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.

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.

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.

Traditional growing methods will produce cannabis reliably. VPD control methods can exceed your expectations, but require a lot of effort.

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.

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.

Condensate from AC units can be used to water plants if the water is treated first.

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.

Domes are a form of VPD control.

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.

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.

Good ventilation is necessary for VPD control.

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.

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.

One method of segmentation involves building smaller rooms.

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.