Vapor pressure deficit (VPD) is a weird scientific concept and kind of hard to understand, even when I’m not high, lol.
But it’s a crucial concept 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–if all this word salad I’m typing 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.
If you start reading the article and it’s too much detail, just use the article’s chart in your grow op. Keep your VPD in the green zone by manipulating the dynamic between temperature and relative humidity, and you’ve solved the VPD puzzle!
We already know 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, for example, it’s at 100% RH.
Before I understood VPD, I only cared that RH not get too high or too low, for very simple reasons.
Too-high RH helps gray mold (botrytis cinerea) grow on my buds.
Too-low RH stresses my plants by sucking a lot of water out of them through their leaves.
When plants experience humidity-related “drought” stress and intake a lot of water, they intake a lot of nutrients. This costs you extra money for nutrients, and can also burn your plants with excess nutrients salts.
What we must understand is that RH and temperature are the primary indoor climate variables influencing water movement within cannabis plants.
Plant internal water movement is driven by evapotranspiration, a process similar to human sweating.
Plants use evapotranspiration to cool leaf surfaces. As leaf temperature increases, or as relatively humidity drops, plants pull more water from their root zone.
Water attempts to leave leaf surface, reducing leaf surface temperature when evaporation happens…but this works only if the water can easily evaporate.
If high temperatures drive increased transpiration in a high-humidity environment, water exits leaves and then sits there on them because RH is too high—that’s when gray mold and other problems start happening.
Consider these examples:
Think of yourself in a desert where RH is 20% on a 105°F day. You’re sweating a lot, and better drink a lot of fluids, but your clothes and skin are dry because 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, your clothes are soaked in sweat, and the sweat doesn’t evaporate, because there’s too much moisture in the air already.
Your plants leaves are kind of like “skin,” and function best in a rather narrow interplay range created by temperature and relative humidity, 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, presses on your marijuana leaves in varying degrees from all sides.
If it puts too much pressure on the leaves, the water 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, creating pathogenic conditions that ruin buds.
Plant scientists have discovered ideal VPD/temperature correlations for cannabis plants.
Manage your grow room’s air temperature and relative humidity to stay in the green range on the chart, and you are a wise grower.
Indoor grow room VPD issues often arise because we want to keep our grow rooms at ideal temperature for marijuana plants. It’s tricky to keep the temperature, RH and VPD exactly where you want them.
Plants grown in grow rooms without added C02 grow best in temperatures between 73-77°F. Plants grown in C02-added rooms do best when grow op temps are 76-85°F.
You find yourself working hard sometimes to control temps and humidity to keep VPD where it needs to be.
Fortunately, we include a vapor pressure deficit chart in this article. I have this chart on my grow rooms’ walls. The chart shows 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 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 an aerated humidity dome in a temperature range of 75-78°F and relative humidity from 68-85% until they’ve developed enough roots to be grown in open air.
- Newly-germinated seedlings usually can survive without a humidity dome, but a dome can be a benefit if your grow room RH is below 47%. Seedlings do best with a 75-77°F temperature range and relative humidity from 58-68% until a week or two after germination. I’ve seen seedlings fall over and die from too-low RH or bad VPD.
- When using the vapor pressure deficit chart, consider using leaf surface temperature rather than ambient grow room temperature. You measure leaf surface temperature using a digital infrared horticultural thermometer.
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 added 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), 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.
Because VPD problems outdoors and indoors 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 bad 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 science-y talk, never fear. Refer to the chart, closely study the videos embedded in the article, and do your best.
The weird thing is that a guy named Paul, the model for one of the characters in the brutal movie No Country for Old Men who runs the New420Guy cannabis seeds company, is growing his 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—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!