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VPD: How Can Vapour Pressure Deficit Boost Cannabis Yields?

VPD, or Vapour Pressure Deficit, is a technique used to discover and implement optimal temperature and relative humidity conditions to achieve greater cannabis plant performance.

VPD stands for Vapour Pressure Deficit. It is a way of calculating the exact combination of temperature and relative humidity to achieve absolute peak performance from a plant. If you secure the right environmental conditions and are able to maintain them, the results are nothing short of astonishing.

If you are able to dial in the right temperature and humidity in your grow-room, this is a must-read.

VPD has been known for quite a while, but fell out of the agro-horticulture circuits because large-scale operations rely on natural climate, as opposed to indoor growing. Only more recently, with the advent of the greenhouse and an influx of hobby cannabis growers, has it become possible to actually control environmental conditions with evermore precision to make practical use of VPD.

What was once only capable by university scientists is now possible to achieve by anyone, anywhere!

All you need is to get a cheap infrared thermometer gun and a relative humidity monitor (which you should already own by now), and start aiming the gun at your canopy, read the leaf temperature, and adjust either the temperature or the relative humidity (RH%)—whichever is more convenient.

It is as simple as that in practical terms. But the complete theory can get very confusing, very quickly.

THE THEORY BEHIND VPD

We will try to keep it as simple as possible. The mathematics behind this concept are quite complex indeed, but we do not need to get into them. Thankfully, there are many charts out there that will make your calculations as simple as crossing two numbers together—temperature x relative humidity.

But… for the sake of accuracy, VPD is the difference (or deficit) between how much water air can hold, compared to how much it is actually holding at that given moment. This difference is expressed in kilopascals, a pressure metric. The higher the ambient temperature, the higher concentration of water molecules can be maintained in the air, until relative humidity reaches 100% and condensation occurs. So, as both temperature and relative humidity shift, the deficit can be greatly increased or decreased.

DO I REALLY NEED TO USE VPD?

Not at all. Depends on how much you appreciate a significant yield increase come harvest.

The principle behind plant growth is: light, food, and water. As you may know, plants consume CO₂ (carbon dioxide) and excrete O₂ (oxygen). They take in water and nutrients mainly through the roots, and transport it to the leaves where the light source then hits to energise the metabolic processes in the developing plant. With VPD, you can control (and hopefully increase) the rate at which these metabolic processes occur—though technically indirectly.

Underneath the leaves, there are tiny structures called stomata. These are openings that mediate the entry of water molecules and carbon dioxide, as well as serving as an exit port for oxygen and water molecules.

The stomata are sensitive to the environment. They can “read” how much humidity is in the air as well as the air column pressure. If needed, they can bring in some extra water through the leaves. Good news for avid foliar sprayers.

On the contrary, if the temperature is too high, the plant will try to transpire as much water as possible as means to cool-off. Much like we as well.

The problem is, when both temperature and relative humidity are either too high or too low, for instance, if the room temperature is too high and the relative humidity is also too high, the leaves are incapable of transpiring. When this happens, your cannabis production rate decreases.

The higher the ambient temperature, the higher quantity of water molecules it can absorb. This increases the surrounding air pressure. It is this air pressure that constricts the stomata from being able to open. They will do their best, but will not be as efficient, and far from working at peak performance.

You may think you have dialed in your grow room perfectly; but in all practicality, there is likely a great margin for improvement. It only makes sense to use VPD to dial in perfect conditions if you are able to fine tune to precise values of both temperature and humidity. In practical terms, most of the time you will simply be adjusting temperature.

BUT I ALWAYS RUN MY ROOM AT OPTIMAL LEVELS!

The more advanced you get, the finer the details get, but also the higher the rewards. Grow rooms are becoming ever more intelligent with the lowering of prices regarding technology. Just a few years back, a full kit of sensors would be prohibitively expensive. In this day and age, you can get continuous readings from your grow room straight to your smartphone, without breaking the bank.

Sure, everyone knows that during flowering, you will want to keep the temperatures at around 25°C and below 55–65% RH to avoid bud-rot. Under 35% RH may be too stressful for the plant. If you run CO₂ supplementation, you can easily run both higher temperatures and humidity levels safely.

But what if you could significantly increase your final yield, simply by knowing with absolute certainty the exact combination of temperature and RH to turbo-charge the stomata’s efficiency? This is the power of VPD.

HOW DOES IT WORK IN PRACTICE?

Start by getting an infrared thermometer gun. These can be found quite cheaply with lower precision tolerances, but are good enough to get the job done and let you see first hand how effective VPD actually is. Or, out of curiosity, to see how far off you are from optimal performance.

You start by pointing and shooting the gun’s laser over your top canopy while you take leaf surface temperature readings. These will vary across your canopy, so take a good few measurements at your most productive regions. Average these values to get a final temperature reading, and then grab your current RH% level.

Simply cross-reference these numbers in a chart like this one, and you will know where you sit regarding VPD. The best of it, no calculations are needed.

If (not surprisingly) you are too far away from optimal levels, just look at the closest, most viable option (temperature or RH%) and tweak your grow room accordingly.

That is essentially it. Do this from time to time to keep a good VPD in its place, and get prepared to be amazed. If you thought you knew your strain well, you might be in for a pleasant surprise.

To really take this to an entirely new level, consider purchasing a continuous measuring unit and see how temperature and RH% varies during a full day-night cycle, permitting you to adjust for ultimate peak performance.

VPD can be a challenge to understand at first, but it is extremely easy to put into use. We highly recommend you try it out, your plant will love you for it.

Understanding Vapor Pressure Deficit: The Weird Humidity Puzzle in Marijuana Growing

Vapor pressure deficit (VPD) is a weird concept and kind of hard to understand, even when I’m not high, lol. But it’s a crucial concept to at least attempt to understand and manage because it significantly impacts how well your plants grow, or whether they grow at all. Let’s dig in to the concept, and see what to do about it in marijuana growing.

And here’s a spoiler alert–even if all this word salad I’m typing just seems like a mishmash of confusing tech talk, I’m including a VPD chart and several very useful videos with this article, and will explain how to use the chart.

We already know that cannabis plants do best in a narrow range of relative humidity (RH). RH is the amount of humidity present in the air at a specific temperature and is expressed as a percentage. When air is completely saturated with moisture, it’s at 100% RH.

For most of us who only look at a humidity monitor in our grow rooms or outdoor gardens and didn’t consider what temperature was present in the environment at that moment, managing relative humidity seems relatively simple.

Indeed, for many years of growing, I only cared that RH not get too high or too low, and for very simple reasons. I knew that too-high RH helped gray mold (botrytis cinerea) grow on my buds and that too-low RH could stress my plants by sucking a lot of water out of them through their leaves. When plants experience drought stress and intake a lot of water, they also intake a lot of nutrients. This costs you extra money for nutrients, and can also burn your plants with excess nutrients salts.

What I didn’t yet understand is that RH and temperature are the primary climate variables influencing water movement within my cannabis plants, but I knew that adequate water movement from roots to shoots was absolutely necessary for maximal plant health and performance.

Plant internal water movement is driven by evapotranspiration, a process similar to human sweating. Plants use evapotranspiration to cool their leaf surfaces. As leaf temperature increases, plants pull more water from their root zone.

Water attempts to leave the leaf surface, reducing leaf surface temperature…but only if the water can easily evaporate. It high temperatures drive increased transpiration, but the water exits the leaves and then sits there on them because RH is too high, that’s when gray mold and other problems start happening.

Think of yourself in a desert where RH is 20% on a 105°F day. You’re sweating a lot, and you better drink a lot of fluids, but it doesn’t feel so horrible because your skin is dry. As soon as you sweat, the sweat evaporates.

But if you’re in a tropical place, where RH is 90% on a 90°F day, it’s miserable. You and your clothes are soaked in sweat, and the sweat doesn’t evaporate, because there’s too much moisture in the air already.

The dynamic interplay between temperature and relative humidity is called vapor pressure deficit. Here’s how it works: air’s water vapor content can be measured as pressure and is part of total air pressure. To make things even more complicated, all gases and vapors in air have their own pressures, called “partial pressures.”

The thing to focus on is the word pressure. It means exactly what you think it means–the air and what’s in it, including water vapor, puts pressure on your marijuana leaves in varying degrees. If it puts too much pressure on the leaves, the water that your leaves are trying to “sweat out” via transpiration can’t get out as easily. And even if it does get out, it doesn’t evaporate as quickly or at all.

Saturation vapor pressure (SVP) is the maximum amount of water vapor that could exist in the air at a specific temperature. The difference between the pressure of water vapor actually in the air (called actual vapor pressure (AVP)), and the SVP of that same air is called the vapor pressure deficit (VPD).

Sorry if this is confusing. Let me put it another way: vapor pressure deficit is the difference between actual vapor pressure and saturation vapor pressure. Plant scientists have discovered ideal VPD/temperature correlations for cannabis plants and it’s important to manage your grow room’s air temperature and relative humidity to stay in range because out of range VPD causes plants to take too much water through the roots, or not enough, creating an interlinking set of problems that harm plants, slowing growth and productivity.

Indoor grow room VPD issues often arise because we want to keep our grow rooms at the ideal temperature for marijuana plants. Plants grown in grow rooms without added C02 don’t like temperatures higher than 76°F, while plants grown in C02-added rooms can handle temperatures as warm as 82-84°F. A crucial fact: warm air can hold more water vapor than cold air.

Fortunately, we’re including a vapor pressure deficit chart in this article. I have this chart on my grow rooms’ walls. The chart shows you the acceptable VPD for various temperatures (the ideal range is colored green).

Use this chart as a guide for controlling grow room temperature and relative humidity so your plants breathe easily and have the optimal amount of internal water flow.

However, it’s not as simple as just ensuring that the temperature of your marijuana garden and the RH in your garden are in the favorable green zone seen on that chart. Here’s what I mean:

  • The vapor pressure deficit chart is for established plants, not for new cuttings or just-germinated seedlings. New cuttings should be in a humidity dome in a temperature range of 74-77°F and relative humidity from 75-95% until they’ve developed enough roots to be grown in open air.
  • Newly-germinated seedlings usually can survive without a humidity dome, but dome can be a benefit if your grow room RH is below 50%. Seedlings do best with a 75-77°F temperature range and relative humidity from 60-73% until a week or two after germination. I’ve seen seedlings fall over and die from too-low RH.
  • When using the vapor pressure deficit chart, the best tactic is to use leaf surface temperature rather than ambient grow room temperature. You measure leaf surface temperature using a digital infrared horticultural thermometer. They don’t cost much, and some models can also be used to test you for COVID-19 fever!
  • One of the biggest challenges is that keeping your VPD in range can sometimes become a juggling act between controlling your air temperature and your dehumidifier…
  • For example, if you’re running a C02-augmented grow op and the ideal temperature for your plants is 82°F, the chart tells you that your RH should be from 70-85%. But what happens if you’re in bloom phase with dense gooey buds on your plants? RH that high is almost certain to cause gray mold. That’s when you have to use your judgment, and often compromise away from ideal temperature, to fight gray mold, powdery mildew and other pathogens or problems that come from the wrong temperature or humidity while also trying to stay VPD within range. It’s a tricky balancing act.

The harsh thing is that if your VPD is out of range, your plants will either drink too much water, or not enough. You may have to do substrate management, as shown in the embedded video.

When I used to run grow ops in the desert, I had to add a humidifier to my grow room to put moisture into the air. Of course, people growing in places where it’s humid year-round, or who have air conditioning equipment that’s not efficient at sucking moisture out of the air (air conditioners can be a form of dehumidifier as a side gig to cooling the air), often have to work hard to add or subtract water from their growing environment. I recommend Quest dehumidifiers and here’s why.

In outdoor marijuana growing, of course, you can’t control relative humidity, temperature or VPD unless you have godlike powers or are growing in a greenhouse with an exhaust fan. But you still should pay attention to the chart. Why?

Because VPD problems often manifest as leaves drooping, looking like your plants are overwatered or wilting, which are two very different problems, but neither one of them could actually be happening. Your leaves might look like that because your plants’ water transport system is shut down due to VPD issues.

For every minute of your light cycle when VPD is out of range, your plants aren’t able to engage in their full metabolic/photosynthesis acceleration. Growth slows, plants mature more slowly, buds develop more slowly, your growing season extends longer than you’d anticipate.

If your head is spinning from all this humidity talk, never fear. Refer to the chart, closely study the videos embedded in the article, and just do your best.

The weird thing is, the dangerously nice guy Paul, who was the model for one of the characters in the movie No Country for Old Men, and runs the New420Guy cannabis seeds company, is growing his new ass-kicking Homer strain outdoors in the Nevada desert right now, and his plants are killin’ it, even though daytime temperatures are over 100 degrees Fahrenheit and humidity is only 10-20%!

Take a look at the VPD chart and you’ll see that Paul’s monster marijuana plants are growing way outside ideal VPD range. That shows you how strong and resilient his Homer strain is.

So…yes you can get a successful harvest of big, sticky buds even if your plants aren’t in perfect VPD every second of their lives. But the more seconds of perfect VPD they have, the better!

Vapor pressure deficit is an important humidity and temperature factor in marijuana growing.