The ultimate guide to Vapor Pressure Deficit for Cannabis

10/02/2023

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Vapor Pressure Deficit (‘VPD’) measures the difference (or ‘deficit’) between the amount of moisture currently in the air and how much moisture the air can hold when fully saturated. For many less experienced growers, VPD feels like an unnecessarily complicated topic which they can afford to ignore and leave to more advanced growers.

But our quick introduction to VPD will show you that the concepts behind VPD are fundamentally quite simple, even if the VPD charts themselves appear technically daunting. A basic understanding of VPD can help you better understand how to optimise your own grow room conditions. The result could be bigger harvests and higher THC/cannabinoid content in your buds!

What is Vapor Pressure Deficit (VPD)?

One way to better understand what VPD really does is to think of it as a measure of the drying power of your grow room atmosphere. Cannabis releases moisture into the atmosphere through small pores in the leaf known as stomata. The release of moisture (transpiration) from the plant to the atmosphere will increase as temperature increases.

Transpiration will also increase as the surrounding air gets increasingly dryer, i.e. as humidity decreases in your grow room, the atmosphere will naturally drag increasing amounts of plant moisture into the dry air. The principle is much the same as the way that a wet towel drys quicker in a hot, dry room than it would in a damp, cold room.

The experienced cannabis grower will already be aware that desired grow room humidity levels should be monitored. Higher grow room %RH (relative humidity) levels can be tolerated in seedling and veg growth than they can be in late bloom. The VPD chart, below, uses different colours to highlight the correct VPD levels in seedlings (green shading), veg (blue shading) and bloom (blue shading in the graph).

In bloom, to avoid mold and bud rot, cannabis growers aim for low humidity (%RH) levels than they would in veg. Often aiming for RH levels of 40%-50% in bloom. The chart indicates that the corresponding temperature range for 40-50% RH would be around 23-24ºC (75F) for ideal VPD.

Leaf vapor pressure deficit chart

At first glance, the VPD chart looks way too busy to be considered user-friendly, and it probably is. To simplify, first consider the green shaded cells. This shows, for a range of grow temperatures (left hand scale), the preferred relative humidity in order to achieve the correct VPD in early veg growth.

Since most people tend to grow in a temperature range around 24ºC/75F you can conveniently ignore the rows with temperatures below 20ºC and those above 30ºC. The cells shaded green represent the preferred VPD during veg. Many growers will pay particular attention to the purple-shaded cells showing optimised VPD during cannabis bloom.

The data for grow room temperatures below 20ºC and above 30ºC is largely irrelevant for most indoor growers. Yes, you can dial in the correct vapour pressure deficit for a grow room at 35ºC but crop quality and yields would be abysmal for cannabis plants grown in such conditions.

Special comments – VPD during cannabis bloom

The purple shaded cells indicate the ideal VPD for flowering cannabis plants. Select your typical bloom temperature on the left-hand axis and from that you can determine the optimum humidity level in bloom. Note how, as cannabis plants grow and mature, they require less humidity. As seedlings they can tolerate much higher humidities. As bloom approaches it’s important to control VPD to minimise risks of mold/bud rot.

The grower should remember that a VPD chart is only a tool to guide them. It’s important to remember the basics too. Keep your grow room temperatures stable, avoid temperature extremes, try to keep your plants in the nutrient and optical sweet spot from cannabis seed to harvest, avoid over watering, grow the best cannabis seeds you can etc.

Many competent cannabis growers that have never seen a VPD chart might find that their preferred grow conditions are generally in agreement with the VPD chart recommendations.

How does VPD affect cannabis growth?

Proper air circulation is vital for your cannabis plants

The VPD chart recommendations show the various grow room temperature/humidity combinations that will produce good VPD for your cannabis grow room. Much of this is common sense. Your plants will dry out if the grow room is too hot and dry. Conversely, they will lose little water in cool conditions with high humidity. The VPD chart shows the preferred humidity level for a given temperature.

Keeping your cannabis plant in optimised conditions from seedling to harvest is the best way to get optimised bud quality with maximised cannabinoid and terpene content.

The VPD chart allows the grower to match preferred humidity to the grow room temperature at each stage of cannabis growth. Though it should be added that many experienced growers achieve good results routinely without having ever seen a VPD chart, simply by adhering to basic rules.

Criticisms of VPD charts

To some growers, especially old-school traditional growers, VPD is a classic example of unnecessary modern over-complication. VPD may seem like something they don’t need to worry about so long as they keep to sensible temperatures and humidity at each stage of growth.

The inclusion of VPD guidelines for extreme grow room temperatures as low as 15ºC (59F) and 35ºC (95F) is another justified criticism by many growers.

Just because VPD may be optimised at extreme temperatures/humidity doesn’t mean that optimised plant growth will be anywhere near possible. For some, it might seem a little more user friendly to publish VPD charts only for typical indoor grow room temperatures.

What is a good leaf VPD for a cannabis grow room?

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VPD is measured as a numerical value around 1. As the chart below shows, the desired VPD value increases gradually as the cannabis plant matures. Grow room relative humidity levels should decrease as harvest approaches.

Assuming leaf temperature 2.8° C below room temp.

Plant Cycle Stage	Min VPD	Max VPD	Temp.	RH
Propagation / Early Veg	0.8 kPa	1.0 kPa	21° C	60%
Late Veg / Early Flower	1.0 kPa	1.2 kPa	23° C	50%
Mid / Late Flower	1.2 kPa	1.6 kPa	23° C	40%

Young cannabis seedlings often prefer high humidity environments. Their young root systems will have limited ability to supply moisture, so keeping a moist humid seedling/cutting environment minimises transpiration (water loss). As plants age and harvest approaches keeping grow room %RH lower minimises risk of rot.

How to calculate Vapor Pressure Deficit for cannabis

Other than hard-core VPD fans, few cannabis home growers tend to routinely calculate the VPD levels in their grow room. Instead, they prefer to monitor grow room temperature and relative humidity.

The experienced grower understands the need for stable optimised grow room temperatures and the role played by temperature/humidity as the plants mature. Often without fully understanding the technical details of VPD, many experienced growers hit the right VPD simply by following solid grow room guidelines and sticking to the basics.

For those that enjoy the technical side to cannabis growing, they can calculate VPD values by using the chart above. You simply need to know grow room temperature, %RH and plant maturity stage. If VPD is too high the cannabis plants will dry out, unable to replace their lost moisture.

FAQ about VDP for cannabis

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Both are important. But the key is to understand that temperature is a key parameter affecting how your plant is able to keep up with the water loss (transpiration). In that respect, VPD is often considered a more comprehensive/complete measurement due to the way it is linked to grow room temperature. The only criticism of VPD is that it can seem a little too abstract and over-complexed to many home growers.

Yes. VPD is derived from both humidity and temperature. That’s why, especially for some professional cannabis growers, VPD is taken very seriously and recorded. Any changes to the trends away from optimised VPD grow conditions will attract instant attention and remedies.

During the night, cannabis plants have no light so there is no photosynthesis. VPD, measured in kPa (kilo Pascals) drops as a result. Some pro-growers like to avoid stressing the plant with a VPD change which is too large. Changes greater than ~0.4 kPa have been reported to result in serious (~20%) yield losses. For this reason, many licensed professional growers prefer to keep night-time VPD fairly close to daytime values. For seedlings ideal night-time VPD is around 0.8 kPa. For cannabis plants in veg, the ideal night value is around 1 kPa. In bloom a night-time VPD around 1.2 kPa is typical). Remember, VPD is about plant transpiration. This happens most during the day, meaning that daytime VPD is perhaps of more importance than night-time VPD.

If your grow room air is ‘perfectly dry’ with no atmospheric water content (0% humidity) then it will have an extreme drying/dehydrating effect on your plant, especially as temperatures rise. This will result in a high water demand from your plant, it may wilt as it struggles to keep pace with the water loss from transpiration.

Monitoring your grow room temperature and humidity isn’t too difficult. But quality focussed growers know temperature and humidity levels in their grow room and will be aware of how those drift as plants age. They will also be aware of seasonal fluctuations and effects of their grow room temperature/humidity and VPD. Knowing whether VPD is too high or too low really can allow you to improve final harvest quality. Some growers will, for example, buy a de-humidifier to allow healthier grow conditions during late bloom if they find that humidity is simply excessive. Others will avoid growing in the hottest seasons, or perhaps will invest in air-conditioning for growing at certain times of year.

Start by using the VPD table to calculate the VPD in your grow room. You will need to measure and monitor both %RH (relative humidity) and temperature to derive the VPD. You may wish to record how this changes from summer to winter. You may also wish to compare readings with a tent of seedlings vs a tent of mature females.

Being able to monitor a grow room parameter such as VPD is the first stage in being able to optimise and control it. You may find that your grow room VPD has always been fine, even without realising that VPD was an important parameter. Or you may find that monitoring VPD exposes some other grow room issue.

For some growers, VPD monitoring allows them to highlight the need for greater control over temperature and humidity. You may have enjoyed years of successful harvests without ever worrying about VPD, meaning that your VPD conditions just happened to be good enough. Or you may have nagging doubts that your harvest quality/quantity is consistently below par. If that’s the case, it could well be worth calculating the VPD in your grow room.

VPD is just one of many considerations when growing cannabis. To some VPD can seem to be one of the more obscure areas of cultivation. If so, we hope this guide has helped clarify VPD and its importance to cannabis growing.