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VPD stands for Vapor Pressure Deficit, and it’s the difference between a leaf’s vapor pressure and the air’s vapor pressure. VPD control is related to the following:

1. Modifying metabolic rate
2. Improving yield quality
3. Determining plant stresses
4. Altering pathogenesis
5. Injecting carbon dioxide

Managing VPD lets me get away with using fewer nutrients while also seeing increased trichome production.

Relative humidity (RH) and VPD are inversely related. This means that when relative humidity (RH) is high, VPD is low, and vice versa. Let’s examine how plants react to changes in VPD:

1. The bulk flow of water changes within a plant’s xylem as VPD changes.
   1. High VPD/Low RH: The plant increases water usage, increasing stress.
   2. Low VPD/High RH: The plant reduces water usage, reducing stress.
2. Nutrients follow the flow of water through the plant tissues.
   1. High VPD/Low RH: Nutrient uptake increases.
   2. Low VPD/High RH: Nutrient uptake falls.
3. Plants open and close their stomata to regulate moisture loss. If you’re using carbon dioxide, you want the plants’ stomata to stay dilated to maximize gas exchange.
   1. High VPD/Low RH: Plants close their stomata, reducing CO2 uptake.
   2. Low VPD/High RH: Plants open their stomata, increasing CO2 uptake.

If your VPD is too low, then your plants aren’t going to acquire enough nutrients, slowing growth; if your VPD is too high, you’re going close the stomata, rendering your extra carbon dioxide ineffective. There’s a sweet spot in the middle.

Yes, but this is true of any environment. How you control your VPD is based on a lot of factors, including:

1. Heat sources
2. Number of plants in the room
3. Environmental controls
   1. Air conditioners
   2. Humidifiers
   3. Ventilation
4. Climate and time of year

Most indoor growers typically use big, powerful AC units to control heat. Air conditioners act as dehumidifiers, so indoor growers need to worry about how to reintroduce moisture into the room. The humidity in greenhouses is generally elevated at most times of the year. The trick is that the humidity can vary wildly based on the climate. You need extra hardware to compensate for seasonal changes.

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Your basic techniques need to be dialed in before you try VPD control:

1. Keep the environment as clean as possible. Higher humidities increases the likelihood of pathogenesis.
2. Be diligent about airflow. Precise VPD control requires optimized air flow.
3. Monitor your water usage. Be careful to not cut off oxygen to the roots.

When everything else is accounted for, the primary technique that complements VPD control is carbon dioxide injection. Fine-tuned VPD controls the behavior of the stomata, allowing increased gas exchange.

When you put clones under a dome, you’re keeping the RH high and VPD low. This reduces stress and gives the cuttings time to form roots. Additionally, most growers keep their vegetative humidity higher to reduce stress. You want to keep your VPD relatively high (low RH) during the flowering cycle. Start with a moderate VPD during the first weeks of flowering, then increase your VPD (lower your RH) towards the end of flowering to prevent pathogenesis.

The biggest drawback to running a low VPD (high RH) is that you can experience pathogenesis if your rooms aren’t clean. Along the same lines, room homogenization is a struggle. There are always microenvironments forming due to the nature of living organisms.

A big mistake people make when they scale up is failing to segment their new space into smaller rooms. Good growing is about having a series of smaller rooms that are easier to control.

There’s not a lot of research out there. All strains can be affected differently in the same environment. Lineages from humid environments will likely prefer humid conditions, and the converse is also probably true.

1. Humidifiers for starters. Personally, I think that ultrasonic humidifiers work best.
2. Double or triple redundancy on your air filtration if you’re making an environment hospitable for fungal pathogens.
3. A controller that integrates your different systems and has multiple setpoints based on factors like time of day.
4. Some sort of CO2 injection system.
5. Grow tents are perfect for experimenting because they have a more manageable environment.

VPD stands for Vapor Pressure Deficit. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor pressure) and the air’s actual vapor pressure. A high VPD value raises a plant’s transpiration rate and increases nutrient movement through the the plant’s xylem, while a low VPD value slows the movement of nutrients through the plant.

Editor’s Note: Vapor pressure is defined as “the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.” Essentially, vapor pressure is how strongly a liquid or solid wants to become a gas at its current temperature and pressure, scientifically known as a chemical’s volatility. Vapor pressure is generally expressed in millibars, but other units of pressure can be used (such as kPA, Torr, etc).

VPD control is related to the following:

1. Increasing or decreasing metabolic rate
2. Improvement in yield quality
3. Determining plant stresses
4. Pathogenesis (more on this later)
5. Carbon dioxide injection (more on this later)

Growers should care about VPD because it impacts yield quality, overall plant vigor, and nutrient utilization. Managing VPD lets me get away with using fewer nutrients, which improves my bottom line. I’m also seeing increased trichome production in the plants, which naturally follows better health.

You reach expert growing level when you learn to manage humidity and VPD. Everybody spends their time managing temperature, nutrients, and whatever else, but the last little thing you learn to manage is humidity, and it is significantly more finicky.

In order to stay on the same page, I should point out that relative humidity (RH) andVPD are inversely related. This means that when relative humidity (RH) is high, VPD is low, and vice versa. When growers want to know how to steer plant growth, they are interested in maximizing growth. So let’s start with how the plants react to changes in VPD:

1. The bulk flow of water changes within a plant’s xylem as VPD changes.
   1. If you have a high VPD, meaning that the RH is low, the plant will increase its transpiration rate and start pulling water faster from the substrate in an effort to stay cool and moist.
      1. If the VPD is too high, the plant will become stressed, leading to inefficiencies.
   2. In the same vein, if the VPD is too low, meaning that the RH is high, the transpiration rate will decrease, slowing the flow of water through the plant.
2. Nutrients follow the flow of water through the xylem and into its various tissues. Nutrients like calcium primarily move with the bulk flow of water through the arteries of the plant.
   1. Therefore, as VPD rises (and the bulk flow of water increases), nutrient uptake will also rise.
   2. If VPD falls (and the bulk flow of water decreases), nutrient uptake will also fall.
3. If you’re injecting carbon dioxide, you want the plants’ stomata to stay dilated for as long as possible. Ideally, the stomata would be fully dilated at all times to maximize carbon dioxide use. Plants open and close their stomata to regulate moisture loss.
   1. If you have a high VPD, or low RH, your plants are going to close their stomata to reduce water loss. When the stomata close, you’re not getting adequate gas exchange, and you’re not making the most of your carbon dioxide.
   2. If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide.

If your VPD is too low, then your plants aren’t going to acquire enough nutrients, slowing growth; if your VPD is too high, you’re going to stress the plant and the stomata will close, rendering your extra carbon dioxide ineffective. Like everything else with growing, there’s a Goldilocks zone. One VPD is too high, one VPD is too low, and one VPD is just right. There are charts of a VPD curve with a three way graph of humidity, temperature, and growth. There’s a sweet spot along the center of the chart.

Editor’s Note: Please note that the above chart is for a hypothetical plant, and the chart for an individual strain will differ. Additionally, take a look at the resources at the bottom of this article to learn more about psychrometry, the science of measuring water vapor!

If you control your VPD, you will start to see a plant’s full potential. I’ve heard it said that you can be the greatest grower in the world, but without getting VPD to where it needs to be, you’ll never grow the best plants. I should point out that any decent facility should have a testing room. We have a testing room here where you can test different things, like new cultivars or new nutrients. Get a grow tent to experiment in because it’s a smaller, more manageable environment.

Growing is about experimentation. A lot of people are afraid to experiment because it can go wrong and, in this business, mistakes cost money. With this much money at stake, people are unwilling to gamble. Many growers thought I was nutty when I started experimenting with VPD, but science doesn’t lie.

Yes, but this is true of any environment. How you control your VPD is based on a lot of factors, including:

1. Are your lights generating heat?
2. How many plants do you have in the room?
3. How are you controlling the environment?
   1. What sort of air conditioners do you have?
      1. What is your ideal temperature? Is it ideal for the plants?
   2. How is the room being humidified?
   3. How is your room ventilated?

VPD control in a greenhouse also depends on some additional factors:

1. Is your greenhouse open-ended? (IE: Polytunnels)
2. What climate do you live in, and what time of year is it?
3. Do you use light deprivation?

One thing worth noting for both indoor and greenhouse grows is that outside conditions will affect your environment. For example, in winter when temperatures are colder, RH naturally tends to be higher. Air conditioners cycle less when it’s cold, so they take out less moisture, making it easier to maintain a high humidity setpoint. Whereas in summer when it’s hot and dry, the air conditioner is running all the time. In those situations, you have to inject moisture into the air constantly in a struggle to keep up with the humidity setpoint.

The challenge with maintaining VPD control indoors is that most growers typically use powerful AC units to control heat from the lights in the room. However, air conditioners also act as dehumidifiers, taking an incredible amount of moisture out of the air. Indoor growers need to worry about how to reintroduce moisture into the room and keep it at proper levels. If you’re growing with light emitting ceramics or LEDs, which don’t put out as much heat as HPS, your air conditioners probably aren’t going to run as much, and you’ll need to adjust your humidity setpoints to compensate. Greenhouses, on the other hand, are basically grow rooms powered by the sun. Generally speaking, the humidity in greenhouses is elevated at most times of the year. Because of this, maintaining high humidity levels may not be difficult. The trick is that the humidity in greenhouses can vary wildly based on the climate. For example, Oregon is an incredibly wet climate. The humidity is close to 80-90% for most of the year, except for three months out of the year when it’s really hot and really dry, like 15-20% humidity. You need extra hardware to compensate for seasonal changes.

In short, a lot of this stuff is room-specific. Any grower will tell you that all rooms operate as their own entities. Each room has its own personality. Even if you set each room up in the exact same way as you did in another location, they might not operate exactly the same.

Growing with VPD control is a lot like the difference between driving the family station wagon and operating a Ferrari. Normal growing techniques are like the family station wagon. They’re an old tank. You can mess up and still get to wherever you’re going. If you want low-medium quality cannabis, you can throw it into a low-tech room and it’ll grow just fine. However, if you really want to see three pounds per light of high-quality cannabis, you have to have everything dialed in like a Ferrari. It can perform amazingly well when it’s properly cared for, but if one thing is wrong, you can lose a drastic amount of performance.

This is not a lazy man’s technique. Your basic techniques need to be dialed in before you try VPD control. Generally when I talk to people and they decide they want to try it out, I suggest the following:

1. Keep the environment as clean as possible. VPD control needs good cleaning techniques and behaviors.
2. Be diligent about airflow. Precise VPD control requires optimized air flow. If a room has poor air circulation, it can experience a lot of problems. This means plants need to be pruned regularly, and extra fans need to be installed.
3. Monitor your water usage. When we switched away from a “standard” growing environment, where the humidity was kept low to reduce pathogenesis, we saw much lower water usage afterwards. You want to be careful that you don’t over water and cut off oxygen to the roots.

When everything else is accounted for, the primary technique that complements VPD control is carbon dioxide injection. Fine-tuned VPD allows you to control the behavior of the stomata, in turn allowing you to increase gas exchange. It’s a big waste of money to buy expensive CO2 devices if you’re only getting half of their potential.

We collect all the condensate water from our air conditioners and put it into tanks. We then filter it, subject it to UV sterilization, and pass it through ultrasonic humidifiers that rehumidify the room. We recapture any overflow and use it to water the plants. The advantage to collecting condensate like we do is that it’s basically distilled water. The AC units distill the water for you, right out of the air. There are no dissolved solids in it. If you have any amount of dissolved solids in the water that you’re using to rehumidify an environment, you will get a fine layer of minerals on all of your equipment. It will look like it snowed in the room because of the calcium deposits. You need to make sure the water is distilled and there isn’t anything in the water.

If you choose to reuse the condensate water, then water treatment is an important step. You will need biological controls to treat the water, things like UV lights and ionizers. You don’t want to pump condensate filled with fungal spores around the room. If you’re already raising the humidity for VPD control, you could have a nightmare situation.

As a matter of fact, most growers use some form of VPD control already, without even knowing it. When you put clones under a dome, you’re keeping the RH high and the VPD low. This, in turn, slows transpiration to a crawl, greatly reducing the stress on the cuttings, which need time to form roots. Typically, most growers will keep their vegetative humidity a little bit higher as well, which reduces stress.

Most growers are concerned about the flowering cycle because that’s where the magic happens. You want to keep your VPD relatively high (low RH) during the flowering cycle. If you assume an average flowering cycle of 8 weeks, start with a moderate VPD (medium RH) during the first 3-4 weeks of your flowering cycle, then increase your VPD (lower your RH) towards the end of flowering. This reduces pathogenesis. One thing you can do when a plant is stressed, say from moving from one room to another, is to raise the humidity. This lowers the transpirational stress and eases their transition into whatever phase or room you have set up. Additionally, HID lights can be stressful for plants, and VPD control gives you the ability to reduce their stress. If you have a dry environment and bright lights towards the top, you’ll see canopy leaves fold in like a taco. Plants do this to reduce light capture and reduce their internal temperature. If you see this happening, you need to ease up on the plants and reduce their stress.

Pathogenesis is a big issue, which we’ve touched on briefly. The biggest drawback to running a low VPD (high RH) is that you can run into a lot of problems with pathogens if your rooms aren’t clean. As a result, many growers reduce their humidity as much as possible. Some growers brag that their humidity is as low as 20%, which is really bad for the plants and slows their growth. Homogenizing a room’s environment is a struggle. In my experience, there are always new micro-environments forming in your room due to the nature of working with living organisms. Keeping on top of it all takes a lot of effort.

Accurate sensor readings are also a problem I keep running into. Keeping the environment at your desired setpoint of temperature and humidity can be tricky. Having the right equipment and the right room layout can make a big difference.

You’re going to need humidifiers for starters. You want to be able to inject humidity into the room without causing any problems such as being too close to one plant. If you have your humidifiers spraying plants directly with vapor, you will end up with undesirable microclimates which could favor pathogenesis. Personally, I think that ultrasonic humidifiers work best. One important thing is to have good air and water treatment protocols. We installed ionizers and UV lights into our air conditioning system in order to reduce spore counts in the air. We also purchased a hydroxyl generator that creates surface-bound hydroxyl radicals that react with pathogens in the air. Double or triple redundancy on your air filtration is especially important if you’re going to be maximizing an environment that fungal pathogens tend to thrive in. If you’re going to manage VPD, you’ll also want a controller that integrates your humidification and dehumidification systems. You want your controllers set up in such a way that when the lights are off, the humidification setpoints for the dehumidifiers are different. Typically controllers that can do this are used for greenhouse management but you can integrate them into your indoor grow room just fine. That being said, if you don’t have the money or the size to put a $20,000 controller into your rooms, it’s not worth the risk for you. This is an advanced technique. When it comes to lights, there’s a lot of debate about what is best and it has always been that way. Whether you’re going with DimLux, Gavita, Nanolux, or Solistek… They’re all pretty close and it’ll be more profitable for you to improve your technique rather than splitting hairs over lighting technology. Editor’s Note: Our other article on VPD mentions a variety of controller manufacturers capable of integrating several sets of data.

A big mistake people make when they scale up is failing to segment their new space into smaller rooms. They build one giant room, making it much harder to manage VPD. Good growing is about having a series of smaller rooms that are easier to control.

If you’re moving into a large room, you need to think about placement of multiple humidifiers and how that’s going to distribute over, under, and through your canopy. You’re always going to generate microenvironments, but you should always work to avoid them.

It’s important to note that plants are their own internal humidifiers, depending on how many plants are in a room and what stage of growth they’re at. Small plants have less surface area and transpire less. Small plants in a big room will require humidity injection to keep the humidity up, whereas plants at full size don’t need as much humidity injection because they’re already transpiring at an increased rate. When you hit the final stages of growth, you may have to run dehumidifiers to take water vapor out.

You should always be worried about pests, of course. No environment is a sterile environment. Some pests prefer higher humidity, and some like lower humidity. I know that different species of mites prefer different conditions. VPD is typically the last thing growers get to learn after they’ve mastered everything else. They should already have a good handle on the basics of growing before attempting it.

There’s not a lot of research out there on cannabis. Between cultivars, it’s hard to say why one strain likes more phosphorus than another one. VPD is no different. I know that all strains can be affected differently in the same environment. Sometimes the effects are subtle. Some strains are more difficult to clone while some strains can do well with a slightly higher temperature. We’ve got a couple of cultivars that need to be kept at a colder temperature. Some strains might do well based on their lineage. Strains that trace back to environments with higher humidities will likely do better in high humidity conditions, while strains that have lineages tracing back to drier environments would likely do better in lower humidity. There is also the question of how sativa-dominant strains and indica-dominant strains compare. Sativa-dominant plants have thin roots and a natural resistance to fungal pathogens that is likely the result of being bred in high humidity environments like the tropics. Indica-dominant plants are bushier and have a natural tolerance to drought, probably from being bred in low humidity environments.

Editor’s Note: A third type of cannabis also exists, known as Cannabis ruderalis. Ruderalis is a variety of cannabis native to Russia and northern Europe, and is known for autoflowering traits. If you have autoflowering cannabis, it may prefer slightly cooler temperatures.

I started growing commercially about 10 years ago in Eugene Oregon. Like most growers, I grew to make a small side income. As time went on, I expanded slowly, improving my size, production and technique. About three years ago the winds began to shift in Oregon and cannabis attitudes started to change for the better. With the advent of HB 3460, I decided to throw my hat in as a professional cannabis producer. I formed Higher Minds Horticulture and began building a brand with only $30k of my own money and 5 maxed-out credit cards. I expanded into a new facility where I am currently the sole owner/operator. When cannabis became recreational here, it was clear that there was an opportunity to get in on the ground floor of a burgeoning industry. After an incredible amount of work, we submitted our application to the state and we were approved as the first pure indoor cultivator in the state, or so claimed my state inspector.

Thus far I have experience with organic, soil-mediated production, both indoors and out. I tried a brief stint with both ebb and flow and deep water culture hydroponics. I’m currently cultivating in Coco-based media utilizing top-feed drain-to-waste and the latest bio-mineral nutrition. We utilize microbials and foliar sprays to enhance our technique and provide an edge. Recently, I’ve taken an interest in plant hormones, although I’m still experimenting and haven’t put them into our main products because of consumer leeriness. Today we’re doing what almost everyone else in this industry is doing: growing a business in uncertain times and unfriendly regulations, managing consumer demand, building a brand, attempting to stay ahead of industry trends, and producing high-quality cannabis every day.

You need to be fast on your feet in a rapidly changing environment. Be relentless and be prepared to spend a majority of your time cleaning and fixing things. Cannabis production isn’t glamorous, despite what news outlets may tell you. It’s hard work; you will smile, you will hurt. Sometimes you will cry. But there is one certainty in this business: it will test your limits, both physically and mentally.