Every aspect of your grow contributes to your environment, and you can manipulate these variables to attain the optimum combination of temperature, humidity and air quality for your space. Vapor Pressure Deficit, also known as VPD, quantifies the synergy of these variables and reflects the optimum temperature and humidity for photosynthetic production.
What is VPD?
VPD refers to the difference in vapor pressure inside and outside of the plant, which affects transpiration and photosynthetic rates. Vapor pressure determines at what point a liquid becomes a gas or vapor. If VPD is too low, a plant will not transpire properly and could become prone to rot and other problems.
If the VPD is too high and the room is too dry or doesn’t have enough humidity, the plant transpires too much, and is not as efficient. In higher humidity/higher temperature situations, additional variables like CO2 may be introduced to compensate for issues. This concept is relatively new for cannabis, but has been a staple of the agricultural industry for 100 years. It’s not a new understanding, but rather a new, fledgling industry that’s adapting from an incomplete basis of information.
Cautions with VPD
It is important to note that VPD is an equation much like photons to yield; it doesn’t tell the entire story, but it is a good baseline to shoot for if you don’t know where to be or you don’t have a feel for your room. The incredibly important factor and deviation from what the VPD chart says is that if you have plants that transpire heavily or you are later in flower, botrytis (bud rot) becomes a critical variable, which will need to be adjusted for.
VPD maximizes photosynthesis but not necessarily the health of a plant; the fastest growing plant is not always the most efficient or healthiest plant. Total photosynthesis is only one piece of the end game. More photosynthesis means more plant growth, but we are oil farmers, and plant growth doesn’t correlate to increased resin production, quality, or concentration of terpenes, in fact, quite the opposite. Health of plant does. If your plant leans too steeply into rapid growth you’re potentially losing potency, terpenes and overall bud quality.
Individual plants react differently to different growing systems in varied environments. Remember, cannabis has a ton of variation, and understanding how your plants work in your room is your number one priority. VPD is simply a guideline to help point you in the right direction.
If you don’t know anything about it, it’s a good guideline for asking "Where am I at? Where should I be?” I think that the chart is most applicable–without dramatic deviation–in vegetative growth and the first two weeks of flower. In vegetative state you’re not as worried about mold and mildew spores. After that, once the plant starts producing resins and there are other variables in play, it goes back to simply just being a guideline to work off of.
Colin’s Environmental Suggestions
Regardless of whether you are just now hearing of VPD or whether you ignore it entirely, your plants will still grow unless you’re way off, in which case you’ll see a big difference. Otherwise, applying VPD as a guideline typically only requires minor adjustments, and proves helpful when you aren’t sure yet about your room’s environment.
It’s also important to note that these recommendations are conditional to growing style, growing medium, density of growth, defoliation, air circulation, ventilation, CO2 levels, levels of light and how much control of temperatures. Individual plants have individual needs and your situation or plants may require different conditions.
Clones & Veg
In veg I like a warm, relatively humid room. In your clone room or veg room you want to make sure your ambient temps are never below 68ºF even with lights off (matters more for clones). In veg, the preferred temperature range I’m shooting for is to have ambient temperatures ranging from 76-80ºF with the hottest spot in the room hovering below 90ºF. I keep this consistent to avoid significant temperature drops when the lights go off. If needed I will heat rooms to keep the rhizosphere warm so it doesn’t slow root growth and metabolism. I try to keep my humidity in veg no lower than 40%, usually shooting for about 60%. I recommend purchasing a humidifier and dehumidifier to have on hand as needed.
Humidity in Flower
In flower your humidity is going to be higher during the light cycle when your plants are transpiring the most. During the first hour of the dark cycle, they continue to transpire, which is why humidity in the room typically peaks right after the lights go off. Humidity doesn’t need to be as high when the plants aren’t photosynthesizing. Medium humidity swings are ok, ranging from 40-60% but be sure your humidity is not below 40%. I’m typically fine with up to 70% but if I’m at 65 I’m trying to get lower because I'm still shooting for 60% as my ideal.
I recommend decreasing humidity gradually throughout flower and then rapidly drop off in correlation with nutrient reduction.
• First Trimester, first 3 weeks of flower: 40-60% and shooting for between 50-60%
• Second Trimester, next 3 weeks: I’m shooting for 40-50%
• Plants are transpiring much more heavily
• There's more foliage in the room
• There’s typically less air circulation.
• I try to keep a slightly lower ambient humidity to compensate for pockets of high humidity. My ambient humidity target is relative to how much the room itself is putting out moisture and holding it in the air. Some rooms are able to create their own ecosystem within themselves if plants are big enough as the canopy will contain moisture from the transpiration. This is why it is imperative to keep good air flow through the canopy, and good circulation within the room.
• Last Trimester, Week 7-Harvest: I’m going low; 40% consistently.
• A drier room typically yields a more resinous plant; I’m ok with a moderately lower photosynthetic rate in exchange for a more resinous product so I’m shooting for 40% consistently; significant humidity fluctuations at this point can exasperate mold and mildew issues. I will lower the humidity if I am running genetics that are prone to botrytis; lower humidity reduces the chance of bud rot significantly, in conjunction with circulation and proper airflow..
• At this point, I’m using dehumidifiers to regulate humidity. These run off of a detector tied to sensors in the room, and the system is triggered to run relative to how I set the parameters.
• If you don’t have sensors/triggers, I recommend running a dehumidifier for 14 hours: an hour before and after lights on.
• If the room is fairly humid, there’s nothing wrong with running the dehumidifier at night, and you might get higher resin production, you just don’t want it to get critically low at night (under 20%).
Temperatures in Flower
In flower, end game dictates temperatures; warmer temps at night and day will get you larger buds. Colder nighttime temperatures will increase colors and likely increase potency and terps but will typically lower your yield a bit. If you're aiming for the most distinction and quality, shooting for 40s at night will bring out all maximize these expression; more dramatic phenotypical expression will come out if you bring nighttime temperatures to the 40s.
Air Quality is King
Regardless of your temperatures and humidity levels and the VPD of your system, air quality is king. Making sure your plants are receiving fresh, conditioned air ensures a healthy and bountiful grow. Stagnant air, stagnant humidity and/or air contamination will lead to a rabbit hole of issues which is why I advise paying attention to the quality of the air in your room above all else. Make sure you have proper circulation as well as ventilation (air exchange).
• If your air circulates properly and leaves properly, you should be able to blow smoke and watch it circulate around the room without flying straight out of your exhaust. But I don’t suggest smoking in your room ;) .
• In addition to typical fan placement I like to use fans on the floor to blow air up through the canopy to avoid stagnant pockets and also help with CO2 distribution.
• I like to get a little tornado coming up from underneath, bringing all that air from underneath the canopy to the top, creating a weather system of sorts.
• Air circulation, especially designing the circulation of your room, makes more sense as the need for it increases in a bigger room. In a 3000 sq. ft. room, if you don’t have fans in the middle of the room blowing up, there’s no way you won’t have stagnant pockets in that room, especially late in flower.
• Significant circulation and ventilation are needed for proper air quality.
Once you dial in your room’s temperature, humidity and air quality you can begin tinkering with minor adjustments to your VPD and system depending on the specific needs of your plants and grow space.
“Explore the space.”