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!