttime measurements
- Giving you an understanding of your ideal temperature range and relative humidity
- Teaching you how to stay in the VPD sweet spot.
The tool has been designed to be highly customisable to you and your grow room.
You can enter information about the lights you are using and what species and strains you are growing.
This enables the tool to create a personalised, week by week, VPD grow calendar for your operation.
5 Ways to Use VPD to Supercharge Your Grow Room
Right then, now we have covered what VPD is, let’s get to business on how to use it to maximise the potential of your grow room.
For optimum plant health, a VDP range of 0.8 – 1.2 kPa (kilopascals) is generally recommended. However, as your plants progress through their life cycle stages, their needs change and therefore their ideal VPD changes too.
With this in mind, we are going to explore the benefits of the VPD sweet spot in each stage of your plant’s life cycle.
1. Use VPD to ensure healthy clones and seedlings
Clones and seedlings are not strong enough to withstand a lot of stress or extreme conditions.
At this stage, your plants have not had the opportunity to grow sturdy and well-developed root systems yet. This impacts their ability to draw up water and nutrients efficiently.
Maintaining a low and consistent VPD range is recommended to nurture your seedlings.
The rate of transpiration will be lowered by the low VPD range and higher humidity. This prevents your young plants from losing too much moisture while they are unable to easily replace it.
A suitable VPD for clones and seedlings in early vegetative stage is 0.8 kPa.
2. Use VPD to promote strong growth during the vegetative stage
Whilst the flowering stage can be considered the most exciting, the foundations for success need to be laid during the vegetative stage. So no scrimping on time or focus here!
Vapour pressure deficit is of particular importance during the vegetative stage because it affects your plants in a number of interlinked ways:
- Regulates stomatal opening
- Determines the rate of CO2 uptake
- Controls transpiration speeds
- Controls nutrient take-up
- Influences plants stress levels
When your plants reach the vegetative stage, they are more robust and have a stronger root system. This allows you to dial down the humidity levels in your grow room to increase VPD.
An appropriate vapor pressure deficit for plants in vegetative growth is 1.1 kPa.
Higher VPD at this stage enables transpiration to speed up and with that comes an increase in nutrient and water take-up.
VPD Pro Tip!
Disperse the boundary layer
The boundary layer is a thin, still and humid layer of air that surrounds the surface of a leaf. It occurs naturally and is created by the plant when it transpires.
If this boundary layer becomes too thick, often due to thick foliage or densely crowded plants, then transpiration will slow down.
Should the air surrounding the leaves become too humid, water vapour will not be able to exit the stomata via transpiration because of the low VPD.
This means that even if you are measuring for optimum VPD, the boundary layer may cause your plants to experience different conditions to those that are being measured. This could raise the risk of mould for your plants.
But fear not! Here’s what you can do…
If you measure your VPD as close to the leaves of the canopy as possible rather than nearer to the stem or roots of your plants then you will achieve a more useful reading.
Following this, take the measures to disperse the boundary layer around the canopy by moving the air to maintain appropriate levels of humidity.
3. Improve yields by managing VPD during flowering
By keeping VPD on the low side until this point, you should hopefully have healthy and robust plants with lots of leaves and large roots.
Your plants will now tolerate a higher VPD, which is great because this is required during the flowering stage. Raising the VPD when plants are established allows for better take-up of nutrients.
It is thought that increasing VPD in the latter stages of flowering can lead to an increase in quality and trichrome production.
Your plants are of the most value during the flowering stage. So a slightly drier atmosphere (higher VPD/lower humidity) brings the added benefit of preventing powdery mildew and mould.
The recommended ideal VPD changes throughout the flowering stage:
- Early flowering: 1.2 kPa
- Mid flowering: 1.3 kPa
- Late flowering: 1.4 kPa
4. Effectively optimise all stages of plant growth with VPD
Whilst indoor growing brings you a high level of control, as hard as you try, your grow room conditions will never remain static.
You may hit the VPD sweet spot in one moment and not quite nail it the next as your plants transpire and release moisture into the air.
Ensuring that you have the equipment and processes in place to manage any increases in relative humidity is important, especially during the night when temperatures drop but humidity increases. The same goes for the changes that can occur when you increase your VPD during the flowering stage.
5. Avoid extremes of VPD for optimal plant health
Achieving the optimal VPD range for your plants is likely to require some tweaks to your grow room environment, but remember to make these changes incrementally.
The rate that your plants move water and nutrients from the root to the rest of the plant is directly linked to VPD, so any sudden changes can have unintended consequences!
You want to aim to avoid the extreme highs or lows of VPD and the effects these have on your plants.
If VPD is too low:
- High levels of humidity will prevent your plants from transpiring.
- Slower metabolism and nutrient take-up will hinder plant development.
- Carbon dioxide take-up will be slower due to slower transpiration caused by high humidity.
- The humidity of the air around the plants will contribute to a risk of mould and mildew.
- Plants will be subjected to stress and unless this is managed, they may eventually die.
If VPD is too high:
- Low humidity levels will cause your plants to transpire too much.
- Plants that respire too much risk suffering from nutrient burn from the excess salts they have taken up with all the water that they need.
- Air that becomes too dry creates an environment in which plants are at risk of spider mites and other insect infestations that favour low humidity conditions.
- Extremely high VPD and water stress will force plants to close their stomata in a bid to retain water. This prevents any further CO2 take-up and restricts growth.
- Plants will experience high levels of stress which if not managed may cause them to die.
VPD is important throughout every stage of your cannabis plant’s life cycle.
But the essential needs of your plants need to be met and maintained before you’re in a place to tweak your grow space’s VPD to generate a substantial impact.
Make any changes to your environment incrementally to avoid unintended consequences.
VPD can really help you dial in your grow and supercharge your room enabling you to grow the best quality and most robust plants possible…
But it’s not a magic solution to all your problems, you still need to keep a close eye on your grow and closely manage any issues that crop up.
Sometimes maintaining the absolutely ideal VPD might be out of reach due to environmental conditions where you are, equipment limitations and other external factors.
Don’t panic, aim for a VPD range that is closer to ideal but also achievable for you and your set-up. In this case, avoiding extremes of VPD is more important than maintaining perfect grow room VPD 24/7.
- 5 Ways to Supercharge Your Indoor Grow Room with VPD - August 24, 2021